74 ANATOMY OF THE CENTEAL NBETOTIS SYSTEM. 



It is evident that the elements which comprise the columns of the 

 spinal cord are of Tery different origin. That cannot be inferred from a 

 simple view of the cross-section. If one is to have even a general idea of the 

 relations, it is at least necessary in every case to make a careful comparison 

 of many cross-sections and longitudinal sections. But, to gain a clear con- 

 cept of the relations, an extended study, aided by embryological and other 

 methods, is necessary. 



Those who are acquainted with the human spinal cord only are sur- 

 prised at the great size of the spinal cord frequently observed in lower verte- 

 brates. The spinal cord is, in these eases, a quite independent organ, whose 

 size depends essentially upon the area which is to be supplied with spinal 

 nerves and only in very small measure upon the development of the parts 

 of the central nervous system. In the lower vertebrates it receives only a 

 few fibers, and even in the higher vertebrates not many fibers from the more 

 anteriorly located parts of the brain. If one would convince himself of 

 this, let him compare in any large fish the small size of the brain with the 

 relatively enormous size of the cord. The fish Gadus wglefinus especially 

 possess a spinal cord which contains almost exclusively spinal elements and 

 very few cerebral. The former are highly developed because the extensive 

 body-musculature and surface require a rich innervation. 



This striking relation may be readily followed even to the mammals. 

 The brain of the horse or ox is much smaller than that of man, but their 

 spinal cord is more than twice as thick as the human cord. 



In this connection compare the accompanying sections (Figs. 35 and 

 36) of the spinal cord of representative vertelyates. All are dra^^^l to the 

 same scale of magnification. 



