290 RESEARCHES ON CORTICAL LOCALISATION AND 



cortex. Ontogenetically it appears shortly after the infra-granular 

 portion of the cortex, and it reaches its maximum development in 

 depth and definiteness in the projection spheres of the cerebrum. The 

 last layer of the cortex to appear ontogenetically is the supra-granular 

 (pyramidal). It is the slowest of all the layers to reach maturity. 

 It is scarcely existent at all in certain regions in some of, if not all, 

 the lowest mammals, and even at its best in the latter it reaches 

 but a slight absolute depth as compared with its depth in practically 

 every region of the neopalHum in the human subject." 



The above description of the mode of development of the cortex 

 cerebri is illustrated in the following figure, which indicates, in 

 graphic form, the five primary cell and fibre laminae of the cortex 

 of foetuses of four and six months, a new-born child, a normal 

 human adult, and a mole. It will be at once seen that the cortex 

 of the human adult differs from that of the insectivore chiefly in 

 the degree of development of the second, pyramidal, or outer cell 

 lamina. In spite of the human brain being one of the largest and 

 that of the mole one of the smallest in the mammalian phylum, the 

 actual average depths of the granular and infra-granular portions 

 of the cortex (laminae 3, 4, and 5) are very similar in the two. 



It may here be remarked that Brodmann, in the course of his 

 prolonged investigations into the structure of the cerebral cortex of 

 the mammalia, has incidentally and independently (1906) made similar 

 observations to those of Watson. He states that, during the ascent 

 of the mammalian series, his layer 3, which is substantially the outer 

 cell lamina or pyramidal layer of the writer and the supra-granular 

 layer of Watson, increases in depth, whilst his layer 6, which is 

 equivalent to the inner cell lamina 5 of the writer and the lower 

 part of the infra-granular region of Watson, on the whole diminishes 

 in depth. Brodmann also makes certain generalisations regarding 

 the variability of the individual layers of the cortex cerebri, which, 

 though they confirm the individual observations of several other 

 writers, are worthy of reference here owing to the wide field covered 

 by the investigations of this author. He remarks that his layer 4 

 (granule layer of other writers) is the most variable ; that his layers 3 

 and 5 (medium and large pyramids, and infra-granular pyramidal- 

 shaped cells) exhibit the greatest histological differentiation ; and that 

 his layer 6 (polymorphic layer or inner cell lamina of the writer) 

 exhibits the greatest variation in breadth. He further remarks that 

 the psychomotor and striate (visuo-sensory) areas of the cortex cerebri 

 are most highly developed in primates. 



