Sauropsida: Class Aves 519 



muscle whose tendon passes upward and attaches to the ventral surface 

 of the humerus, effecting the downstroke of the wing. Removal of the 

 pectoralis by dissection exposes to view a comparatively small and 

 thin muscle (subclavius or pectoralis minor) whose tendon goes to 

 the shoulder joint and, there passing through a smoothly lined aperture 

 (foramen triosseum), makes a turn of about 180 degrees and at- 

 taches to the dorsal surface of the humerus. Mechanically, the arrange- 

 ment is equivalent to a pulley. When the subclavius contracts, the 

 direct downward pull is transformed by the curving tendon into an 

 upward pull on the wing. This is an extraordinary feature of the bird. 

 In other vertebrates the pectoral limb is raised by direct upward pull 

 of muscles which are situated on the animal's back. 



Aside from the muscles of the wing and leg, the bird has no heavy 

 muscles. Movements of the neck are effected by a complex system of 

 small muscles attached to the cervical vertebrae. In the trunk, loss of 

 mobility as a result of fusion of vertebrae is attended by great reduc- 

 tion in the dorsal muscles. The more prominent muscles which remain 

 in the trunk are those concerned with the breathing movements of 

 the thoracic wall and the usual thin muscular layers of the abdominal 

 wall. 



Respiratory System 



Birds, of all vertebrates, possess the most complex and most highly 

 efficient respiratory system, and it is unique in its mechanism and 

 mode of operation. Its efficiency is due to the following factors: the 

 lungs, although not of large volume relative to the body, are minutely 

 subdivided internally so that the respiratory surface is enormously 

 increased; the ultimate subdivisions are so arranged that there are no 

 "dead ends" in which the air cannot be wholly changed; capacious air 

 sacs annexed to the lungs provide for complete change of air through- 

 out the lungs at both inhalation and exhalation. 



The trachea divides, at the base of the neck, into a right and a 

 left bronchus. Each bronchus enters the anterior end of its lung and 

 passes back through the central region of it (mesobronchus), giving 

 off several large branches, the secondary bronchi (Fig. 407). These 

 are connected with one another by numerous tubes (parabronchi) 

 which are smaller and of uniform diameter (Fig. 408). Adjacent para- 

 bronchi are connected by very minute tubes, air-capillaries. The 

 interstices among the air-capillaries are occupied by a close network of 

 blood-capillaries. Thus blood and air are brought very close together. 

 The bird's lung, as seen in dissection, is bright red because of the 

 abundance of blood in it, and its tissue has a minutely spongy texture. 



Of the numerous air-sacs connected with the lungs, there are five 

 pairs which are fairly constant in position and occurrence (Figs. 409, 



