306 THF, INTfRNAL F.NVIRONMF.NT OF THF BODY Part III 



space for nerve cells is more significant than their arrangement or character 

 which are not unique. They are similar in higher mammals, and the brains 

 of normal human beings greatly resemble one another no matter how different 

 the mental ability of their owners. After years of study, no structure has 

 been found in the human brain which is actually different in kind from those 

 present in the brain of a chimpanzee. In man, the size of the cerebrum com- 

 pared to the rest of the brain is far greater than it is in the apes. An adult 

 human cerebrum weighs around three times that of an adult gorilla. The 

 layer of gray matter, called the cerebral cortex, is about one-eighth of an 

 inch thick. By counting the cells in small areas and using such counts as a 

 basis of computation, it is figured that the human cerebral cortex alone 

 contains some 9280 millions of nerve cells. Most of these are provided with 

 long nerve fibers, chiefly axons, that extend for relatively great distances and 

 branch in different directions, connecting each cell through the junction-like 

 synapses with the cells in many different centers. The total number of such 

 connections and nervous pathways is inconceivably great. 



Fiber Tracts. The wires of a telephone exchange are grouped in cables 

 and distributed on a switchboard according to a system. In like manner, the 

 nerve fibers that have similar functions extend in bundles or tracts through 

 the white matter of the brain and cord (Fig. 16.11), and from there they have 

 synaptic connections with other nerve cells which continue into nerves. In 

 spite of the complexity of their arrangement, the make-up of the main nerve 

 tracts has been analyzed. The courses of various series of nerve cells have been 

 traced from receptors, such as those involved in a pinched toe, to the appro- 

 priate center of adjustment in the cortex. Likewise, the motor pathways 

 have been traced, in this case from the cortex to the muscles which move 

 the foot. 



In general, nerve cells in the cortex of the right side of the brain com- 

 municate with muscles on the left side of the body, and likewise those on the 

 left side of the brain communicate with the right side. Throughout the white 

 matter of the spinal cord and brain there are intercrossing fibers. As a result 

 the spinal cord and the two parts of the brain are bound together structurally 

 and functionally by an unthinkably complex network. Untangling these facts 

 began centuries ago and is not finished. Much has been learned from dissec- 

 tions, microscopic examinations, and experimental studies. Hundreds of 

 observations on how the human nervous system works have been made 

 when injuries to it made this possible. 



Functions of Cerebral Cortex. The human cerebral cortex is the location 

 of intelligence, of reasoning powers, of consciousness and of memory. The 

 brain acts as a coordinated whole. However, it is well known that different 

 areas of the cerebral cortex function differently. 



At the extreme rear of each cerebral lobe is the visual area (Fig. 16.22). 



