1. Handle heifers carefully when they are first milked. A cow must like to be milked if there is to be a com­ plete response to milking. Consequently, special care is necessary so that newly-freshened heifers do not come to associate unpleasant sensations with milking. 2. Avoid the unusual during milking. Such things as strange noises may attract the cow's attention and consequently prevent complete response to milking. 3. Do not treat the cow roughly at any time, particu­ larly just before and during milking. 4. The milker must be a person who does not arouse the cow's suspicion. Some people, by their actions, create nervousness in cows and cannot be good milkers. 5. Do not wash or massage the udders or stimulate cows in other ways to let down their milk before milking is to begin. 6. Milk those cows first which let down their milk in response to preparations for milking, such as start­ ing the milking machine, clanging of utensils, etc. 7. Milk rapidly. If milking takes more than seven min­ utes for most cows, the effectiveness of the hormone that is essential to letting down milk will be partly worn out and incomplete milking will result. 8. Do not practice prolonged stripping because this will ultimately make a stripper of the cow. 9. Operate milking machines according to the manu­ facturer's directions. Increasing the vacuum or alter­ ing the rate of pulsations from those recommended by the manufacturer may make milking unpleasant for the cow. 10. Do not leave the milking machine on the cow after the milk has ceased flowing because this may injure the delicate lining of the teat cistern. 11. Develop the technique of knowing by feel when the gland has been emptied of milk.

Accepted for publication June 19, 1942 5M-4-44 The Cow's Udder

W. E. Petersen

ROM AN ECONOMIC standpoint the A better knowledge of the structure, F cow's udder might be said to be the growth, development, and functioning most important unit in agriculture be­ of the cow's udder not only will bring cause nearly one fifth of the total farm a better appreciation of the work the income in the United States comes from cow is doing but will also form the milk. As a contributor to human well­ basis for better management of this being, the udder of the cow is most im­ important gland. The purpose of this portant, for nutrition auth~?rities agree bulletin is to set forth the more im­ that milk and m ilk products are essen­ portant known facts about the growth tial in a good diet. Because milk is so and dev lopment, the structure, and the essential in the diet, the cow h as been functioning of the cow's udder. referred to as the "foster mother of the human race." From the standpoint of activity and complexity of its processes, the u dder Growth and of the high-producing cow is unexcel- Development 1 d by any other gland. Each year, good cows usu ally produce 10 times their w ight in milk, and exceptionally good Because of the hund•reds of years of cows have produced 20 times their selection for high milk production, the weight. mammary glands of the good dairy cow There is still much to be learned are proportionately better developed abou t the complex processes of mak­ than in any other species. How this de­ ing milk in the udder. Few dairymen velopment has taken place is best un­ appreciate th complexity of the ma­ derstood by briefly considering the more chine that they are d aling with daily. important known facts about (1) the

FIG. l. A well­ developed udder 4 MINNESOTA BULLETIN 361

FIG. 2. (Above) A section of the udder of a calf at birth

(Below) A sec· lion of an udder of a lS·months· old heifer

evolution of the mammary glands, (2) The development of the mammary development before birth, and (3) de­ gland from this point on has been com­ velopment after birth. bining more glands into one, develop­ Evolution of Mammary Glands­ ment of a gland cistern into which the From studies of the most primitive ducts drain, and a reduction of the num­ mammals, as well as from embryologic ber of ducts in the teat to just one in studies, there is good evidence that the the case of the cow. mammary glands are modified skin Development before Birth-Science (sweat) glands. In the simplest of the produces further proof that the mam­ living mammals, the egg-laying echidna mary gland is a skin gland. When the of Australia, the mammary glands look bovine embryo is about two-thirds inch like sweat glands located on the abdo­ long, the single layer of skin cells in the men. There are no teats. The milk from region of the udder starts m ultiplying each of the 100 to 150 glands oozes out and becom es many cells deep. From this of a pore much like sweat gland pores, multiplication of the cells, the embry­ and the young lick the milk off the hair ologist has traced the development of upon which it collects. th teats and the d ucts of the gland. Evidence of the next step in the evo­ At first the cells form solid masses. One lution of the glands is furnished by mass becomes the teats, and others another egg-laying mammal, the Duck­ (sprout';) extend into the tissue. These bill platypus, in which a number of solid masses of cells canalize or open ducts join to form a common opening up in the center to form the teat cistern in the skin, but there is not a teat. In and ducts. animals such as the opossum and kan­ Development after Birth-At birth garoo comes the first evidence of the there is no essential difference in the development of teats. In the simpler development of the mammary glands of forms there are many teats and many the bull and heifer calf. The small gland ducts running through the teat, each is mostly fatty tissue with a very limited to an opening in the end. duct development. In the male there is THE COW'S UDDER 5 little change in the mammary gland de­ two quarters on either side are also in­ velopment with maturity, but in the dependent of each other, and the milk normal heifer progressive changes take produced in one quarter cannot be place with increase in age. drained out of the other. There is, how­ With each recurring estrus or heat ever, no discernible membrane separat­ cycle, the gland becomes larger. The ing the two quarters of a half. growth is due to an increase in the fat Size of Udder and Quarters-Other tissue and an increase in both the size things being equal, the amount of milk and number of ducts. A section through a cow can produce depends on the size the udder of a 15-months-old nonpreg­ of her udder. Poor udder quality or fail­ nant heifer is shown in figure 2. The ure to secrete the necessary hormones tissue is mostly fat and the milk cisterns may prevent a cow with a large udder are as yet poorly developed. A micro­ from being a high producer, but a large scopic examination would reveal no udder is essential for high milk produc­ alveoli (milk secreting structures). De­ tion. Udder size varies greatly. Udders velopment in this stage is due to the from mature cows have been recorded secretion by the ovary of a hormone known as estrogen. After pregnancy the estrogen secre­ tion increases causing a more rapid de­ velopment of the duct system, and an­ other hormone (progesterin) secreted by the yellow body (corpus luteum) of the ovary comes into play. In rats progesterin causes the development of the alveoli and in all probability does the same for the cow, although this has not been proved. While estrogen and progesterin are the main hormones involved in the growth and development of the mam­ mary gland, they are by rto means the only ones. Secretions of the pituitary gland, the thyroid, and several other glands are needed for a fully developed udder. Nutrition also plays a part in mammary gland development.

Structure

EXTERNAL STRUCTURE The normal cow's udder consists of fo ur glands or quarters arranged two on each side dividing the udder into two fairly distinct "halves. " These halves are separated by a membrane and, ex­ cept for some connecting veins, are in­ FIG. 3. A section through the rear quarters of the udder. from the rear. Note sharp line dependent of one another (Fig. 3). The of demarcation between halves 6 MINNESOTA BULLETIN 361

FIG. 4. Cross sections of the udder from different angles (Le ft ) Cros s section approximate ly half way up. Front quarters are to the right. The right front and the left rear quarters were inject ed with pink dye; the right rear and left front quarters w ere injected with blue dye (dark areas). (Right) Cros s section through gland cisterns. Fore udder is shown in the lower part of the photo­ graph. Note the greater porosity compare d to the photograph to the left.

weighing but a few pounds to over 80. the body, especially when the udder is Individual quarters vary greatly in distended with milk. size. On the average the front quarters When fibers in the supporting liga­ make up about 40 and the rear quar­ me nts become stretched or broken, the ters 60 per cent of the total weight of udder breaks away from the body and the udder . Ther e are cases where the becomes pendulous. Strong udder at­ reverse may be true. The rear quarters tachments are of great importance be­ are not as w ide as, but much deeper cause broken attachments and pendu­ than, the forequarters. The rear quar­ lous udders are unsightly and m ore sub­ ters also are usually longer from front ject to injury. to rear than they are wide (Fig. 4). The Teat-Normally each quarter has This makes it impossibl to judge the a teat for expressing the milk. Teats relative sizes of the quarters from ex­ vary greatly in size and shape, the ideal ternal examination. teat being 4 inches long and 1% inches Attachment of Udder- The main sup­ wide. Absence o.f hair from the skin of porting structure holding the udder to the teat is the only distinguishing line the body is the m edian suspensory liga­ between the teat a nd the udder when ment. This is a tough ligament arising the quarter is funnel shaped. At the end from fibers coming out of the lower ab­ of the teat is a n opening known as the dominal wall and sending fibers into streak canal which connects with the each half of the udder . Additional sup­ inside cavity of the teat. The streak port for the udd r is provided by the canal is kept closed by ringlike muscles lateral suspensory ligament, a tough (sphincter muscles) that encircle it at m embrane covering most of the top of the end of the teat. Milking is effected th udder and extending part way down by applying nough pressure to the teat the sides. The skin covering th udder filled with milk to force open th s ring­ also assists in supporting the udder to like muscles. If these muscles are too THE COW'S UDDER 7

Milk

Gland ci~ern

Demarcation be~e~n 1 ted cistern ct qlond cistem 1 ~Wall of the teat

FIG. 5. A section through the teat and gland cistern strong, the cow is a hard milker. If secretory structure, and lobes. Consid­ they are weak, the milk :nay leak out. eration of the circulation and nervous Extra TeatS'--From 20 to 55 per cent connections logically follows. of cows have supernumerary or acces­ The Teat-The teat consists of a wall, sory teats. Supernumerary teats usually a cistern, and a streak canal as illus­ do not secrete milk, but occasionally trated in figure 5. While the wall of the they may be quite troublesome. The teat varies in thickness, a quarter of an supernumeraries are most commonly inch is representative when the teat is located b hind the rear teats. However, distended with milk. Th outside of the they may be between the fore and rear teat is covered with skin while the in­ teats, to the fore of the front teats, or side is lined with a delicate m mbrane. they may actually branch from ither The rest of the wall consists of muscle front or rear teats. and connective tissue as well as blood vessels. The teat wall is elastic, con­ INTERNAL STRUCTURE tracting so that the walls become thicker wh n emptied of milk and expanding The internal structure of the udd r when filled with milk. can best be dealt with by tracing back­ The streak canal is from one-third to ward the path over which the milk has on -half inch long. It is lined with a come, beginning with the teat and going continuation of the outer skin, which is back through the gland cisterns, ducts, arranged in longitudinal folds that in- 8 MINNESOTA BULLETIN 361

the chief place for the secretion of milk, although some secretion may take place in the lining of the ducts and ductules. The arrangement of alveoli in relation to a small duct and its branching duc­ tules resembles a bunch of grapes in which the grapes are the alveoli and the stems are the ductules. The inside of each alveolus is lined with a single layer of cells that synthesize and secrete the milk. As these cells elabora te the milk, it is secreted into the alveolus wher e it is stored until withdrawn by milking. The outer walls of each alveo­ FIG. 6. A cross section of the end of the teat showing the streak canal partly opened. lus are connected with a fine network The light-colored area is the sphincter muscle of blood vessels from which the cells that closes the streak canal get milk-making materials. Lobes- Each quarter is divided into terlock when the canal is closed and several small units known as lob s. A which allow the canal to open without lobe is that part of the udder that is stretching the lining (Fig. 6) . Beginning drained by a duct. Therefore, a quarter at the upper end of the streak canal, contains as many lobes as it has ducts. the teat cistern extends upward through The lobes are further divided into lob­ the teat to the gland cistern from which ules. A lobule repr sents the part of the it is separated by a circular fold of the lobe drained by a branch of the main m embrane lining the teat. This fold duct. serves as a distinguishing line between Connective Tissue- Other things be­ the teat cistern and the gland cistern. ing equal, the producing capacity of an In some cases the fold may extend into udder is proportional to the number th cistern so far that it interferes with the free passage of milk from the upper chamber into the teat cistern. - The Gland Cistern- As shown in fig­ ure 5, the gland cistern is an irregu­ larly sh aped chamber for collecting and storing milk tha t m ay vary in capacity from one-half pint to one quart. The cistern is lined w ith th same ty pe of m embrane as is the teat cistern. Fre­ quently its outer walls m ay contain small milk secreting glands. Milk Ducts- Radiating out from th cistern walls are from 20 to 50 or more large ducts which branch out innumer ­ FIG. 7. Microscopic section of normal gla nd able times until fin ally the branches be­ after milking com e microscopic in size. Th y are th n Alveoli are show n s urrounde d by sing le layers of secre tory e pithe lial ce lls . Two layers of cells known as ductules. The secretory struc­ a re sepa rate d by connective tissue w hich contains blood vessels by me ans of w hich the precursors t ure of the ver y sm allest bra nch s or of milk a re carried from the external pudic a rt ery ductules ends in an enlargement known to the seat of milk synthesis. The d ark a reas are d roplets of milk fat s tain e d with dye. A t lower as the alveolus (Fig. 7). Th alveoli are left is connective tissue. THE COW'S UDDER 9

FIG. 8. Circulation to and from the udder of alveoli that it has. The number of sists of the arteries which bring in the alveoli present in any part of the ud­ blood, the veins which carry it out, and der varies greatly depending upon the the lymph system . Figure 8 diagrams amount of other tissue present, which the principal featur s of the circulatory may be determined by inheritance or system which supplies the udder with udder disease. blood. In th lower part of the gland, as the Arte1ies--The blood for each half of ducts become larger, there is more con­ th udder comes in through a single nective tissu than in the upper part artery which enters the udder from where the ducts are smaller. The walls above at a point midway between the of th larger ducts contain larg amounts halves and about one-third way from of connective tissue. the rear side. Here the artery divides Frequently the alveoli are destroyed into two branches, one for the fore by mastitis and r placed by connective quarter and one for the rear. As it tissue which in this cas is th sam as progresses through the quarter, each scar tissue. branch divides and su bdivid s innumer­ An udder that contains a large able times until the smallest branches amount of conn ctive tissue feels firm or capillaries afford close contact with and "meaty." The udder of good quality every cell to supply it with blood. is soft and spongy. Veins-After the arterial blood has Circulation- Since all milk comes giv n up its oxygen and nutrients to the from blood, th circulation of blood in cells and has picked up from them the the udder is of gr at importance. Abou t carbon dioxide and other wast prod­ 400 gallons of blood must pass through ucts, it must go back to th lungs to b the udd r for every gallon of milk pro­ purifi d and ren w d. Here the udder 's duced. Th circulation to th udd r con- system of v ins com s into operation, 10 MINNESOTA BULLETIN 361

FIG. 9. Section of left half of an udder show- ing main arteries Note that the external pudic a r­ tery divides into two branches, one supplying the rear quarter and the other the front quarter with blood.

the venous capillaries taking the stale as the veins. In the udder the flow of blood from the cells and passing it along lymph is first downwards from the through larger and larger branches until smaller vessels and then upward and it reaches the main veins which carry it backward into the lymph glands, one to the lungs. As illustrated in figure 8, of which is located on the upper rear the' veins leading from the udder follow side of each half (figure 10). The lymph three different routes. One, known as gland is usually about the size of a the abdominal mammary or milk vein, walnut and can be felt from the rear. goes forward just under the skin of the When there is infection in the udder, belly. A larger one extends upward par­ the lymph glands may become many allel to the main artery, while the third times the normal size. and smallest of these veins runs back­ Most of the veinlike structures under ward from the udder and then upward the skin of the udder are lymph vessels und r the skin to enter the body through and not blood vessels as commonly sup­ the pelvic arch. Eith r of the two larger posed. The swelling of the udder and veins can carry away all of the blood abdomen before and after calving is due that enters the udder. Tying off of the to the backing up of lymph in these mammary vein does not reduce the milk tissues. · production as the other veins have ample Nerve Supply to the Udder-The capacity. nerve supply to the udder is too com­ Lymph System-Lymph or tissue plex for detailed discussion here. It juice is produced in all tissues and is must suffice to point out that ach half returned to the blood stream through a of the gland proper is supplied with special system of ducts and lymph nerves from a nerve trunk which enters glands which filter the lymph. Lymph the udder near th main artery and ducts in the uader reach every minute branches to follow the arterial system part. These ducts consolidate into larger of the gland. and larger ducts in much the same way The skin of the udder and teats is THE COW'S UDDER 11 richly supplied with nerves coming from the spinal column by way of the abdominal wall. The nerve endings in the skin of the udder and teats are of great importance in milking because they pick up sensations that ultimately result in the letdown of the milk. Pre­ sumably, endings for all skin sensa­ tions-touch, pain, cold, and hot-are to be found here.

ABNORMALITIES OF THE UDDER

In addition to functional supernumer­ ary teats there are many abnormalities of the udder, a few of the more impor­ tant of w hich will be briefly discussed as follows: FIG. 10. Lymph system of the udder. Note Blind Quarters-Frequently heifers that the lymph vessels (just under the skin) run upwards and backwards to the mam- will come in with one or more quarters mary lymph gland completely inactive. For some unknown reason the gland tissue failed to develop and nothing can be done to correct the canal. It requires skilled surgery to deficiency. remove a spider. Fistulated Teats- The teat often may Absence of Teal: Cisterns-Occasion­ have an extra opening, known as a fis­ ally, for unknown reasons, one or more tula, through which milk may leak. teats may not h ave cisterns, the teat This fistula may be adjacent to the being a solid mass of tissue. There is streak canal or may be located on any no remedy. part of the teat. A veterir.ary surgeon Obstruction of the Gland Cistern­ can cauterize and get rid of teat fistulas. Although rare, the udder cistern may Leaky Teats-Frequently the sphinc­ be separated from the teat cistern by a ter muscles around the streak canal may membrane. A skilled veterinary surgeon be injured or ar too weak to withstand can correct this condition by cutting the the pr ssure of accumulating milk and membrane. the milk will leak out. This condition Accessory Glands on Teats or Udder­ can also be corrected by skilled surgery. Sometimes small secreting glands with­ Hard Milkers-Hard milkers have too out teats may be found on the teat and strong sphincter muscles or too much udder. In some cases there is no opening connective tissue around the streak ca­ and the gland will be engorged with nal. Skilled surgery can correct this milk following calving. The swelling condition. soon subsides as the milk is resorbed. Teat Spider--Sometimes a wartlike When the milk is released through an growth, commonly known as a spider, opening (fistula), th e condition becomes takes place in the teat cist rn. It may a nuisance and the gland should be de­ be located on any part of the teat cis­ stroy d by a veterinarian. tern wall and causes difficulty when the The udder may be affected by injuries growth is large enou gh to stop the and various diseases, including mastitis, milk from getting through the streak non of which can be discussed here. 12 MINNESOTA BULLETIN 361

tively, of the milk, are found nowhere else in the body. Of the proteins, casein Functioning averaging about 3 per cent and milk albumin about 0.5 per cent are likewise not found outside of milk. Milk globulin THE COMPLICATED processes of the representing about 0.05 per cent of the functioning udder may be divided milk is apparently the same as blood into the following heads: starting and globulin and may simply filter in from maintaining lactation; circulation and the blood. The blood precursors (blood blood precursors of milk; equilibrium substances used for making milk) of of milk and blood; time milk is secre­ these milk substances have been the ted; and the "letdown" or ejection of subject of a great deal of study during milk. the past few years. As a result some­ thing is known about them, although STARTING AND MAINTAINING much is yet to be learned. LACTATION Studies of the uptake of materials from the blood by the udder have re­ The fully developed udder does not vealed that large volumes of blood pass secrete milk unless a hormone known through the udder of a high-producing as prolactin (and possibly others) is cow. As about 400 pounds of blood must present. Prolactin, which is produced by pass through the udder for each pound the pituitary gland, normally is not of milk produced, about 10 tons of blood secreted until about freshening time. must daily pass through the udder of a However, in some cows it may be se­ cow that produces 50 pounds of milk. creted before that time as is evidenced Milk Fat-Milk fat comes mainly, if by coming into milk before calving. not entirely, from the neutral fat of the What prompts prolactin secretion at blood, being changed by the cells of calving time is not known. the udder. The exact method by which Not only is prolactin needed to start the blood fat is changed into milk fat lactation, but its continuous secretion is not known, but apparently a com­ is necessary to keep lactation going. plicated mechanism is involved in which Because an injection of prolactin will the fatty acids are broken down into increase milk flow, it is evident that shorter ones. Analysis of the fat in the many cows fall off rapidly in milk flow udders of milking cows shows that it is because of inadequate secretion of this intermediate between milk fat and body hormone. It is also probable that too fat, while in the dry udder the fats are little prolactin prevents many cows with more nearly like body fat. It is also well-developed udders from reaching noteworthy that the active udder con­ their milk producing capacity. tains several times as much fat per unit of weight as does the dry udder. CIRCULATION AND BLOOD Milk Sugar-The udder uses two PRECURSORS OF MILK bloc·C substances for making milk sugar. These are blood sugar (glucose) and In recent years it has been definitely lactic acid. From these two substances proven that all of the major ingredients and an extract out of an active udder of milk are synthesized (made up) in milk sugar has been synthesized in the the udder from constituents of the blood laboratory. As the udder contains about and not merely filtered out of the blood 0.2 per cent of glycogen (animal starch), by the gland as. was once thought. Milk it is probable that this substance enters fat and milk sugar, representing, on into the making of milk sugar and that an average, 4 and 5 per cent, respec- the process is very complicated. THE COW'S UDDER 13

Milk Proteins-Casein and milk albu­ The normal blood substances that min are different from the proteins of may be found in the same concentration the blood, and they are made up in the in milk as in blood are urea, uric acid, udder from some blood substances. Only creatine, and creatinine. amino acids and proteins of the blood The substances that will be found only can be considered as possible precursors in the blood and milk if they are pres­ of milk proteins. Only about a third ent in the feed are many and of great enough amino acids are taken out of importance to dairymen, for among the blood by the udder to account for them are the feed-flavoring constitu­ milk proteins, and about enough urea ents. Among the many feed substances is produced by the udder to account that may find their way into the blood for the breakdown of the amino acids. and from there into the milk are: alco­ It is, therefore, apparent that most, if hol, ether, chloroform, iodine, turpen­ not all, of the milk proteins must come tine, paradichlorbenzene, the flavoring from the blood proteins. As the globulin ingredients of onions, garlic, turnips, of the milk is apparently the same as mustard, Frenchweed, silage, and many the globulin of the blood, it is obvious others. that this protein passes through the membranes of the gland. Chemical anal­ EQUILIBRIA BETWEEN MILK ysis of simultaneously drawn arterial AND BLOOD and mammary vein bloods also indicates that this protein is taken out of the While milk and blood have the same blood in sufficient quantities to account osmotic pressure, they are not in equi­ for the milk proteins. librium; that is, they do not contain the The method by which blood globulin same amounts of given parts. As a mat­ is changed into casein and milk albumin ter of fact, none of the major constitu­ is not known. It undoubtedly is a com­ ents of milk are in the same concentra­ plicated process. It is possible that at tion as in blood. The mammary gland least some, if not all, of the amino acids absorbs some blood constituents prefer­ taken up by the gland are also used in entially for a greater concentration in the synthesis of milk proteins. milk than in the blood and holds back Salis of Milk-All of the salts of milk other constituents in the blood. There come from the blood but not by a simple is approximately 80 times as much diffusion process because the amounts sugar, 20 times as much fat, 15 times and relationship of salts in milk are as much calcium, 7 times as much phos­ greatly different from those of blood. phorus, and 4 times as much magnesium The salt content of milk in relation to in milk as there is in the blood. Blood that of the blood is very closely asso­ contains 8 times as much sodium, 4 ciated with the phenomena of equilibria times as much chlorine, and 2 times as or balances between milk and blood. much protein as milk. Sodium bicar­ Diffusion from Blood into Milk­ bonate, a prominent constituent of While the principal constituents of milk blood, is not found in normal milk. are synthesized in the mammary gland, The great concentrations of calcium there are a number of substances in and phosphorus in the milk are signi­ blood that merely diffuse into the milk ficant because these two substances pass and are found in the same concentra­ through ordinary membranes less read­ tion in both milk and the blood. Among ily than sodium and chlorine, which are these are a number of normal sub­ held back to a large extent by the mam­ stances of blood and many that come mary gland. Another significant fact is into the blood only if they are present that when the calcium or phosphorus in the feed. content of the blood is lowered, there 14 MINNESOTA BULLETIN 361 is no lowering of these substances in the or to a change in the permeability of milk. When body calcium is too low in the membranes. In acute mastitis the the milking cow, the blood calcium will milk characteristics are also affected by be so lowered by the demands for the the breaking off of gland tissue. milk that milk fever is produced. Re­ A large number of substances have duced blood phosphorus, as a result of been injected into the duct system of the demands for milk, produces no such cows' udders to study their effect upon dramatic symptoms, but if continued, the milk. Among these are various salts milk production will be lowered. and salt mixtures, sugar solutions in If blood sugar content is lowered as various concentrations, blood serum, by the administration of insulin, the distilled water, and many other sub­ milk sugar is lowered. Milk fat is also stances. All of these, except distilled lowered when the blood fat is mark­ water and very dilute salt solutions, edly reduced by starvation. have produced severe disturbances with Milk and blood will tend to come into a marked lowering of milk secretion equilibrium when there are any dis­ and milk that resembled acute mastitis. turbances within the udder. Leaving the The mammary gland is unusual in its milk in the udder for long periods of reaction to injected substances. Isotonic time, injecting substances into the duct salt solutions and blood serum are non­ system of the udder, and mastitis will irritating to body tissues but produce cause the milk to come toward the com­ severe reactions in the mammary gland. position of blood. In all of these condi­ Distilled water is very irritating to body tions the protein, sodium, and chlorine tissues but has little effect when in­ contents of milk will be increased. In jected into the udder. A realization of addition sodium bicarbonate from the the effect of injected solutions upon the blood will pass into the milk to make it udder is important in considering vari­ alkaline instead of slightly acid, which ous injected agents in the treatment for is the reaction of normal milk. This mastitis. change causes the blue reaction to brom­ thymol-blue as a test for mastitis. Large TIME OF MILK SECRETION numbers of white blood cells and cata­ lase of the blood will also pass into the It was formerly believed that most of milk. Calcium, milk sugar, potassium, the milk was actually secreted during and phosphorus of the milk will pass the milking. It was thought that the into the blood to lower the levels of milking act stimulated a rapid secreting these constituents in the milk. The activity of the cells because the udder casein content of the milk is also mark­ and teats became distended shortly after edly reduced, the increase in protein milking began. It is now known that the being due to increments in globulin and old ideas were wrong and that the dis­ albumin. tention of the teats and udder is due to Colostrum milk is more nearly in the "letdown" of milk and not to a equilibrium with the blood than is nor­ rapid secretion. Proof that all of the mal milk. The reason for this is that milk given is present in the udder when colostrum milk has been in the gland milking begins was furnished by slaugh­ for a comparatively long time. For the tering cows at milking time, removing same reason, the milk of cows that are the udders, and then milking or analyz­ being dried off is also coming into ing them for the milk content. These equilibrium with the blood. experiments have shown that all of the The characteristics of mastitis milk milk expected at a milking was present may be due either to an interruption in the udder. of normal secretory function of the cells Not only is the milk secreted in the THE COW'S UDDER 15 interval between milkings, but the rate and from which it does not drain by is slowed up and secretion may even gravity. The alveolus filled with milk stop completely as milking time ap­ may be likened to a tiny eye dropper proaches because of the pressure built filled with fluid. The fluid will not flow up by the accumulating milk in the out of the eye dropper until pressure alveoli. When pressure of the milk is applied to the bulb. The alveolus has within the gland reaches 30 to 40 mm. tiny muscle cells around it that contract of mercury, milk secretion is completely under the influence of the oxytocic hor­ stopped and resorption of the milk mone to force the milk out through the begins. tiny ducts and ultimately down into the The knowledge of the effect of pres­ gland cistern from which it is drawn sure explains why more frequent milk­ by milking. ing increases milk production. The more Lefdown Is a Reflex Acf-The let­ often a cow is milked, the less pressure down of milk is a reflex act, that is, there will be at any time to retard or the response is not voluntary but auto­ check milk secretion. It also explains matic. It involves the sensory nerves the curative effects of air inflation of which carry the stimulus to the pitui­ the udder on milk fever. The air pres­ tary which in turn ejects the hormone sure not only stops milk secretion to into the blood. About 40 seconds are halt absorption of calcium from the normally required after the stimulus blood but also actually causes the blood until the milk is let down. The normal to take back calcium from the milk to stimulus for the letdown of milk is the hasten recovery. The fact that milk se­ milking act, but cows may be condi­ cretion stops and resorption of the milk tioned to respond to a number of differ­ begins when the pressure within the ent stimuli. If cows are fed immediately gland reaches one fourth to one third before milking for some time, this may of blood pressure also explains why the become associated with milking, and best way to dry off cows is to abruptly the feeding may become the stimulus stop milking. for the letdown of the milk. Cows fre­ quently become conditioned to let down their milk to the starting of the milking THE LETDOWN OF MILK machine, noises from the milking uten­ To be a good producer a cow not only sils, washing of the udders, and other must be capable of synthesizing much stimuli that they may cor.'e to associate milk but also must be able to let down with milking. her milk and then be milked in such a Interferences with Response-The re­ manner that all of the milk is obtained flex act of response to milking is easily at each milking. When all of the milk interfered with. If the cow is frightened thought to be in the udder is not ob­ or angry or has her attention called to tained by milking, the cow is usually any strange factor, there will be but said to have "held up her milk." This partial or no response to milking. Ex­ is an incorrect statement of what hap­ periments where rats were placed in pens. A cow cannot hold up her milk the manger, inflated paper bags were but under certain conditions she fails exploded every 30 seconds, a cat was to "let it down" completely. The let­ placed upon the back, or the cow stuck down of milk is a positive act requiring with needles resulted in a complete the secretion of a hormone (oxytocin) lack of response to the milking act. from the hind part of the pituitary Placing grain feed where it could be gland. seen but not reached by the hungry It is normal for the milk to be stored cow also caused complete failure of in the tiny alveoli where it is made response to the milking act. People who 16 MINNESOTA BULLETIN 361 have mistreated cows or by their ac­ of milk from milking to milking and tions arouse the cow's suspicion prevent from day to day. Marked differences in complete response to milking. Cows that the amounts of milk, provided milking are anesthetized also fail to respond. is done at regular intervals, are in most There is now evidence to show that a cases accounted for by incomplete let­ hormone from the adrenals, known as down of the milk at times. When this adrenalin, interferes with the action of happens the smallest milkings are usu­ oxytocin. Adrenalin is secreted when ally followed by an unusually large the cow becomes excited. Cows were milking. stimulated to let down their milk; then Inherited Lack of Ability-How large they were excited or adrenalin was in­ a percentage of cows fail to completely jected and milking was incomplete. respond to milking by total letdown of Time When Hormone Is Active-Ex­ the milk is not known. However, inves­ periments have shown that the activity tigations have revealed cows that under of the hormone is limited to a relatively apparently normal conditions do not short time after it is secreted into the completely let down their milk at any blood. Usually one cubic centimeter of a time. Cows of this type are "short tim­ standard solution of oxytocin injected ers," that is, they dry off early. Their intravenously will be effective in letting udders also are what is known as down milk for not over 10 minutes. As "meaty," which really is not the case soon as the hormone is secreted into because the hardness of the udders even the blood, its destruction begins and it after milking is due to the retained is, therefore, important that milking be milk. One such cow was found normally completed before the hormone's effect to retain 6. to 8 pounds of milk under is lost. the best of milking conditions, as such Effect of Incomplete Letdown-When amounts of milk could be obtained reg­ the milk is not ejected from the alveoli ularly after milking by injection of the at milking time, only the milk that has oxytocic hormone. drained down into the gland cistern and Why such cows fail to let down their larger ducts is obtained by milking. Be­ milk completely is not known. It may cause of the milk retained in the alveoli, be due to improper response of the a pressure is soon built up by the ac­ nervous mechanism or to inadequate cumulation of the newly secreted milk hormone production in the pituitary and the rate of milk secretion is low­ gland. ered. In addition the pressure may be­ Strippers-In nearly every herd there come great enough that resorption of are cows that let down their milk the milk begins. When resorption be­ slowly, particularly in advanced lacta­ gins, the drying off process sets in and tion. On the basis of facts now at hand the cow will soon stop secreting milk. this can be explained by a slow ejec­ Experiments in which the cows were tion of the hormone from the pituitary stimulated by washing the udders to let gland. Experience has shown that the down their milk 20 minutes before milk­ habit of strippers can be established by ing caused as much as 15 per cent drop improper milking methods. If too much in milk production in two weeks. Dur­ time is taken in milking, many cows ing the experimental period there was will adjust themselves to the situation also a marked variation in the amounts by a gradual secretion of the hormone.