388 DISSECTION OF THE PIGEON 



ii. The external auditory apertures are a pair of 

 circular openings, below and slightly behind 

 the posterior or outer angles of the eyes. 

 Each is surrounded by a circlet of small, back- 

 wardly directed feathers — the auricnlars. 



Each leads inwards, and slightly backwards 

 and downwards, to the tympanic cavity. 



II. DISSECTION OP THE PECTOEAL MUSCLES ■ 



Bemove the shin from the ventral surface of the thorax and 

 from one shoulder, so as to fully expose the first pectoral 

 muscle. Glean this muscle along its whole length, defining its 

 boundaries carefully. 



1. The first or great pectoral muscle is large and triangular, 



and forms the greater part of the side of the breast. 

 It arises from the whole length of the ventral 

 half of the keel of the sternum, from the whole 

 length of the clavicle, and from the lateral part of 

 the body of the sternum. Its fibres run forwards 

 and outwards, converging to a broad flat tendon, 

 which is inserted into the greater tuberosity of the 

 humerus. Prom the anterior border of the muscle, 

 near the shoulder, a slip is sent off to the skin. 



The great pectoral is the main depressor of the 

 wing, and the most important of the muscles of 

 flight. 



Divide the great pectoral muscle, transversely to its fibres, 

 and about the middle of its length. Turn the two halves of 

 the muscle aside, so as to clearly define its origin and insertion : 

 then cut away the muscle entirely. 



2. The second pectoral muscle is a smaller and more 



deeply placed muscle, similar in shape to the first 

 pectoral, and completely covered by it. 



It arises from the dorsal part of the side of the 

 keel of the sternum, and from the inner part of the 



