NERVES OF THE DOGFISH 377 



nerves. As the nerve trunk approaches the place of its sepa- 

 ration into the chief branches of distribution, these elements be- 

 come more distinct. The derivative of the third spinal nerve 

 retains its individuality throughout, and never actually unites 

 with the others. The sensory elements of the spinal-nerve con- 

 stituents are coalesced and become intimately associated with 

 the occipital-nerve derivatives. The motor elements of the first 

 and second spinal nerves are distinct. 



In this condition the hypobranchial nerve forms a flat hori- 

 zontal band at the ventral border of the dorsal trunk muscles, 

 mesial to the ramus intestinalis X. Gradually it shifts laterally 

 dorsal to the ramus intestinalis, and finally, passing ventrally 

 around the lateral border of it, breaks up into its chief branches. 

 There are three of these branches, two directed anteriorly and 

 one posteriorly. In the readjustment of the constituent ele- 

 ments to form the peripheral divisions, the sensory elements from 

 the first and second spinal nerves combine with the motor con- 

 stituents of the occipital and second spinal nerves to form a 

 trunk that sends a small motor branch posteriorly into the third 

 spinal nerve and thence into the brachial plexus, and a large 

 branch anteriorly along the mesial dorsal border of the limb- 

 girdle (fig. 49) . When the posterior end of the fifth epibranchial 

 cartilage is reached, the nerve runs along its mesial border, then 

 shifting to the ventral border of the fifth ceratobranchial bar 

 and dividing into two or three divisions that later reunite, it 

 finally unites with the second anterior division of the hypobran- 

 chial nerve, just outside the ventrolateral wall of the pericardium, 

 on the dorsal border of the coraco-arcualis communis muscle, 

 which it innervates. In its course along the border of the coraco- 

 arcualis muscle the motor hypobranchial trunk divides into a 

 dorsal and a ventral branch. The latter, the larger, turns sharply 

 through the muscle ventrally, sends branches into the coraco- 

 mandibularis and enters and passes anteriorly within the coraco- 

 hyoideus muscle. The dorsal branch runs along the dorsomesial 

 border of the coraco-arcualis communis, and after passing 

 through this muscle, between the first and second divisions of the 

 coracobranchialis muscle, is distributed to the latter. 



