A HISTORY OF VERTEBRATES: BIRDS AND MAMMALS 



475 



on the ventral surface of the humerus, is responsible for the powerful 

 downstroke of the wings. In some species this muscle accounts for one- 

 fifth of the body weight. One might expect that dorsally placed muscles 

 would be responsible for the recovery stroke, but instead, another ventral 

 muscle, the supracoracoideus (pectoralis minor of some authors), is re- 

 sponsible for the upstroke by virtue of a peculiar pulley-like arrange- 

 ment of its tendon of insertion. The origin of the supracoracoideus is 

 on the sternum dorsal to the pectoralis. Its tendon passes through a 

 canal in the pectoral girdle near the shoulder joint and inserts on the 

 dorsal surface of the humerus. Muscles within the wing are responsible 

 for its folding and unfolding and the regulation of its shape and angles 

 during flight. Other muscles attach to the follicles of the large flight 

 feathers of the wings and tail and control their positions. 



Major Features of the Visceral Organs. Less obvious but no less 

 important adaptations for increased activity and flight are present in 

 many of the internal organs. Increased activity and a high metabolic 

 rate necessitate a large intake of food. The digestive system (Fig. 24.5) 

 is compact, but it is so effective that, in some of the smaller birds, an 

 amount of food equivalent to 30 per cent of the body weight can be 

 processed each day! Moreover, most of the food that is selected has a 

 high caloric value. Birds eat a variety of insects and other animals and 

 such plant food as fruit and seeds. They do not attempt to eat such 

 bulky, low caloric foods as leaves and grass. Food taken into the mouth 

 is mixed with a lubricating saliva and passes through the pharynx and 

 down the esophagus without further treatment, for birds have no teeth. 

 In grain-eating species, such as the pigeon, the lower end of the esoph- 

 agus is modified to form a crop in which the seeds are temporarily stored 



Eustachian, tube-- 



Uretcr 

 Uropygca-l 

 bland 



^ — Esophagus 



External na.ris 



Tongixe 

 Larynx 



Main bronchas 



Lar^e 

 intestine 

 Pancreas 

 Duodtnum 



Pectoral mu.scles 



Bile ducts- 

 Figure 24.5. A lateral dissection of a pigeon to show the major visceral organs. 



