Integrative Systems 173 



an appendage has some relation, but not an exact one, to the number of 

 somatic segments which contribute to the development of the append- 

 age. In the skate, a fish having the pectoral fin enormously expanded 

 anteroposteriorly, there are about 25 nerves in the cervicothoracic 

 plexus. In general, there is great variation in the number of nerves in 

 the plexuses of the paired limbs, the range being usually between 2 

 and 10. 



The anatomic consequence of the interconnecting of the several 

 nerves of a plexus is that a single muscle of the appendage receives 

 nerve-fibers from several, or perhaps all, of the spinal nerves which 

 enter the plexus. This may mean that one muscle is compounded of 

 material derived from all of the several segments corresponding to the 

 nerves which contribute to its innervation. It is certain that the em- 

 bryonic segmentation of the muscle-forming mesoderm is at least much 

 modified and usually quite obliterated in the course of development of 

 the complex group of muscles of a tetrapod appendage. It is not certain 

 that the plexus is functionally significant. 



In most fishes and in tailed aquatic amphibians, the tail is the chief 

 swimming-organ. The segmentally arranged muscles continue from the 

 trunk back to the tip of the tail, and the spinal cord is coextensive with 

 the muscles, giving off nerves throughout the entire length of the tail. 

 In animals whose tails are not locomotor and are of minor functional 

 importance, muscles are present only in a short proximal region of the 

 tail, and a greater or less extent of the caudal region of the neural 

 tube fails to produce nervous tissue, persisting in the adult as a slender 

 nonfunctional vestige, the filum terminale. In many cases (e.g., frog 

 and man), the longitudinal growth of the vertebral column exceeds 

 that of the spinal cord, with the result that the more posterior spinal 

 nerves, whose foramens of exit are established at an early stage, are 

 dragged backward by the elongating vertebral column and form a 

 bundle of nerves extending back alongside the cord and inside the verte- 

 bral canal. This bundle, together with more or less of the filum ter- 

 minate, is called the eauda equina (Fig. 156). 



AUTONOMIC NERVOUS SYSTEM 



The basic activities of cells may proceed independently of excitation 

 or control by nervous mechanism. A gland-cell may secrete or a muscle- 

 cell may contract in complete absence of any connection to a nerve-cell. 

 A heart continues its rhythmic pulsation after all nerves connected to it 

 externally have been cut. But there is much nervous tissue upon or in 

 the walls of the heart itself. A small fragment of muscle completely 

 detached from the wall of the heart and placed in a suitable chemical 



