THE EMISSARY FORAMINA OF THE CRANIUM IN PRIMATES By G. I. BOYD Department of Anatomy, Winnipeg, Canada IN a previous paper' I gave a description of the emissary foramina of the cranium based on an examination of 1478 human skulls and of skulls of 124 anthropoids, including 50 Simiidae. Since then I have had the oppor- tunity of examining a larger number of Primate crania in the American Museum of Natural History, New York, and in the British Museum of Natural History, London, and have been able to confirm some of my previous findings. I have included my previous figures in the present series.

GORILLA The parietal foramen was found present forty-two times in 159 skulls of both sexes and all ages (26-35 per cent.). It was usually situated far back in the parietal bone or in the apex of the occipital bone (6 per cent.), in or close to the median plane. On four occasions there was a double foramen, the foramina being placed one in front of the other. The mastoid foramen was present on one or both sides in twenty-five out of 144 skulls in which the temporal bone was intact (17-4 per cent.). The foramen was present on both sides in six skulls, twelve times on the left side only and seven times on the right side only. The condyloid foramen was present twelve times in 113 skulls (10.6 per cent.), three times on both sides, four times on the left only and five times on the right only. A squamosal foramen was found present on both sides in one skull (0-6 per cent.). No difference was found in the sex incidence.

CHIMPANZEE In 137 skulls the parietal foramen was present twelve times (8 7 per cent.), situated in the parietal bone, usually in the median plane. Twice the foramen was double. The mastoid foramen was present eighteen times in 128 skulls (14 per cent.), four times on both sides, eight times on the left side only and six times on the right side only. The condyloid foramen was present nineteen times in 115 skulls (16.5 per 1 "The emissary foramina of the cranium in Man and the Anthropoids." J. Anat. 1930, vol. LXV, pp. 108-21. Anatomy Lxix 8 114 G. I. Boyd cent.), four times on both sides, eight times on the left side only and seven times on the right side only. In most cases the foramen was large. The postglenoid foramen was never present.

ORANG-UTAN The parietal foramen was present ten times in 98 skulls (10.2 per cent.), but only three times was it actually in the parietal bone; in the other seven cases it was situated in the apex of the occipital bone. The mastoid foramen was present eighty times in ninety-eight skulls (81-6 per cent.), sixty-five times on both sides, seven times on the left side only and eight times on the right side only. The mastoid foramina were large, and on some occasions were duplicated on one side. The condyloid foramen was present once in ninety-five skulls on the left side only (1 per cent.). The postglenoid foramen was found once, paired, and a squamosal foramen once, on the left side only (together 2 per cent.).

GIBBON The parietal foramen was present eight times in 113 skulls (7 per cent.), but only twice was it actually in the parietal bone: in the other six cases it was situated in the apex of the occipital bone. The mastoid foramen was present sixty-eight times in 107 skulls (63.5 per cent.), forty-two times on both sides, thirteen times on the left side only and thirteen times on the right side only. It was largest and most frequent in the Siamangs, occurring as a paired foramen in twelve out of fourteen skulls and being absent in the other two. The mastoid foramen was smallest in Hylobates lar, so much so that it was sometimes difficult to decide if there was a foramen present or not. The condyloid and postglenoid foramina were invariably absent.

OTHER MONKEYS The parietal foramen was found once in 100 skulls of different species, both Old World and New World monkeys. The mastoid foramen was found present in sixty-six skulls, usually on both sides. The condyloid foramen was present in four skulls, on one side only. The postglenoid foramen was present in fifty-four skulls, usually on both sides.

LEMUROIDEA In 104 skulls of true lemurs the postglenoid foramen was present in all but one case (99 per cent.) and was large in size, larger than the jugular foramen. The mastoid foramen was present fifty-eight times in seventy-eight skulls in which the mastoid portion was available for examination (74.4 per cent.), forty-seven times on both sides, three times on the left side only and eight times on the right side. The foramen was very variable in size. The Emissary Foramina of the Cranium in Primates 115 In thirty-two skulls of Indris and Propithecus, the postglenoid foramen was present in all: the mastoid foramen was present twenty-five times bi- laterally and twice unilaterally (84 per cent.). In fifty-five Galago skulls the postglenoid foramen was present in all cases and was large in size. The mastoid foramen was present four times bilaterally and twice unilaterally (10.9 per cent.), and was small in size. In twenty-nine skulls of Perodicticus the post- glenoid foramina were small and sometimes absent. (In seventeen skulls the foramen was identified with certainty on one or both sides, while in eight others its presence was doubtful.) The mastoid foramen was present on two occasions (6 per cent.). In forty skulls of Nycticebus there was present in every case a foramen which was taken to be the postglenoid foramen but which was situated farther medially than the usual site. The mastoid foramina were small and difficult to identify. In sixteen skulls of Loris and three skulls of Chiromys neither postglenoid nor mastoid foramina were found.

TARSIOIDEA In all of the seven skulls of Tarsius large postglenoid foramina were present; mastoid foramina were small.

DISCUSSION The parietal foramen would be present thirteen times in a group of one hundred Simians composed of equal numbers of the four genera, gorillas, chimpanzees, orang-utans and gibbons (Table I). The Simian family stand in an intermediate position between Man, in whom the parietal foramen wals found present in 60*5 per cent. of skulls, and the other monkeys, in which the foramen was rarely present (1 per cent.) (Table II). It has been found convenient to include under the term " parietal foramen" foramina which were situated, not actually in the parietal bone, but in the apex of the occipital bone. This was done because such foramina are openings in the skull roof and correspond to the human parietal foramen. An occipital foramen, when present in Man (1-6 per cent. skulls), was found at or below the inion. While in Man the parietal foramen varies in position within fairly well-defined limits, in the Simians it is found in or close to the median plane, in the posterior half of the parietal bone, or even in the apex of the occipital bone. In the Simian group the parietal foramen was present most often in the gorilla (26 per cent.) and least often in the gibbon (7 per cent.). If the strictly "parietal" foramina only be considered, the difference is even more marked; the Simians can then be ranged in order: gorilla 20 per cent., chimpanzee 8 per cent., orang-utan 3 per cent., and gibbon 2 per cent. The gorilla also approached Man in that the parietal foramen was sometimes double; in the other apes the foramen was invariably single. In several gorilla skulls there were, as in Man, incomplete parietal foramina. 8-2 116 C. I. Boyd It is possible that the greater frequency of occurrence of the parietal foramen may be correlated with the larger cranial capacity and size of brain, because a gradation may be found from the Lemuroidea, in which the foramen is absent, up to Man, in whom it is present in 60 per cent. of crania, and in whom it is bilateral in 20 per cent. The mastoid foramen was present in the Simian group in 44 per cent. of cases compared with 68 per cent. in Man and 66 per cent. in the other monkeys. This relative infrequency is due to the small percentage found in the gorilla and chimpanzee. In orang-utans, however, the frequency (81 per cent.) and Table I. Actual and percentage frequency offoramina on one or both sides in the Simiidae. Parietal Squamosal or Mastoid Condyloid postglenoid "True " . Fora- P.F. Fora- Fora- Fora- mina Skulls % mina Skulls % mina Skulls % mina Skulls % Gorilla 42 159 26-4 20-0 25 144 17-4 12 113 10-6 1 159 0'6 Chimpanzee 12 137 8-7 8-7 18 128 14-0 19 115 16-5 0 137 0 Orang-utan 10 98 10-2 30 80 98 81-6 1 95 1-0 2 98 2-0 Gibbon 8 113 7-0 2-0 68 107 63-5 0 103 0 0 113 0 Percentage 13-1 8-4 44-1 7-0 0-6 ofgroup of Simians Table II. Percentage comparison ofPrimates. Foramina present on one or both sides

A I "True" Squamosal or Skulls examined parietal Mastoid Condyloid postglenoid Man, 1478 60-5 68-0 77-0 0-6 Anthropoidea 426 Simiidae 8-4 44-1 70 0-6 113-159 gorillas 20-0 17-4 19.6 0-6 115-137 chimpanzees 8-7 14-0 16-5 0 95-98 orang-utans 30 81-6 1.0 2-0 103-113 gibbons 2-0 63-5 0 0 100 other monkeys 1-0 66-0 4-0 540 Lemuroidea 78-104 true lemurs 0 74-4 0 990 165 other Lemuroids 0 Variable 0 Variable 7 Tarsiu8 0 All small 0 All large size of the mastoid foramen was remarkable. Among Lemuroids the frequency varied enormously. In a group of true lemurs the percentage was 74. The condyloid foramen was in Man the one most constantly present and largest in size (77 per cent.). In the Simians it was only present in 7 per cent. of cases and in the other monkeys in 4 per cent. Among the Simians it was most frequent and largest in size in the chimpanzee (16.5 per cent.), and next in the gorilla (10.6 per cent.). It was absent in all Lemuroids. The postglenoid foramen was rarely present, alike in the Simians and in Man (0.6 per cent.). This was in, marked contrast to the other monkeys in which the foramen was present in 54 per cent. of cases, and the true lemurs in which it was present in 99 per cent. Whereas in most Lemuroids and in The Emissary Foramina of the Cranium in Primates 117 Tarsius the foramen was invariably present and often large, in Perodicticus it was small and sometimes absent. In the skulls of Loris and- Chiromys examined no foramina were found.

SUMMARY In an examination of the Primates in relation to the frequency of the emissary foramina of the cranium, a graded series can be demonstrated from the Lemuroids, in which the parietal foramen is invariably absent, up to Man, in whom it is present in 60 per cent. of skulls. The Simian group are dis- tinguished from the other monkeys and approach Man, not only in the greater frequency of the parietal foramen, but also in the rarity of the postglenoid foramen. Among this group, the gorilla shows a greater frequency of the parietal foramen and the chimpanzee and the gorilla of the condyloid foramen. The orang is remarkable for the size and frequency of the mastoid foramen. The present paper confirms the significance of the previous findings, which were based on an examination of a smaller number of anthropoid crania.