THE MYOLOGY OF AMIURUS CATUS. 333 



plate, and from the under sui'face of the portion of the clavicle over- 

 lapping this. It passes below this overlapping portion of the clavicle, 

 in the channel between it and the coracoid, and uniting with the 

 first portion, is inserted with it. 



Innervation. — The same as for the abd. super/. 



Action. — This muscle abducts the first ray, and thus assists in 

 abducting the entire fin, but at the same time it gives to the first 

 ray the rotation which is necessary to complete its abduction and 

 fixation. This rotation is brought about by the muscle being in- 

 serted into the upper surface of the ray. 



The position of the second portion of this muscle appears some- 

 what anomalous, inasmuch as it is apparently in the upper surface of 

 the arch, the abd. super/., and even the other portion of the abd. 

 prof., lying in its lower surface. An examination of the structure 

 of the arch explains the anomaly. The posterior portion of the arch 

 which unites with its fellow by suture is not the posterior portion of 

 the clavicle as it has been usually described, but is an enlargement 

 of the coracoid. Now this latter lies really on the inferior surface 

 of the arch, and therefore the upper surface of this enlargement is 

 applied to the under surface of the clavicle, and accordingly a muscle 

 lying upon its upper surface may yet lie on the under surface of the 

 clavicle. Though the two portions of the deep abductor are widely 

 separated at their origins, yet their union before insertion indicates 

 that they originally constituted one muscle, homologous with the 

 deep abductor of Perca. 



4. Adductor superficial^. 



Arises from the inner surface of the ascending portion of the 

 clavicle and from the bridge-like spiculum of bone near its base ; the 

 deeper fibres arising from the radialia. It is inserted into the 

 superior surfaces of the bases of all the rays, except the first, dividing 

 into a separate tendon for each ray. 



Innervation. — It is supplied by a branch from the combined second 

 and third spinal nerves. 



Action. — Adducts the fin. When the fin is abducted the rhythini» 

 cal and successive action and relaxation of the superficial abductors 

 and adductor will produce an undulatory movement of the fin. 



