COMPARATIVE ANATOMY OF THE NERVOUS SYSTEM 315 



it is distributed, and a more ventral visceral branch. In all three forms, open 

 up the remaining visceral pouches as directed for the ninth nerve and determine 

 the distribution of the visceral branch of the vagus. With the walls of the sinus 

 well spread open note the four branches crossing the floor of the sinus to the 

 visceral pouches. Dissect out each of these and observe that each bears a gang- 

 lion, beyond which it divides into three branches: an anterior pretrematic 

 branch, a middle pharyngeal branch, and a posterior post-trematic branch. 

 The pharyngeal branch seems to be missing in the skate. As in the case of the 

 ninth nerve, the pre- and post-trematic branches embrace the visceral pouch, 

 which lies between them ; all three branches have the same functions as described 

 for the ninth nerve. We thus see that the vagus nerve supplies the remaining 

 visceral arches, beginning with the fourth, and the remaining visceral pouches, 

 beginning with the third. After supplying the gill apparatus the visceral branch 

 of the vagus passes on into the pericardial and pleuroperitoneal cavities, supply- 

 ing the heart, digestive tract, and other viscera. 



Our study of the cranial nerves shows that they bear a definite relation to the original 

 segmentation of the head and to the gill apparatus. The fifth, seventh, ninth, and tenth 

 cranial nerves are gill-arch nerves, each associated with one or more particular visceral 

 arches. In the evolution of the vertebrates each nerve continues to supply its particular 

 arch or arches and all derivatives thereof. Since the muscles of the visceral arches are 

 visceral muscles, the motor components of the gill-arch nerves in all vertebrates are visceral 

 motor in nature. The somatic motor components for the fifth and seventh nerves, as already 

 stated, are separated into special nerves, the eye-muscle nerves. There are no somatic 

 motor elements for the ninth and tenth cranial nerves, since the myotomes which should 

 give rise to voluntary muscles for their segments are degenerate in all vertebrates above 

 cyclostomes (Fig. 69, p. 300). 



5. The occipital, hypobranchial, and first spinal nerves. Very carefully 

 expose the spinal cord posterior to the medulla by shaving away the cartilage 

 of the neural arches in thin slices. On the dorsolateral surface of the cord note 

 the little swellings, the dorsal or spinal ganglia, attached to the cord by the 

 sensory or dorsal root. In the skate the spinal ganglia are elongated. Between 

 the first spinal ganglion and the root of the vagus note two or three small roots 

 springing from the side of the medulla. These are the occipital nerves. They 

 innervate some muscles of this region and also help to form the hypobranchial 

 nerve described below, and contribute to the cervicobrachial plexus. On press- 

 ing the spinal cord to one side the ventral or motor roots of the spinal nerves will 

 be seen arising from the ventrolateral region of the cord, at the same level as 

 the occipital nerves. The ventral roots are formed by the union of several small 

 rootlets coming from the cord. The most anterior ventral roots are situated 

 anterior to the dorsal roots which belong to the same segment. 



In the dogfish the union of dorsal and ventral roots to form a spinal nerve 

 is not easy to follow. It may usually be seen by carefully paring down the carti- 

 lage along the side of the spinal cord. In the skate the union is easily followed; 



