MAMMALIA 649 



The heart has four principal chambers, as it does in the chicken. 

 It is fairly large and compact. The pericardial sac holds it in posi- 

 tion in the body. There is only one systemic arch leading from the 

 heart, and it turns to the left. The anterior part of the body is sup- 

 plied largely by the branches of the innominate artery which repre- 

 sents the right aortic arch (Fig. 350). The two preeaval veins with 

 their branches collect blood from, the anterior, and the single postcaval 

 vein and its branches from the posterior, parts of the body. They 

 bring it to the right auricle of the heart (Fig. 351). The hepatic portal 

 system is present, serving to collect blood from the visceral organs and 

 deliver it to the liver. There is no renal portal system. The blood is 

 composed of the fluid plasma, white corpuscles, and red corpuscles. 

 The latter are small, disc-shaped, biconcave, and without nuclei, as is 

 common among mammals. 



Respiratory System 



In mammals, the special organs of respiration are the lungs, 

 wherein the carbon dioxide is taken from the blood and the oxygen 

 from the air is supplied to it. The nasal passages lead through the 

 nasopharynx to the pharynx and from here through the glottis to the 

 laiynx, thence by trachea to the lungs. The air is warmed as it 

 passes in through the nasal chambers and pharynx. The air passes 

 over the true and false vocal cords of the larynx in the production 

 of sound. The walls of the trachea are prevented from collapsing by 

 about forty-five C-shaped rings. The trachea divides into right and 

 left bronchi just before reaching the lungs. Each lung is completely 

 invested with a sac of delicate, transparent serous membrane, called 

 pleura, and is divided into lobes by deep clefts. The left one is com- 

 posed of two large unequal lobes and one small lobe while the right 

 consists of four unequal lobes. 



Breathing is effected by increasing and decreasing the size of the 

 thoracic cavity. To do this the ribs are moved forward and spread 

 while the diaphragm which usually arches anteriorly is contracted to 

 a flat position. As the chest cavity is thus enlarged, a vacuum is 

 produced, and to balance the pressure, air naturally rushes into the 

 lungs from the outside. When relaxed the wall of the thorax and 

 the diaphragm both return to their original position and force the 

 air out. The intaking of the air is called inspiration and the dis- 

 charge is expiration. 



