NERVOUS SYSTEM OF VERTEBRATES. 



CHAPTER XI. 



THE SOMATIC MOTOR DIVISION. 



This division of the nervous system directly controls the 

 actions of the typical body muscles; namely, those derived 

 from the dorsal mesoderm or somites. It would be expected 

 that each segment of the body which has such muscles would 

 have a pair of somatic motor nerves. Inasmuch as several somites 

 in the occipital region degenerate in most vertebrates without 

 producing muscle, these nerves are wanting in those segments. 

 Somites i, 2 and 3 produce the eye muscles and these are inner- 

 vated by the cranial nerves numbered III, IV and VI. Between 

 these and the first somatic motor nerve of the spinal or trunk 

 region is a gap owing to the absence of a variable number of 

 postotic myotomes. In cyclostomes, where the postotic somites 

 all develop into myotomes, one or more nerves are absent (Petro- 

 myzon), apparently because one or two somites only partially 

 develop. In one cyclostome (Bdellostoma) it is now known that 

 a complete series of somatic motor nerves is present in this region, 

 one nerve for each postotic myotome. In the adult of this animal, 

 however, the eye muscle nerves are wholly lacking. Finally, 

 it is to be mentioned that one rudimentary somite is known in 

 selachians anterior to those which produce the eye muscles. It 

 is probable that the segment to which this somite belonged pos- 

 sessed muscles in primitive or ancestral vertebrates. The somatic 

 motor division is to be thought of as incomplete owing to the 

 loss, in various segments, of the muscles which it should innervate. 

 Otherwise this division is the simplest and least modified of the 

 four functional divisions of the nervous system. 



In the trunk region (Fig. 102) the somatic motor nerve fibers 

 arise from the cells of the ventral horn of the spinal cord and make 

 their exit from the ventral surface of the cord as the ventral roots 

 of the spinal nerves. In cyclostomes these ventral nerves arise 



