604 TEXTBOOK OF ZOOLOGY 



rings which in different kinds of birds will vary from 90 to 120, 

 depending on the length of the neck. This inferior larynx is the 

 true voice box in the bird because it is here that the vocal apparatus 

 is located. The two semi-lunar internal tympanic memhranes, one 

 as a fold on each side of the cavity, are caused to vibrate by the air 

 in sound production. In song birds there is a membrane with a 

 second glottis across the lower end of the trachea. The bony bar 

 supporting it is called the pessulus. 



The air goes next into two hronchi which enter the tissue of the 

 lungs. Within the lung each primary bronchus divides into secondary 

 bronchi and beyond this each of these gives off smaller tertiary bron- 

 chial branches. The further branches are considered a part of the 

 lung. The lungs occupy only about one-seventh of the thoracic space. 

 Bronchioles which are small branches of the tubules carry the air into 

 the air spaces or alveoli of the lungs where most of the respiratory 

 exchange of gases is made with the blood. Birds have, some bladder- 

 like extensions of the bronchial tubes in the form of air sacs which 

 increase the respiratory surface as well as making the body more 

 buoyant in the air or on water. These sacs are arranged in the 

 neck, thorax, and abdomen, and extend into the cavities of bones. 

 The principal ones are : a single anterior thoracic, a pair of cervical 

 sacs along the neck, a pair of posterior diaphragmatic sacs behind 

 the diaphragm, a pair of anterior diaphragmatic sacs, and a pair 

 of abdominal sacs. 



Circulatory System 



This system, as a whole, includes the blood vascular system and 

 the lymphatic system. The first consists of heart, arteries, capil- 

 laries, and veins, making a closed system through the body, while 

 the latter is composed of spaces, vessels, and capillaries which empty 

 into the blood vascular veins near the heart. The lymphatic system 

 is somewhat of an open system. 



The heart of the chicken and most other birds is relatively large 

 and is located near the median line of the thoracic cavity. The 

 double-walled membranous sac, the pericardium, surrounds and holds 

 the heart in place. There are two distinct, thin-walled auricles, and 

 two distinct muscular ventricles. Blood is drawn into the right 

 auricle from the systemic veins ; right and left precava, and the 

 single postcava. This blood needs aeration and passes through the 



