THE ELASMOBRANCII FISHES 



227 



brandies. The ])liarynu:eal is unlike tliat of the ninth only in that it first passes 

 ))osteriorly and then cni-ves forward. The nerves posterior to the seventh cleft 

 have undergone considerable cliange. The third division of the vagus, tlie 

 ramus intestinalis (bi.X), breaks up into a numl)er of important divisions, 

 some of which go to the lieart, others to the oesophagus, and still others to the 

 stomach and associated parts. 



OCCIPITOSPINAL NERVES 



Arising ])osterior to the vagus are certain small nerves known as the occipi 

 tospinales (w-z, figs. 200a. 200b, and 204) . These nerves are unusually numer 

 ous in Heptanchiis and con- 

 sist of both ventral and 

 dorsal branches. The first 

 pair to arise ventrally {w, 

 fig. 200b) is near the mid- 

 ventral line and not unlike 

 the sixth or abducens nerve 

 in origin. Posterior to this 

 are three others (.r, y, z), 

 making in all four ventral 

 pairs. Dorsally in Heptan- 

 chiis maculatus (fig. 200a) 

 there are two pairs, the first 

 of which joins the "(/" divi- 

 sion of the ventral nerve. 

 These nerves supply the subspinalis and dorsal interarcuales muscles (fig 

 204) and are like the spinal nerves with which .r, ;/, and z form a plexus. 



Fig. 204. Cervical and brachial plexus, IJcptancJtiis 

 cinereus. (From Max Fiirbringer.) 



hr.p., brachial plexus; cr.p., cervical plexus; d.r., dor- 

 sal or sensory nerve; ^., dorsal root ganglion; sp., spinal 

 (mixed) nerve; v.r., ventral or motor nerve; «', x, y, s, 

 occipitospinal nerves. 



SPIXAL NERVES 



A spinal nerve arising from the spinal cord in Heptanchus cinereus (M. 

 Fiirbringer, 1897) is composed of a single ventral (motor) and a dorsal 

 (sensory) root. The ventral root comes from cells in the ventral horn of the 

 cord* {v.h., fig. 201) and passes outward to be joined by the dorsal root (fig. 

 204) ; while the dorsal root extends to the dorsal horn of the cord (<:/./(., fig. 

 201) through the dorsal root ganglion (g., fig. 204). After the union of dorsal 

 and ventral divisions the mixed nerves thus formed, in the region of the neck 

 and the pectoral and pelvic fins, are pressed together. 



The cervical {cr.p., fig. 204) and pectoral plexuses {hr.p.) may be consid- 

 ered together since they form a continuous bundle. In Heptanchus cinereus 

 these plexuses (fig. 204) are made up of the first seven spinal nerves together 

 with occipitospinales which were above mentioned. At the region of the scapula 

 the nerve trunk separates, the cervical nerves {cr.p.) going to the hypobran- 

 cliial or ventral longitudinal muscles, and the pectoral plexus {hr.p.) to the 

 pectoral fin. 



