400 DISSECTION OF THE PI&EON 



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 auriculars. 



Bach leads inwards, and slightly backwards 

 and downwards, to the tympanic cavity. 



II. DISSECTION OP THE PECTOBAL MUSCLES. 



Bemove the skin 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 out- 

 wards, converging to a broad flat tendon, which is 

 inserted into the greater tuberosity of the humerus. 

 From 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. 



Cut through the great pectoral muscle^ transversely to its 

 fibres, and about the middle of its length. Twrn the two halves 

 of the muscle aside, so as to clea/rh/ define its origin and inser- 

 tion : 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 



