570 Dynamic Theory. 



from two to ten times as large as those in the top la} T ers, and those be- 

 tween are correspondingly of intermediate size. In the upper layers 

 there are from 100 to 120 cells to one square millimeter of surface, and 

 of a millimeter in depth. In round numbers this amounts to about 

 70,000 cells per square inch in the outer layer of gray matter. This, 

 multiplied by 670, the average number of square inches of the outer 

 surface of the cerebrum, gives an aggregate of 46,900,000 cells for this 

 layer, for a depth of JL of a millimetre, or ^ part of an inch, which is 

 about ^ of the whole thickness of the cortex. It is probably far 

 within bounds to estimate for all the layers of the cortex an aggregate 

 of 500,000 per square inch, and it is not unlikely there are nearer a 

 million, so that we may safely assume that we are possessed of from 

 300,000,000 to 500,000,000 of these organs in the cerebrum alone. 

 (For description of the cells see chapter 52.) The shape of the cells 

 is probably influenced by the manner in which they are crowded to- 

 gether. Each cell has from one to five fibrous connections. In the 

 adult the cortical substance is seen to be stratified, and observers make 

 out five or six layers. .( See fig. 278. ) In addition to the insulation of 

 the individual cells and fibres, which has been mentioned, there are 

 groups of related cells which, as groups, are thought to possess more or 

 less insulation from each other. Luys is of opinion that there is a de- 

 gree of insulation as between the several layers. The layers are differ- 

 ently colored, some being white or gray, others yellowish-red. 



The white substance of the brain, under the cortex, consists, as al- 

 ready stated, of the white nerve fibres closely packed together in paral- 

 lel bundles and masses, yet all insulated from each other by the medul- 

 lary sheath, as described in chapter 52. By means of these insulated 

 conductors, a stimulus reaching one mass of gray cells, if strong enough, 

 is conveyed to another. The fibres which converge to the crura cerebri, 

 internal capsule, and optic thalamus, keep company with the commissural 

 fibres belonging to the corpus callosum, from the layers of the cortex 

 toward the middle of brain, and when they reach the outside edges of 

 the lateral ventricles, they are seen to separate, the former passing under 

 the ventricles to their respective ganglia, while the latter pass across to 

 the opposite hemisphere over the top of the ventricles. 



It has already been observed that the sensory and motor functions of 

 the posterior and anterior columns of the spinal cord are continued into 

 the same relative parts of the medulla oblongata and the cerebral pedun- 

 cles. The appendages of the brain directly connected with these parts 

 partake of their general functions. Accordingly, the corpora quadri- 

 gemina and optic thalami, being on the sensory tracts, with perhaps 

 some exceptional connections elsewhere, are regarded as being in the 

 main sensory ganglia, while the corpora striata being placed on the 

 motor parts of the peduncles, possess motor functions, that is. the move- 



