86 I'liLI.TKV DISEASES AND TlllJK TKF.AT.MKNT. 



as in nianimals but each passes to the posterior end of its hing 

 and where it opens into the abdominal air sac. This relation is 

 shown in fig. 13. 



Til the liuij;- the primary bronchus gives off branches (sec- 

 ondar\- bn>nchij some of which end blindly (cf. fig. 13). The 

 secontlary branches give off branches (tertiary bronchi) all of 

 which end blindly (cf. fig. 14). This tubular system makes up 

 the air-containing portion of the lungs. It is imbedded in a 

 net-work of almost naked blood vessels which make up the 

 spongy tissue of the lungs (cf. fig. 14). The aeration of the 

 blood takes place through the w^alls of these vessels. The rela- 

 tions of this tubular system of the lungs are shown in fig. 14. 



The air sacs arc very large, thin-walled sacs which open into 

 the bronchial tubes as described. They function chiefly as res- 

 ervoirs of air so that fresh air is supplied to the lungs twace dur- 

 ing each breath. The air passes through the lungs into them 

 during inspiration and during expiration the lungs are filled 

 with the air forced back from the sacs. Some aeration of blood 

 takes place in the sacs and they also help to reduce the relative 

 weight of the body. In addition to aeration of the blood, the 

 respiratory apparatus eliminates most of the waste moisture of 

 the body and is. therefore, the temperature regulator. In mam- 

 mals this function is performed by the sweat glands and the se- 

 cretion of the kidneys. Birds have no sweat glands and the 

 secretion of the kidneys contains relatively little moisture. 



The air passages are lined with mucous membrane and this 

 membrane is the seat of several diseases. Diseases are easily 

 transferred from one part of the respiratory system to another, 

 since the passages and also the lining membranes are contin- 

 uous from the nostrils to the air sacs. There are also diseases 

 of the vascular part of the lungs. Some of these diseases are 

 caused by unfavorable conditions as exposure to cold, draughts 

 of air, or moist air or to improper food. Others are due to 

 specific organisms. Most of the latter are contagious. Expo- 

 sure to unfavorable conditions also reduces the ability of the 

 birds to resist infectious diseases. 



