A HISTORY Of VERTEBRATES: BIRDS AND MAMMALS 477 



series of air sacs that extend into the abdomen, thorax, and even up into 

 the neck and into many of the bones (Fig. 24.6). Most of the air sacs are 

 connected through recurrent bronchi with the same air capillaries that 

 diverge from the main bronchus. Since the air capillaries are connected 

 to bronchi at each end, it is possible that air flows directly through them 

 rather than ebbing and flowing. The exact path that the air takes, how- 

 ever, is uncertain. Some investigators have postulated that during in- 

 spiration most of the air passes through the main bronchi into the air 

 sacs, especially the posterior sacs, and that during expiration it returns 

 to the lungs through the recurrent bronchi and then passes through the 

 air capillaries to the main bronchi. Whatever the path taken, the absence 

 of blind passages would indicate that little stale air is held in the lung. 

 The composition of the air in the air capillaries must be very similar 

 to the external air. 



The How of air through the lungs is brought about by the contrac- 

 tion of muscles in the thoracic and abdominal walls, probably aided by 

 the action of the wings during liight. The thoracic and abdominal cavi- 

 ties and their contained air sacs are alternately expanded and contracted, 

 but the lungs themselves are relatively inelastic and do not change 

 greatly in volume. 



A mechanism for the production of sounds is associated with the 

 air passages. Membranes are set vibrating by the movement of air; how- 

 ever, the vibratory membranes are not in the larynx at the anterior end 

 of the trachea, but in a syrinx at its posterior end (Fig. 24.6). Muscles 

 associated with the syrinx vary the pitch of the notes. 



The bird heart is completely divided internally. Venous blood re- 

 turning from the body and going to the lungs passes through a right 

 atrium and right ventricle as it does in mammals (Fig. 27.3), whereas 

 the arterial blood returning from the lungs en route to the body flows 

 through the left atrium and left ventricle. The sinus venosus of primi- 

 tive vertebrate hearts has been absorbed into the right atrium, and the 

 conus arteriosus contributes to the arterial trunks leading to the lungs 

 and body. An interesting feature of the vessels supplying the body is the 

 unusually large size of those going to the powerful flight muscles. The 

 complete separation of venous and arterial blood within the heart, 

 the rapid heart beat (400 to 500 times per minute in a small bird such 

 as a sparrow when it is at rest), and an increase in blood pressure make 

 for a very rapid and efficient circulation. This is of the utmost im- 

 portance in a homoiotherm, for the tissues need a large supply of food 

 and oxygen, and waste products of metabolism must be removed quickly. 

 Nitrogenous wastes are removed from the blood by a pair of kid- 

 neys, drained by ureters, and basically similar to those of reptiles (Fig. 

 24.5)' A large volume of excretory products can be removed, chiefly as 

 uric acid which can be eliminated with little loss of water. Birds have 

 lost the urinary bladder and the right ovary and oviduct of more primi- 

 tive tetrapods, possibly as one means of reducing body weight. 



The sense of smell is less important in vertebrates that spend a 

 considerable part of their life off the ground than it is in terrestrial 

 species, so it is not surprising to find that the olfactory organ and olfac- 

 tory portions of the brain are reduced in birds. Sight, on the other hand. 



