The elasmobranch fishes . elasmobranchfish03dani Year: 1934 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 dis
The elasmobranch fishes . elasmobranchfish03dani Year: 1934 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.) , lateral abdominal vein; , 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 (, 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 () 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 th
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