SEA-FISHERIES LABORATORY, 289 



the fourth. R. post-trematicus (post. 4).* The last branch 

 continues the downward course, gives off a motor branch 

 behind, and then courses forwards and downwards in close 

 contact with the roof of the pericardium. This may be 

 the R. cardiacus (r. car. ?), but it does not actually pass on 

 to the sinus venosus, and certainly contains a number of 

 motor fibres distributed outside the heart. The true R. 

 cardiacus may therefore have been overlooked, especially 

 as S tannins proved by stimulation experiments that the 

 vagus of the Plaice sent fibres to the heart. 



3.— The Spinal Nerves (Fig. 27). 

 The Fourth Spinal Nerve. 



We describe this spinal nerve first since in most 

 respects it may be taken to represent the structure of most 

 of the other spinal nerves. The visceral fibres are not 

 taken into account. 



The fourth spinal nerve arises by two roots — a dorsal 

 largely sensory (77. 4) and a ventral motor (v. 4). Each 

 root leaves the neural arch of the third vertebra by a 

 separate foramen, as described by Stannius, and passes at 

 once into a single large extra-vertebral ganglion (g. 4). 

 The motor fibres for the R. medius and R. ventralis per- 

 forate the ganglion, but those for the R. spinosus pass 

 upwards internal to it. One lateral, one ventral and two 

 dorsal branches arise from the nerve. The two last are 

 the R. communicans (sensory) and the R. spinosus (motor), 

 whilst the former are respectively the R. medius (sensory 

 and motor) and the R. ventralis (sensory and motor). 



1. R. communicans (r. com. 4). — This sensory ramus 



* We differ from Herrick in naming two of the branches of the vagus. 

 His fourth post-trematic (fig. 4, post 4) is apparently the pharyngeus iv., 

 and his " branches of the vagus for the inferior pharyngeal teeth " (ph. v., 

 same figure) are equivalent to our post-trematic iv. 



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