534 



COMPARATIVE ANATOMY 



an individual muscle fiber is innervated by more than one nerve. Evi- 

 dence of such plural innervation of a single muscle fiber is lacking. In 

 fishes other than teleosts, the lumbo-sacral plexus receives fibers from a 

 collector nerve, which unites anterior spinal nerves with the plexus. 



The problem of the origin of a nerve plexus was difficult in the days 

 when a primary and indissoluble connexion between nerve and muscle was 

 assumed. It is simpler, now that neuro-muscular connexions are known 

 to be secondary. Even in those regions where no plexuses are formed, 

 each spinal nerve innervates at least three myotomes, its own and parts 

 of the two adjacent. This tendency to muscle piracy is carried to a 



Sensory nerve fibers. 

 Muscle fibers. 



Motor plate. 



Medullated nerve fibers. '' 



Nerve fiber bundle. 



Fig. 441. — Motor nerve endings of intercostal muscle ilbcrs of a rabbit. X150. 

 (From Bremer's "Text Book of Histology.") 



greater extreme in the nerve plexuses, in which fibers from one nerve may 

 innervate more than three adjacent myotomes. 



The numerous variations in plexuses, therefore, demonstrate that the 

 peripheral distribution of a given neurite is not unalterably predetermined, 

 though it must be admitted that the factors which determine the direction 

 of growth of a given neurite are still uncertain. In the light of the evi- 

 dence which we now possess, it does not seem necessary to explain the 

 formation of a plexus by assuming a crowding of adjacent nerves in the 

 formation of an appendage. The crowding is secondary, not primary. 



Since both brachial and lumbosacral plexuses have their origin and 

 meaning in relation to appendages, it is significant that both brachial 

 and lumbo-sacral plexuses occur in snakes. 



In mammals the cord ends in a terminal thread, the filum terminale, 

 the length of which is proportional to that of the tail. The spinal cord 

 in the fetus stops growing sooner than the vertebral column. As a result, 

 the more posterior spinal nerves, which have their foramina of exit in 



