CRANIAL NERVES OF SIREN LACERTINA 331 
The second spinal nerve arises by a single dorsal and two to 
three ventral roots. It possesses a small ganglion and divides 
into characteristic dorsal and ventral rami of mixed constitution. 
The ventral ramus passes posteriorly through the longitudinal 
musculature dorsal to the pharynx, comes into contact with the 
ventral ramus of the first spinal nerve as mentioned above, but 
does not unite with it until a level is reached a little posterior to 
the point where the ramus intestino-accessorius X breaks up 
into its larger divisions. There the two spinal nerves curve 
sharply laterally and ventrally around the lateral border of the 
pharynx and, running anteriorly at the dorso-lateral border of 
the hypobranchial musculature, unite into a common trunk. 
Driiner states that the two rami unite shortly after they emerge 
from the spinal column, a little posterior to the transverse process 
of the first vertebra. This may occur in exceptional cases but the 
writer has seen no indications of it. Rather the fusion takes 
place in the mode characteristic of the Urodela. | 
The nerve resulting from this fusion is the hypobranchialis of 
mixed constitution (fig. 44, hgl.). Its general cutaneous fibers 
are given off shortly after the formation of the nerve. The nerve 
runs at first at the dorso-lateral border of the hypobranchial 
musculature, but more anteriorly sinks ventrally until it lies at 
the ventro-lateral edge. Posteriorly, the nerve supplies the three 
anterior segments of the abdomino-hyoideus muscle, the sterno- 
hyoideus and omo-hyoideus muscles, and anteriorly the genio- 
hyoideus muscle only, the genio-glossus muscle, as Driiner has 
observed, being absent in Siren. As the two rami which form the 
hypobranchialis are passing posteriorly from their emergence from 
the spinal column they give off a few small branches to the longi- 
tudinal musculature, musculi intertransversales, through which 
they run. Shortly before they recurve to their point of union 
they give off a few small dorsal branches, some of which anasto- 
mose with the branch of the ramus intestino-accessorius X sup- 
plying the trapezius muscle, and others innervate the basi-scapu- 
laris muscle. From the ventral ramus of the second spinal nerve 
a branch is contributed to the brachial plexus. Otherwise the 
latter is formed from the ventral rami of the third and fourth 
spinal nerves. 
