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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 Stannius proved by stimulation experiments that the 
vagus of the Plaice sent fibres to the heart. 
3.—THE SprnaL 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 (d. 4) and a ventral motor (v. 4). Hach 
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, 
N 
