No. I.] PECTORAL APPENDAGES OF PRIONOTUS. 1 79 



The sensory root (Fig. 4, s.r.) of the third spinal nerve is 

 more than ten times as large as its motor root (Fig. 4, vi.r^j and 

 extends forward between the motor and sensory roots of the 

 second, and first spinal nerves. The motor and sensory roots 

 of the first and second spinal nerves, and the motor root of the 

 third are about equal in size (Fig. 4). 



A nerve of the brachial plexus (Fig. 3, c) unites the second 

 and third spinal nerves near their origin. 



The somewhat flask-shaped enlargements of the first three 

 spinal nerves {g',g", and^'") just after they arise from the 

 cord, contain the spinal ganglia. 



The second spinal nerve (II) passes through a foramen 

 formed by the clavicle and scapula. At the posterior border 

 of the pectoral fin the nerve bends nearly at right angles to 

 penetrate and follow the triangular space between the proximal 

 ends of the paired bones which form the skeleton of each ray 

 and the distal border of the brachial ossicles where the rays 

 articulate with the ossicles. The nerve extends forward and 

 downward nearly to the anterior border of the fin where it 

 unites by cross fibers with the two or three branches from the 

 nerve (Fig. 3, 3) which supplies the posterior free ray. A 

 branch of this nerve (II) is sent to each half of every fin ray 

 (Fig. 3, n.f.r. of II)' except the two or three anterior ones 

 which are innervated by branches (Fig. 3, n.f.r.) from the 

 nerve of the posterior free ray (Fig. 3, 3) as already noted. 



The third spinal nerve (Fig. 3, III) passes under the pectoral 

 girdle and divides into three large branches (Fig 3, 1,2, 3) quite 

 near its origin. Each of these divisions, anterior (Fig. 3, i), mid- 

 dle (2), and posterior (3), is much larger than the entire trunk 

 of either the first (I) or second (II) spinal nerves. They lie on 

 the inner surface of the muscles of the fin, just beneath the skin. 



The middle branch (2) divides into two parts (Fig. 3, A and 

 B); the former innervates the second free ray, counting from 

 the head, while the latter passes to the posterior surface of the 

 third free ray and with the posterior branch (3) of the third 

 spinal nerve (III) supplies that ray. The anterior branch of 

 the third spinal nerve (i) furnishes the nerve supply for the 

 first free ray. 



