CLASS MAMMALIA. MAMMALS 



469 



the submaxillaries. They pour watery and 

 mucous secretions into the mouth cavity to 

 moisten and lubricate the food. 



The posterior continuation of the mouth 

 cavity is called the pharynx. In the floor of 

 the pharynx is the respiratory opening, the 

 glottis, which is covered by a bilobed car- 

 tilaginous flap, the epiglottis, during the act 

 of swallowing. The pharynx leads downward 

 into the narrow, muscular esophagus. Fol- 

 lowing this is the stomach, behind the 

 diaphragm; then comes the U-shaped duo- 

 denum, into which open the pancreatic duct 

 from the pancreas and the bile duct from 

 the liver. 



The small intestine, which is several feet 

 in length, leads into the large intestine 

 (colon), which is continued as the rectum. 

 At the junction of the small and large in- 

 testines, a short blind sac, the cecum, is 

 given off. The cecum ends in a conelike sac, 

 but there is no appendix as in man. 



Circulatory system 



The blood corpuscles of the cat are unlike 

 those of the lower vertebrates, but charac- 

 teristic of those in mammals; they are 

 smaller and round,* instead of oval, bicon- 

 cave, and without nuclei. The heart (Fig. 

 342) is completely 4-chambered, as in the 

 pigeon, but the main blood vessel, the aorta, 

 arising from the left ventricle, has only the 

 left arch, whereas in birds the right arch 

 persists. The right systemic arch of the cat 

 is represented by the innominate artery, 

 which is the common trunk of the right 

 carotid and subclavian arteries. A hepatic 

 portal system is present, but no renal portal 

 system. The elongate spleen, a dark reddish 

 organ, is on the left side behind the stomach. 



The lymphatic system is important in the 

 cat and other mammals. The fluid portion 

 of the blood, because of the blood pressure, 

 escapes through the walls of the capillaries, 



* There is only one exception — in the camels 

 they are oval in shape. 



into the spaces among the tissues, and is 

 collected into lymph vessels. These vessels 

 pass through the lymph glands and finally 

 empty into the large veins in the base of the 

 neck. The lymphatics which collect nutri- 

 ment from the intestine are called lacteals. 



Respiratory system 



The cat and all other mammals breathe 

 air by means of lungs (Fig. 341 ) . The glottis 

 opens into the larynx, from which a tube 

 called the trachea or windpipe arises. The 

 trachea is held open by incomplete rings of 

 cartilage; it divides into two bronchi, one 

 bronchus going to each lung. These distrib- 

 ute air through smaller and smaller branches 

 that end in microscopic air sacs (alveoli) 

 of the lungs. The alveoli are surrounded by 

 a capillary network of blood vessels in which 

 the oxygen and carbon dioxide exchange of 

 gases takes place. The larynx is supported 

 by a number of cartilages and across its 

 cavity extend two elastic cords called the 

 vocal cords, with which the cat's squalls are 

 produced. The lungs are conical in shape 

 and lie freely in the thoracic cavity sus- 

 pended by the bronchi. 



Air is drawn into the lungs by the en- 

 largement of the thoracic cavity. This is ac- 

 complished both by pulling the ribs forward 

 and thus separating them, as in most rep- 

 tiles, and by means of the diaphragm. The 

 diaphragm is normally arched forward; 

 when it contracts, it flattens, thus enlarging 

 the thoracic cavity. The increased size of 

 this cavity results in a slight negative pres- 

 sure around the lungs; the lungs conse- 

 quently expand and air rushes in (inspira- 

 tion). Air is forced out of the lungs (expira- 

 tion ) by the elastic contraction of the lungs 

 when the diaphragm and rib muscles relax. 



Excretory system 



The urine excreted by the two kidneys is 

 carried by two slender tubes, the ureters, 

 into a thin-walled muscular sac, the distensi- 



