502 A MANUAL OF VETERINARY PHYSIOLOGY 



the fibres of the brain goes on for years after birth, but ceases at 

 forty, and in old age diminishes. We have indicated but briefly 

 the field which awaits the veterinary histologist of the future ; 

 in the absence of direct experimental inquiry — which nothing 

 can supplant — important information may be gained by an 

 application of the methods of Flechsig. 



Function revealed by Structure. — The development of nervous 

 tissue from its earliest laying down to its maturity has, as we have 

 seen, been turned to account in the interpretation of its function. 

 The next step forward, on somewhat similar lines, was the interpreta- 

 tion of function by a comparison of the structure of the various parts 

 of the cortex in normal and insane persons. Of the structure of the 

 cortex an outline has already been given ; the four, or, as some prefer, 

 six, layers of cells and fibres into which this has been divided have 

 received at the hands of Bolton* another classification, which has been 

 generally adopted. It is spoken of as the five-layered type of cortex, 

 and the following are described in man : 



1. Outer fibre lamina : Two-thirds developed at birth. 



2. Outer cell lamina : Half developed at birth. 



3. Middle cell lamina : Three-fourths developed at birth. 



4. Inner fibre lamina : Two-thirds developed at birth. 



5. Inner cell lamina ; Fully developed at birth. 



Watson f has studied the development of these layers in many 

 orders of mammals. He shows that layers 4 and 5 are the earliest 

 to appear, and whether in the highest or lowest mammals there is 

 very little difference in their thickness (Fig. 153). Layer 3 is the 

 next in order of development. Layers 1 and 2 are the last to appear, 

 and the slowest in development. In the lowest mammals layer 2 is 

 generally poorly developed, and even at best obtains but a slight 

 absolute depth as compared with the human brain. 



The great difference between the human brain and that of the 

 insectivora lies chiefly in the degree of development of layer 2 ; 

 whereas, notwithstanding the difference between the brain of man 

 and that of the mole — one of the smallest in the mammalian series — 

 layers 3, 4, and 5 are of much the same thickness (depth) in each 

 animal. Fig. 153 illustrates approximately the relative depths of the 

 cerebral cortical layers in the normal human adult and the mole (after 

 Watson) . From the point of view of intelligence, layer 2 is of the 

 utmost importance ; it is under-developed to different degrees in 

 idiots and imbeciles, and in chronic and recurrent lunatics the same 

 fact is observed, though in a lesser degree. The examination of the 

 various layers of the cortex in cases of congenital blindness has 

 shown that certain portions of layer 3 are atrophied ; this has 

 enabled the cortex to be mapped out into a projection centre for 

 visual impressions, known as the visuo-sensory area. At the periphery 

 of this area is another, which can be distinguished by an abrupt 

 modification in the arrangement of the layers. It is known as the 



* ' Further Advances in Physiology,' edited by L. Hill, M.B., F.R.S. 

 Goulstonian Lectures, J. S. Bolton, M.D. : Brain, part exxix., vol. xxxiii., 

 1910. 



f 'The Mammalian Cerebral Cortex, with Special Reference to its 

 Comparative Histology,' G. A. Watson, M.B., CM., Archives of Neurology, 

 vol. iii., 1907. 



