FUNCTIONAL DIVISIONS OF NERVOUS SYSTEM 365 



It is possible, therefore, to make out four main divisions 

 of the nerves according to their function : 



those which convey sensory impulses from the outside 

 world, somatic sensory, or afferent ; 



those which convey sensory impulses from the inner 

 world, visceral sensory, or afferent ; 



those which convey motor impulses to the smooth muscles 

 of the viscera, visceral motor, or efferent ; 



those which convey motor impulses to the striped muscles 

 of the body-wall and limbs, somatic motor, or efferent. 

 Each of these functional systems are called components, and 

 as the same components can be found in several different 

 nerves, it is interesting to study the nerves according to the 

 components which they contain. In this way a classification 

 of nerves is obtained which, as it were, runs at right angles to 

 the classification according to the segment of the body in which 

 they lie. Further, the different components occupy special 

 parts of the central nervous system, and the evolution of the 

 latter, and especially of the brain, has been largely controlled 

 by the positions and relations of these " centres." 



In an ordinary spinal nerve of any vertebrate above the 

 Cyclostomes, there are two roots : one dorsal and one ventral, 

 and they join to form a mixed nerve. The mixed nerve also 

 sends a branch (ramus communicans) to a sympathetic ganglion. 

 Now, the dorsal root is made of fibres of afferent (sensory) 

 neurons, and the ventral root is composed of efferent (motor) 

 ones. Accompanying the anatomical division into dorsal and 

 ventral roots, there is therefore an important physiological 

 distinction. 



The cell-bodies of the afferent neurons are situated in the 

 ganglion which is always present on the dorsal root in all 

 chordates above Amphioxus. This means that the receptor 

 cell itself does not convey the impulse to the central nervous 

 system, this function being served by the afferent neuron of 

 the ganglion of the dorsal root. (In Amphioxus, and in the 

 nose of all vertebrates, on the other hand, the primitive con- 

 dition characteristic of many invertebrates persists : that is, 

 the receptor sensory cell itself produces an axon which runs 



