434 DISSECTION OF THE PIGEON. 



b. Paired apertures. 



i. The nostrils are a pair of oblique slit-like aper- 

 tures, between the beak and the cere, 

 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. 



Each leads inwards, and slightly backwards 

 and downwards, to the tympanic cavity. 



II. DISSECTION OF THE PECTORAL MUSCLES. 



Remove the shin from the ventral surface of the thorax 

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

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

 boundaries carefully. Ligature the large blood-vessels. 



1. The first or great pectoral muscle is large and trian- 



gular, and forms the greater part of the side of the 

 breast. It arises from the whole length of the ven- 

 tral 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 for- 

 wards and outwards, converging to a broad flat 

 tendon, which is inserted into the deltoid ridge 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. Turn the two halves 



of the muscle aside, so as clearly to 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. 



