172 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 is not matter 

 or energy, and which, therefore, is not within the scope of physio- 

 logical explanation. In what follows, therefore, attention is called 

 only to the mechanical side, the facts that have been discovered 

 regarding the anatomical structure and 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 minute differences the extent of which has 

 not been wholly determined, it is an interesting fact that the cor- 

 tex everywhere has a similar structure. It consists of four or five 

 layers more or less clearly distinguishable (see Fig. 76) : 



1. The molecular layer, lying immediately beneath the pia 

 mater, and having a thickness of 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 some relatively large pyramidal cells, particularly in 



