102 HUMAN BIOLOGY 



remembered that most animal cells are of microscopic 

 proportions and quite invisible to the unaided eye, the 

 extraordinary character of the nerve cell or neurone must be 

 evident. 



The number of neurones in the central nervous system of 

 man is inconceivably great. A single instance will suffice 

 to illustrate this statement. The gray layer that covers the 

 exterior of the human hemispheres is of great uniformity in 

 thickness and in structure and thus lends itself easily to an 

 estimation of the number of neurones contained in it. This 

 number on good grounds is believed to be nine thousand 

 two hundred and eighty millions (9,280,000,000), a number 

 which, prodigious as it is, is approximately only about 

 one three thousandth of the twenty-six millions of millions 

 (26 X 10^-) of cells estimated to be present in the body 

 of the adult human being. It is clear from this one number 

 alone that it is no exaggeration to say that the human brain 

 contains millions upon millions of neurones. The interrela- 

 tions of these elements must establish a system whose 

 intricacies are unbelievably great. 



Notwithstanding the enormous number of neurones in the 

 central nervous organs of man, these elements conveniently 

 fall in accordance with their functions into three classes. 

 These classes are the sensory or receptive neurones, the 

 motor or effective neurones, and the communicating or 

 internuncial neurones. They can be most clearly illustrated 

 in the spinal cord where nervous relations are relatively 

 simple as compared with the brain. 



The spinal cord gives out from its sides right and left a 

 regularly arranged series of spinal nerves. As these nerves 

 emerge from the cord they are seen to arise by two independ- 

 ent roots, one dorsal and the other ventral. The dorsal 

 root has upon it an enlargement or ganglion. It has been 

 known now for somewhat over a century that these two 

 roots differ in the kinds of fibers that comprise them. A 

 dorsal root is made up of sensory or receptive fibers. These 

 are distributed to the skin and to the sense organs concerned 

 with the deeper parts of the body, such as those in the 

 muscles and the tendons. A ventral root, on the other hand, 

 is made up of motor or effective fibers which are distributed 



