2i8 ZOOTOMY. 



164. The interclavicular air-sac, situated at the 

 bottom of the pit bounded at the sides by the furcula and 

 pectoral muscles, in front by the crop and by the carina : 

 it is distinctly bilobed, being formed by the fusion of two 

 sub-bronchial air-sacs, and extending on either side of 

 the gullet : immediately dorsal to each of its lobes is one of 

 the paired prebronchial air-sacs. 



165. A prolongation of the interclavicular air-sac on each 

 side into the corresponding axilla, placed just external 

 to the coracoid, and covered immediately by the pectoralis 

 major. 



166. The subclavius or pectoralis secundus, a 

 broad flat sheet of muscle, arising from the' dorsal portions 

 of the keel and median portions of the body of the sternum : 

 it is a bi-pinnate muscle, that is, is divided longi- 

 tudinally by a tendinous band to which the fibres of either 

 side converge : anteriorly it is seen to pass forwards and 

 outwards between the furcula and the axillary prolongation 

 of the interclavicular air-sac. 



167. The coraco-brachialis longus, a small muscle 

 lying external to the anterior portion of the subclavius, and 

 passing forwards and outwards dorsal to (beneath in the 

 present position) the. axillary portion of the interclavicular 

 air-sac. 



1 68. The pectoral arteries and veins, large vessels 

 distributed to the pectoral muscles, and, at present, pre- 

 venting further reflection of the pectoralis major. 1 



1 The injection of the pigeon's vascular system is best performed in 

 the following way. As soon as the bird, killed with chloroform or 

 potassic cyanide, is dead, pluck the breast, expose the pectoral vessels of 

 one side as directed in Section XXII., cut through these vessels as near as 

 possible to the reflected pectoralis major, and allow to bleed. All this 

 should be done with the greatest rapidity, as birds' blood coagulates 

 very quickly, and it is essential to success to allow as much as possible 



