OF THE SCOTTISH NATIONAL ANTARCTIC EXPEDITION. 225 



tendon inserted into the humerus. These two portions have different actions ; the 

 anterior carries the wing forward to a position at right angles with the trunk, while the 

 posterior draws the limb back in the effective part of the swimming stroke, and by its 

 insertion into the anterior margin of the humerus it " brings about that rotation of the 

 wing round its long axis which, combined with the backward stroke, gives rise to the screw- 

 like motion of the organ observable when the bird is progressing through the water."* 



The nerve supply is provided for by a special branch from the large cord of the 

 brachial plexus. 



The dermo-humeralis arises from a strong fascia which covers the external oblique 

 muscle ; at the posterior margin of the sternum it is reinforced by a number of fibres 

 arising from the subcutaneous tissue covering the knee joint. It passes forward parallel 

 to the outer border of the pectoralis major to be inserted along with the posterior fibres 

 of that muscle into the anterior margin of the humerus. Its action apparently is to 

 co-operate with the external part of the pectoralis major in depressing the wing. It is 

 supplied by a twig from the nerve to the pectoralis major. 



The 'pectoralis medius passes over a pulley formed by the bones of the shoulder and 

 acts as the great elevator of the wing ; by some authors it has been called levator 

 humeri. It also is supplied by a twig from the nerve to the pectoralis major. 



The pectoralis minor is inserted into the outer margin of the tricipital fossa ; its 

 chief action appears to be to rotate the humerus in a direction opposite to that of the 

 rotation produced by the action of pectoralis major. In addition, however, it must help 

 in producing the effective, propulsive stroke of the wing. Its nerve of supply is a twig 

 from the nerve to pectoralis major. 



The arrangement of the fascia in regard to these muscles is striking and peculiar. 

 Tracing the deep fascia of the neck downwards, it passes deep to the clavicle, to which 

 it is lightly attached ; and, passing to the thoracic region, it lies superficial to the 

 pectoralis medius and minor and becomes continuous with the strong fascia which 

 covers the external oblique. Superficial to this fascial layer lie the pectoralis major and 

 dermo-humeralis, and from it they both obtain a portion of their origin. The super- 

 ficial aspects of pectoralis major and dermo-humeralis are in close contact with skin — so 

 close, indeed, that they are marked by regular rows of pits which accommodate the 

 rounded ends of the feather quills. 



From this arrangement it appears to us possible that the dermo-humeralis, pectoralis 

 major, and clavicle are in reality cutaneous structures. That the dermo-humeralis is in 

 series with the dorsal cutaneous muscle and panniculus carnosus is certain ; that it is 

 closely associated with the pectoralis major in position, in action, and nerve supply is 

 also certain ; but whether the pectoralis major can justly be associated with the same 

 group is, on the present evidence, not certain, although it appears to us probable. Un- 

 fortunately the evidence from nerve supply is so weakened by the association of 

 pectoralis medius and minor with the same nerve as to be valueless. 



* Quoted from Professor Morrison Watson's memoir. 



