The Respiratory and Vascular Systems. 43 



The chief features of the pharynx depend on its relation as a common 

 or general portion of the digestive tube with the tubes of the respiratory- 

 system. It is divisible into an oral portion, representing the direct 

 connection of the oral cavity with the oesophagus, a dorsal or nasal 

 portion, connected with the nasal fossae, and a ventral or laryngeal 

 portion, containing the aperture of the larynx. 



THE RESPIRATORY SYSTEM. 



In all air-breathing vertebrates the lungs arise embryonically as 

 ventral outgrowths of the digestive tube, and are secondarily con- 

 nected with the outside of the body through special perforations of 

 the anterior portion of the head and through the oral cavity. As 

 indicated above, this connection in a mammal is represented by an 

 extensive nasal cavity bearing on its lateral walls the olfactory sense- 

 organs. It is distinguished as an accessory respiratory tract from the 

 true respiratory tract formed by the trachea and its terminal divisions, 

 the bronchi. The respiratory system as represented by the lungs and 

 related tubes, is nominally ventral to the oesophagus, but this relation 

 is chiefly true of the trachea. In the thorax (Plate VII) the bronchi are, 

 in general, interposed between the oesophagus and the heart, the lungs 

 being expanded laterally into the paired pleural cavities. 



THE VASCULAR SYSTEM. 



In the rabbit/ as in all vertebrates, the vascular system embraces a 

 central, muscular organ of propulsion, the heart, and a series of branched 

 tubes, the bloodvessels, the latter being of three different kinds : (a) thick- 

 walled, elastic, distributing vessels — arteries; (b) microscopic terminal 

 canals in the peripheral organs — capillaries ; and (c) thin- walled collecting 

 vessels — veins. 



The chief mammalian feature in this system consists in the division 

 of the heart into two portions (Plate VII), each consisting of a receiving 

 chamber, or atrium, and a driving chamber, or ventricle, and the arrange- 

 ment of their vascular connections in such a way that two complete, 

 circulations are established. One of these is the long, or systemic circula- 

 tion. It is concerned with the distribution of blood to the various parts 

 of the body, with the exception of the lungs. It is established by the 

 left ventricle, the aorta, the carotid and subclavian branches of its arch, 

 and the parietal and visceral branches of its thoracic and abdominal 

 portions. The blood is collected from the anterior portions of the body 

 through paired internal and external jugular and subclavian veins, com- 

 municating with the right atrium of the heart through paired superior 

 cavals; from the posterior portions of the body through the unpaired 

 and also asymmetrical inferior caval vein, the latter passing forward on 

 the right of the median plane and entering the posterior end of the right 

 atrium. The second, short, or pulmonary circulation, is concerned with 



