182 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



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 differences in the shape and stratification 

 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. 81). 



1. The superficial, plexiform, or molecular layer, lying imme- 

 diately 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, probably, a distributive or associa- 

 tive function. In this layer, also, end many of the dendrites of the 

 larger nerve cells of the deeper layers and the terminal arboriza- 

 tion of entering nerve fibers (axons) from other regions. 



2. The layer of pyramidal cells. This layer is characterized 

 by the presence of numerous pyramidal cells (see D, Fig. 84), 

 which in general increase in size in passing from the upper to the 

 lower strata. The apices of these cells are directed toward the 

 external surface. The dendrites from the apical process terminate 

 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. This thick lamina of 

 cells is sometimes subdivided into three layers of small, medium, 

 and large pyramidal cells. 



3. The granular or stellate layer composed of many small cells, 

 some of which are pyramidal and some stellate in form, with short 

 branching axons. These latter belong to Golgi's second type of 

 nerve cell. 



4. The deep pyramidal layer or layer of large or medium-sized 

 pyramidal cells, similar in form to those in layer two, and the axons 

 of which pass into the medulla or white matter of the cerebrum 

 as nerve fibers. 



