192 FOUNDATIONS OF BIOLOGY 



by one component cell process, the nerve fiber; that is, by 

 way of a definite cell path through the nerve. The same is 

 equally true of the cord and the brain, which differ from 

 nerves largely in the circumstance that they comprise more 

 cell processes and also the cell bodies themselves. In other 

 words, the brain and cord comprise the elements of both 

 ganglia and nerves. 



A given nerve may conduct impulses both to and from the 

 central organ if it contains afferent and efferent cell paths, or 

 fibers. As a matter of fact all the peripheral nerves primarily 

 are mixed nerves, because typically they arise by two roots 

 from the central organ; the DORSAL ROOT containing only 

 sensory (afferent) fibers and the VENTRAL ROOT only motor 

 (efferent) fibers. This condition is preserved by the spinal 

 nerves of higher forms since each arises by two roots. But 

 some of the cranial nerves, in response to the profound modi- 

 fications which have been wrought in the head region, have 

 only one root, and so are either solely sensory, as those to the 

 sense organs, or only motor, as those innervating the muscles 

 which move the eye. (Fig. 107.) 



So far we have considered the central system the brain 

 and spinal cord and its lines of communication with the 

 body as a whole, the peripheral system, or cranial and spinal 

 nerves. In point of fact, however, the peripheral system 

 gives rise to an auxiliary series of ganglia and nerves which 

 are charged with the innervation of certain of the internal 

 organs, particularly the alimentary canal and arteries, which 

 are not directly under voluntary control. This AUTONOMIC 

 SYSTEM in the higher Vertebrates consists essentially of a 

 double nerve chain situated chiefly within the coelom just 

 ventral to the spinal column. It communicates with the 

 central system by way of the sensory roots of the spinal and 

 some of the cranial nerves. (Fig. 106.) 



