174 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



mechanism in the brain matter which is capable of giving us a 

 reaction in consciousness. The methods of physiology are adapted 

 to the investigation of the nature of this mechanism, but the reac- 

 tion in consciousness deals with a something which so far as we 

 know is not matter or energy, and which, therefore, is not within 

 the scope of physiological or, indeed, scientific explanation. In 

 what follows, therefore, attention is called only to the mechanical 

 side, the facts that have been discovered regarding the anatomical 

 structure and the physical and chemical properties of the nervous 

 mechanism. 



The Histology of the Cortex. The finer structure of the 

 different regions of the cortex has been the subject of much investi- 

 gation, but in this connection it is only necessary to recall the 

 elementary facts so far as they are useful in physiological explana- 

 tions. Leaving aside minor differences in the shape and stratifica- 

 tion of the cells, it is an interesting fact that the cortex everywhere 

 has a similar structure. It consists of four or five layers more or 

 less clearly distinguishable (see Fig. 75) : 



1. The molecular layer, lying immediately beneath the pia 

 mater, and having a thickness bf about 0.25 mm. In this layer, 

 in addition to the supporting neuroglia, there are found a number 

 of very small nerve cells of several types lying with their processes 

 parallel to the surface of the brain. The axons and dendrites of 

 these small cells terminate within the layer, so that they take no 

 direct part in the formation of the white matter of the brain, but 

 have, so to speak, a distributive or associative function. In this 

 layer, also, end many of the dendrites of the larger nerve cells of 

 the deeper layers and the terminal arborization of entering nerve 

 fibers (axons) from other regions. It must be conceived, there- 

 fore, as containing a fine feltwork of nerve fibrils, dendrites and 

 axons or their collaterals, and as a region, therefore, in which 

 many of the incoming impulses along afferent fibers are, so to 

 speak, distributed into outgoing ones. On histological grounds 

 Cajal was inclined to believe that this layer represents the location 

 of the most important psychical reactions. 



2. The layer of small pyramidal cells of about the same thick- 

 ness as the last. This layer contains a number of small nerve 

 cells, mostly of the pyramidal type, with the apex directed toward 

 the external surface. The dendrites from the apical process termi- 

 nate in the molecular layer, while the axon arising from the basal 

 side of the cell passes inwardly to constitute one of the nerve 

 fibers of the medullary portion of the cerebrum. 



3. The layer of large pyramidal cells. This layer, much thicker 

 (1 mm.) than the preceding, is not sharply differentiated from it. 

 It contains large pyramidal cells, particularly in the Rolandic area. 



