32 POULTRY DISEASES 



axillary cell which extends to the humerus, breastbone and 

 ribs. This is the only air sac that is single, the balance exist- 

 ing in pairs. The cervical air sacs lie on the last cervical 

 vertebra and extend into the cervical verte])ra. The infero- 

 posterior thoracic air sac (ventral, phrenic or diaphrag- 

 matic). The supero-posterior thoracic air sacs (dorsal phrenic 

 or diaphragmatic air sacs). These air sacs or cells lie be 

 tween the lungs and the abdominal viscera at the rudimen 

 tary diaphragm and aid in keeping separate the thoracic and 

 abdominal viscera. The abdominal air sacs, which are by far 

 the larger of all the air sacs, extend from the anterior to near 

 the posterior border of the abdominal cavity. Part of the 

 alidominal viscera lies between them. They extend to the 

 pelvis and thigh bones. The cavities of the head receive their 

 air from the nasal passages. 



Thymus Gland 



The thymus gland is present in young birds and is located 

 along each jugular vein in the neck. The parts of this gland 

 are elongated and lobulated. 



Carotid Glands 



These are two round or oblong glands abundantly supplied 

 by blood vessels and lie at the base of the carotid arteries. 



Adrenal Glands 



These small reddish-like bodies lie at the anterior end of the 

 kidneys. 



THE CIRCULATORY SYSTEM 



The heart is located in the thoracic cavity with the base 

 directed in a cranio-dorsal direction. The apex directed 

 downward and backward lies in the anterior portion of the 

 fissure formed by the two lobes of the liver. It is surrounded 

 by a pericardial sac which is often attached by means of con- 

 nective tissue to the adjacent air sac. The structure of the 

 heart is similar to that of mammals, except that the tricuspid 

 valves of the right auriculo-ventricular opening are rei)laced 

 with a strong double nuiscular plate w^hich extends from the 

 outer chamber wall. The papillary muscles are absent in the 

 right chamber. The aorta is given off from the left ventricle 

 and the pulmonary artery from the right ventricle as in mam- 

 mals. The bicuspid valve frequently is provided with three 

 points. The venae cavae (two anterior and one posterior) 

 empty into the right auricle. The pulmonary veins empty 

 through one common opening into the left auricle. 



