246 



THE ELASMOBRANCH FISHES 



located on each side of the yoimg of Hexanchus and Chlamydoselachus. In 

 Acanthias there are only two or three of the ventral nerves present and two 

 dorsal rami. These nerves united with the first and second spinals are marked 

 hb. in figure 220. In Torpedo a single ventral occipitospinal nerve as such is 

 present. 



The more posterior of these nerves unite with the first group of spinal 

 nerves to form the cervical plexus which in turn joins the pectoral plexus, the 

 fused trunks of which may run for a short distance, with, although it is no 

 part of, branches of the vagus. The whole group forming the two plexuses may 



Fig. 224. Nervous collector, Chlamydoselachus. (From Brans.) 



l.a.v., lateral abdominal vein; pl.p., pelvic plexus; sp.^^^^, twenty-fifth and thirty-eighth 

 spinal nerves. 



be composed of relatively few nerves (five in Spinax) or it may include many 

 (twenty in Torpedo). The nerves of the cervical plexus (cr.p., fig. 222) sepa- 

 rate from the pectoral plexus and pass in front of the girdle to supply the 

 hypobranchial muscle as in Scyllium and Squatina, while those of the pec- 

 toral plexus (ir.p.) pass through the girdle and are distributed to the pec- 

 toral fin. 



SPINAL NERVES 



In the region of the cord proper a sensory root and motor root (solid line, fig. 

 220) unite to form a mixed spinal nerve, much as in Hepianclius. Each of 

 these roots after leaving tlie cord passes backward within the neural canal, 

 then perforates the basal or dorsal intercalary cartilages as single roots. 

 Shortly before perforating the basal plate the motor branch may bifurcate 

 (fig. 220) and send a branch dorsally to join a branch from the ganglion {gn.) 

 of the dorsal root; or this motor root may pass by the ganglion without receiv- 

 ing from it sensory fibers. This branch passes dorsally to the dorsal longi- 

 tudinal bundles. When no sensory fibers join this root a sensory l)undle runs 

 dorsally from the ganglion. The other branch of the l)ifurcated ventral root 

 joins a sensory root from the ganglion, the two united passing as a mixed 

 nerve ventralh'. 



Posterior to the pectoral fin and in the region of the lateral al)dominal vein 

 the ventral rami of the various spinal nerves often form a connected strand, 



