Responses of Higher Animals: The Nervous System - 461 



nerve is a mixed nerve, since it carries sen- 

 sory fibers from the receptors to the cord, 

 and motor fibers from the cord to the mus- 

 cles, in a particular body segment. 



The junction between each spinal nerve 

 and the cord is effected by two roots: a dorsal 

 root and a ventral root (Fig. 25-13), and these 

 two roots are different as to function. All 

 sensory fibers in the nerve enter the spinal 

 cord via the dorsal root; whereas all motor 

 fibers emerge from the cord via the ventral 

 root (Fig. 25-13). The dorsal root is also dis- 

 tinguished by an appreciable swelling, the 

 dorsal root ganglion, which contains the cen- 

 trons of all the sensory neurons of the cor- 

 responding side of a particular body seg- 

 ment. Each of the neurons shown in Figure 

 25-13 represents a type, of which many are 

 present in the actual nerve — so that a spinal 

 nerve innervates all the receptors and effec- 

 tors on the corresponding side in a particular 

 body segment. Not shown in the diagram are 

 the association neurons in the gray matter of 

 the cord, which serve to transmit impulses 

 from the sensory neurons to motor neurons, 

 not only in the same segment of the body, but 

 also in segments that lie above and below a 

 particular spinal nerve. 



It is not difficult to demonstrate the func- 

 tional differentiation between the dorsal and 

 ventral nerve roots. If a dorsal root is cut, 

 the animal suffers an anesthesia, which is 

 limited strictly to one side of one segment 

 of the body. No sensations can originate from 

 the receptors of this segment, nor can any 

 reflexes be initiated if stimuli are applied in 

 this area of the body. However, the muscles 

 of the segment are not paralyzed, since these 

 muscles can take part in other reflexes, pro- 

 vided these reflexes originate in other seg- 

 ments of the body. 



If the ventral root of a spinal nerve is cut, 

 there is a localized unilateral paralysis in- 

 volving only the muscles supplied by the 

 particular nerve. After the operation these 

 muscles cannot participate in any reflex. The 

 segment suffers no anesthesia, however, and 

 reflexes involving other muscles of the body 



can originate from the receptors of the seg- 

 ment. Essentially, the cutting of one or more 

 ventral roots simulates the main symptoms 

 of infantile paralysis, and, in fact, the virus 

 of infantile paralysis does produce a degen- 

 eration of the motor neurons that pass out 

 from the spinal cord via the ventral roots. 



When a nerve is cut, the fibers that have 

 been divorced from their cell bodies gradu- 

 ally degenerate. Later, however, a new set 

 of axons and dendrons may grow out along 

 the remnants of the old fibers; and this trail 

 of degeneration and regeneration makes it 

 possible to trace out the origin and distribu- 

 tion of a given set of nerve fibers, even within 

 the mazes of the central nervous system. 



The Cranial Nerves. In primitive vertebrates, 

 the cranial nerves develop on a segmental 

 basis, but the brain of modern vertebrates 

 has become so highly modified that the seg- 

 mentation is obscured. The cranial nerves 

 originate from the brain — or more precisely, 

 from the sides of the brain stem (Fig. 25-10). 

 Most vertebrates possess twelve pair of cra- 

 nial nerves; Table 25-1 shows the cranial 

 nerves of man. Reptiles, birds, and mammals 

 possess the same nerves as man; but only the 

 first ten pair are fully developed among the 

 fish and amphibians. Some of the cranial 

 nerves (I, II, and VIII) are purely sensory; 

 others are mainly motor (III, IV, VI, XI, and 

 XII); and the others (V, VII, IX, and X) are 

 mixed nerves, containing sensory and motor 

 fibers in fairly equal proportions. The sen- 

 sory neurons of the cranial nerves, like those 

 of the spinal nerves, have their cell bodies 

 localized in ganglia situated near the nerve 

 roots. 



The cranial nerves function like the spinal 

 nerves, except that in many cases the cra- 

 nial nerves are more closely associated with 

 the higher centers of the brain. This is espe- 

 cially true of the main sensory nerves — from 

 the eyes, ears, tongue, and nose — in which 

 the fibers are routed almost directly to the 

 cerebral cortex. 



Main Conducfion Paihs: Somatic Reflexes. 

 The simplest possible reflex in a vertebrate 



