488 PHYSIOLOGY 



ment of the various layers and the elements constituting them. By 

 this method we can mark out the cortex into areas, which agree closely 

 with the localisation of functions as arrived at by experimental methods 

 or by a study of the systems of fibres in the brain or of the functional 

 defects observed under pathological conditions. 



The localisation arrived at in this way is represented in the diagrams 

 taken from Campbell (Fig. 222). Thus in the motor area, the precentral 

 or ascending frontal convolution, we find below the granular layer the 

 well-marked pyramidal cells or Betz cells, which are larger than any 

 other element in the cerebrum. The layer of pyramidal cells is also 

 very thick, while the granular layer is but slightly developed. In this 

 area the actual average thickness of the different layers is as follows : 



Molecular or outer fibre lamina .... 0-34 mm. 

 Pyramidal or outer cell lamina . . . .0-90 mm. 

 Granular or middle cell lamina .... 0-22 mm. 



Betz or inner fibre lamina 0-22 mm. 



Polymorphous layer or inner cell lamina . . 0-31 mm. 



In the visuo-sensory area the granular layer is the thickest, and is 

 divided into two layers by the band of tangential fibres forming the line 

 of Gennari. In the association areas, both those, such as the inter- 

 mediate precentral and visuo- psychical, which are normally associated 

 with motor or sensory processes, as well as the higher association 

 centres of Flechsig, i.e. the frontal and parietal temporal lobes, the 

 most marked feature in the section is the great development of and 

 the large number of cells observed in the outer cell lamina or pyramidal 

 cell layer. It will be noticed, too, that the audito-sensory area is but 

 small in extent and lies almost entirely within the lips of the fissure 

 of Sylvius, while the greater part of the superior temporal convolution, 

 which on the left side is associated with the auditory images and 

 associations necessary for the comprehension of speech, partakes of 

 the character of an intermediate or psychic sensory area. 



The same method may be applied to a comparison of the relative 

 development of the cerebral functions in different types of animals. 

 A. comparison of the brain of the dog, ape, and man shows that while 

 the absolute amount of brain substance devoted to the elementary 

 functions of movement and sensation remains practically the same 

 throughout, in man these areas are, relatively to the whole brain, 

 very much diminished in size, the greater part of the brain surface 

 being taken up with the nervous material of the type which is con- 

 nected with the functions of association involved in the higher pro- 

 cesses of reflection, intelligence, and volition. 



If we draw still lower animals into the sphere of our observations 

 we are enabled to form some idea as to the relative significance of 



