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AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



functional nerve-cells with age. Kaiser, 1 as is shown in the accompanying 

 table, found in man increasing numbers of large nerve-cells in the ventral 

 horns of the spinal cord at the ages named : 



Number of Developed Cells in the Cervical Enlargement of Man at Different 



Ages (Kaiser). 



Age. Number of Nerve-cells. 



Fetus, 10 weeks 50,500 



" 32 " 118,330 



New-born child 104,270 



Boy, 15 years 211,800 



Male, adult 221,200 



Here, as in the frog, the apparent increase must be looked upon as due to 

 the gradual development of elements present from an early date. And it 

 must be further remembered that in this case the cells thus maturing after 

 birth probably belong in a large measure to the group of "central cells," 

 the function of which is to increase the complexity of the pathways within 

 the cord. 



Fig. IK". Diagram illustrating the extent of the cerebral cortex. The outer square i,* , shows a sur- 

 face approximately one-fiftieth of 2352 sq. cm. in extent ; the inner square (.1) has two-thirds of this area, 

 and is the proportion of the cortex sunken in the fissures. 2352 sq. cm. are approximately the areaof the 

 entire cortex in a male brain weighing 1360 grams. 



Increase in the Fibres of the Cortex. — The area of the cerebral cortex 

 (see Fig. 11!)) varies according to several conditions, but in general the more 

 voluminous the cerebral hemispheres the greater it- extent. That which 

 covers the walls of the sulci, — the sunken cortex — has in man about twice 

 the extent of that directly exposed on the surface of the hemispheres. 



In the cortex of the human cerebral hemispheres it has been shown by 

 Vulpius 2 that the number of fibres in the different layers is greater in the 



1 DU Functional der Oanglienzellen dea Haismarkes, Haag, 1891. 

 -' Vulpius: Archiv fur Psychiatric und Nervenkrankhcilen , 1892. 



