512 MANUAL OF ZOOLOGY sect. 



are given off from the aorta just beyond the semi-lunar 

 valves. The corresponding vein opens into the terminal 

 part of the left pre-caval. The pulmonary artery divides 

 into two, a right and a left, each going to the corresponding 

 lung. 



The aorta gives origin to a system of arterial trunks by 

 which the arterial blood is conveyed throughout the body. 

 It first runs forwards from the base of the left ventricle, 

 then bends round the left bronchus, forming the arch of the 

 aorta (Fig. 307, a), to run backwards through the thorax 

 and abdomen, in close contact with the spinal column, as 

 the dorsal aorta (d. ao) . 



The system of caval veins which open into the right 

 auricle consists of the right and left pre-cavals and of the 

 single post-caval. From the liver the blood is carried to the 

 post-caval by the hepatic veins. 



The hepatic portal system consists, as in other vertebrates, 

 of a series of veins conveying blood from the various parts 

 of the alimentary canal to the liver, the trunks of the system 

 uniting to form the single large portal vein (Fig. T,oj,p.v). 

 There is no renal portal system. 



Respiratory Organs. — The larynx (Fig. 30S) is a cham- 

 ber with walls supported by cartilage, lying below and 

 somewhat behind the pharynx, with which it communicates 

 through a slit-like aperture. It contains the vocal cords. 

 Leading backwards from the larynx is the trachea or wind- 

 pipe (Fig. 308, //'), a long tube, the wall of which is sup- 

 ported by cartilaginous rings which are incomplete dorsally. 

 The trachea enters the cavity of the thorax and there divides 

 into the two bronchi, one passing to the root of each lung. 



The lungs (Fig. 304) are enclosed in the lateral parts of 

 the cavity of the thorax. Each lung lies in a cavity lined 

 by a membrane — the cavity of the pleural sac or pleural 



