EMBRYOLOGY OF TADPOLE AND CHICK 

 THE PERIPHERAL NERVOUS SYSTEM 



573 



The peripheral nervous system has been discussed in some detail in 

 the chick, and will be taken up in comparative anatomy later. 



In the tadpole there are some forty pairs of spinal nerves but only 

 ten pairs in the adult. They arise by a dorsal and ventral root, which 

 unite to form the trunk of the spinal nerve, after which this trunk divides 

 into a dorsal and a ventral ramus, while a ramus communicans connects 

 the trunk with the sympathetic system. 



There are also ten cranial nerves instead of twelve as in the higher 

 forms. 



Fig. 334. 



Transverse and frontal sections of frog embryo to show position and division of 

 neural crest in head region. 



The V, VII, IX, and X are called branchiomeric nerves on account 

 of their close relationship to the branchial clefts. 



The cranial nerves take their substance from three embryological 

 elements, namely: (1) the cell masses derived from the neural crests 

 as described in the study of the chick, (2) cells from ectodermal patches 

 on the surface of the head, and (3) from the cell processes which extend 

 outward from the neuroblasts in the ventro-lateral walls of the spinal 

 chord. 



They differ, therefore, from the spinal nerves, for in these (2) is 

 lacking". 



The V, VII, IX and X cranial nerves arise by a single root (though 

 this may be mixed, i. e., it may be both sensory and motor in function) 

 passing into a large ganglion, beyond which a large horizontal branch 

 is given off w r hich in turn branches into two rami which pass anteriorly 

 and posteriorly to the gill cleft with which the particular nerve is asso- 

 ciated. 



As in the chick, so in the frog, the cranial nerves develop from the 

 neural crests left on each side of the central canal after the neural folds 

 fuse, and the indented ectoderm again returns to its normal condition. 



