SENSE ORGANS OF AMIURUS. 371 
of the arch of the first vertebra in contact with it. The fourth nerve 
is, however, quite normal in its emergence, escaping through the arch 
of the third vertebra towards its union with the arch of the fourth. 
Further particulars as to the neural canal in this region are to be 
found under’ the description of the auditory organ. There also the 
nature of the saccus paravertebralis is described in which the ganglia 
of the first four spinal nerves lie, 
The ventral branches of the first four nerves go to form the 
brachial plexus, according to the diagram, Fig. 5, Pl. IV. The dorsal 
branches, especially of the second and third, are of large size, and join 
the ramus lateralis V.,as already described. As the ventral branches 
of the second and third pass outwards towards the plexus, they are 
extremely close together and may lie in the same sheath in a groove 
between the ventral edges of the strong anterior part of the fourth 
tranverse process (Fig. 13, Pl. [V.) and the transverse process of the 
supraclavicle. The ventral branch of the fourth is much slenderer, 
and after escaping from the neural canal gains the posterior aspect of 
the part of the fourth transverse process referred to. 
After the ventral branch of the first nerve leaves the saccus pura- 
vertebralis, it rests on the trapezius muscle which it supplies, and 
then divides into medial and lateral branches. The former (1 Fig. 
5), is intended for the pharyngo-clavicular muscles, the latter 
reinforced by a branch from the second nerve is destined for the 
supply of the abductor muscles and the deep adductor. (2 and 3, 
Fig. 5). 
The remainder of the second nerve joins the third ; the superficial 
adductor is supplied from this junction, a slender cutaneous filament 
courses to the skin in front of the fin, and a large nerve enters the 
defensive spine of the fin. The fourth nerve assists in the supply of 
the superficial adductor, it sends a delicate filament to the skin below 
the fin, and is distributed also to the upper part of the ventral mus- 
culature there. Fig. 5 also represents the method in which the 
following myotomes are supplied by the fifth, sixth and seventh 
nerves, and the nature of the communications between these. The 
ventral branch of the fifth runs down the intermuscular septum 
between the third and fourth myotomes of the ventral musculature 
and the following nerves conduct themselves similarly, supplying 
chiefly the myotomes in front of them. 
