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, PI. 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 

 'tran verse process (Fig. 13, PL IV. ^ and the ti'ansverse 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 venbral branch of the first nerve leaves the sarcus para- 

 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 nmsculature 

 and the following nerves conduct themselves similarly, supplying 

 chiefly the myotomes in front of them. 



