369 THE CELLS OF MEYNERT IN THE VISUAL CORTEX OF THE MONKEY By W. E. LE GROS CLARK Department of Anatomy, University of Oxford In 1872, Meynert called attention to conspicuous large cells which are present in the visual cortex of the human brain and which are characterized by possessing well marked horizontally directed basal processes. He noted that cells of this type are located at the level of the white line of Gennari, and also in the infragranular layers of the cortex. The former are not so large or conspicuous as the latter, though in the visual cortex of some species of New World monkeys they may be aggregated to form a definite lamina- traversing the central zone of the white line of Gennari. Those in the infragranular layers, however, are particularly obtrusive in stained sections, and since it is to these that the reference 'solitary cells of Meynert' is now commonly applied, such a usage of the term may conveniently be followed here. The first detailed account of the morphology of the solitary cells of Meynert was provided by Cajal (1899). He described them as being situated in his 7th layer of the visual cortex of the human brain (a layer which evidently corresponds to at least a part of the lamina ganglionaris of more modern terminology) and he noted that they are commonly disposed in a single row, and at some distance from each other, in a plexus of fibres which are mainly horizontal. The cells were described as being charac- terized by having a base which is very broad in proportion to its height, and in possessing stout basal dendrites which extend for considerable distances parallel to the surface. In many lower mammals the cells of Meynert seem hardly to be present in the visual cortex; at least, there are no cells in the infragranular layers which stand out conspicuously by reason of their size, shape and their basal dendritic expansions. Thus, in a recent intensive study of the cyto-architecture of the visual cortex of the rabbit, based on Golgi preparations, O'Leary & Bishop (1938) make no record of cells of exceptional size and with extensive horizontal dendrites in the infragranular layers. In some reports of the cortical cyto-architecture of lower mammals, based on Nissl preparations, references have from time to time been made to the presence of cells of Meynert, but it is not always certain that they can be correctly identified in such material. However, an exception occurs in the primitive mammal, Tupaia, for in the visual cortex of this genus scattered cells of unusually large size are very conspicuous elements in the deeper laminae of the visual cortex (Clark, 1925). The significance of this may be related to the concurrent development in Tupaia of an advanced degree of differentiation of the visual cortex with remarkably massive anterior colliculi in the mid-brain, for, as will be shown, there is evidence that these cells provide cortico- mesencephalic fibres. OBSERVATIONS Histological. In silver-irmpregnated sections of the visual cortex of Macaca Meynert cells are very conspicuous. In both pyridin-silver and Bodian preparations they be- come deeply stained, while the cytoplasm of other cells remains unstained or only slightly impregnated. The significance of the differential staining is not clear, -and it 370 W. E. Le Gros Clark may be related simply to cell size. In any case, however, it permits of a ready re- cognition of the Meynert cells so that their distribution and enumeration can be easily determined. In methylene blue preparations they are also evident and are seen to contain well-defined Nissl granules scattered throughout the cytoplasm, but they do not stand out so conspicuously from other, smaller cells of the infragranular layers. In the present study, use was made of a series of Golgi preparations of the macaque visual cortex, in order to observe the relation of the cells of Meynert and their pro- cesses to other cells of the cortex. In Text-fig. 1 is a composite drawing made from these preparations. Each cell in the drawing has been accurately drawn from an individual cell in one of the sections and placed in its correct position relative to the laminar arrangement. In general, the Meynert cells are situated in- the lamina multiformis (lamina VI). Occasionally they are to be seen in the boundary between lamina V and lamina VI, and less commonly in lamina V. A similar stratigraphical location has been noted in Papio by Schuster (1910) and in Cebus by v. Bonin (1938). It appears, therefore, that in the monkey their level in respect of the cortical layers is not quite the same as in man, for according to Cajal (as noted above) and also according to the recent studies of Conel (1939), in the human brain they are characteristically found in lamina V. Measurements of fifty Meynert- cells selected at random in silver preparations, made with an ocular micrometer, show that their size varies from 24 x 14 p to 43 x 24 1, with an average of 29 x 20,u (the long axis being vertical). They are all cells of the pyramidal type, and, as in the human brain, they are characterized by an unusually broad base and by the great extent of thie horizontal basal dendrites. The apical dendrite, a remarkably stout process, runs vertically upwards to reach the zonal lamina at the surface of the cortex. It thus extends through almost the whole thickness of the visual cortex which (on the basis of measurements made on sections of twenty different specimens) has been found to have an average depth of 1*3 mm. Its course is straight except for slight sinuosities, and it usually forms a component of one of the radial fasciculi which are so well defined in the visual cortex. Its coarseness serves to distinguish it from the fine fibres of which these fasciculi are mainly composed. No branches are-given off by the apical dendrite above the level of lamina V. At its origin, however, lateral branches are frequently given off which pursue a horizontal course in lamina V or VI, or an oblique upward course which is usually confined to lamina V. Occasionally in lamina V one or two collaterals arise which first run obliquely upwards through the upper level of lamina V and the lower level of lamina IVc, and then turn vertically upward in one of the radial fasciculi. These collaterals can be traced as far as the stria of Gennari (lamina IVb). Of the basal dendrites of the Meynert cells, those which proceed horizontally are the most conspicuous. They confine themselves to lamina VI and the deeper levels of lamina V. In several instances they could be traced for a course of over 0-25 mm., and in the case of one cell the main horizontal dendrite on one side was followed under the microscope for a distance of 0 37 mm., and even then its actual termination was not identified. Occasional short branches arise from the proximal part of the main basal dendrite, but they limit their distribution to lamina VI. Other quite short dendrites leave the basal surface of the Meynert cell and extend downwards for short distances. From one of these the axone may arise, or it may take origin directly from the cell-body (P1. 1, figs. 4, 5). v4.) Q, I I Ca i .4) I I I 04) I I 0 0 I bE . o I O 4) OCs 0E T42 ~0 O ce 4) 4 4 4) .gce*.E 0 4- l bo ... o4 Cd _e 11t .. 11 11 ___1 _ 1-4 .- C -4 ~~~~~~~0-4t p > ;>-Q. H~~~~~~~~o~-q- -4-- ~- 41 Anatomy 76 25 372 W. E. Le Gros Clark The Meynert cells are usually single, but occasionally two or three are found to- gether. Counts were made in silver sections of the visual cortex in order to compute the number of Meynert cells in a given area of cortex. It was found that they are distinctly more numerous on the lateral surface of the occipital lobe than in the neighbourhood of the calcarine sulcus on the medial surface. In other words, the density of their distribution is greater in that part of the visual cortex which is con- cerned with macular vision. The figures obtained in these counts are as follows. Lateral surface of occipital lobe. In a number of sections representing altogether a total linear cortical measurement of 161 mm., 184 Meynert cells were counted. Thus the average distance between adjacent cells is 0-86 mm. This corresponds to a density of approximately 130 cells in each square cm. of cortex. Medial surface of occipital lobe. In a number of sections representing altogether a total linear cortical measurement of 77 mm., 58 Meynert cells were counted. Thus the average distance between adjacent cells is 1*33 mm. This corresponds to a density of approximately 56 cells in each square cm. of cortex. In order to compute the total number of Meynert cells in the visual cortex of the macaque monkey, it was necessary to estimate the total surface area of the visual cortex. For this purpose, a male animal (O.M. 212) of average size was selected. The third molar was in process of eruption, the body weight was 3980 g., and the brain weight 88 g. The brain was fixed in Kaiserling solution. Both occipital lobes were cut coronally in sections of 1 mm. thickness with a freezing microtome. The sections were then placed in glycerin. By transmitted light the white line of Gennari was clearly visible as a dark band, and allowed the limits of the visual cortex to be defined with accuracy.