520 THE RABBIT CHAP. 



soon as the pleural cavities are perforated, air pasess in and 

 equalises the pressure : the elasticity of the lungs then 

 comes into play, causing them to collapse. When the 

 muscles of the diaphragm contract (p. 507), air is drawn 

 into the lungs, and this process is aided by the external 

 intercostal muscles (p. 506) and, in forced respiration, by 

 other muscles of the body-wall also. The mechanism of 

 respiration may therefore be compared with a suction-pump, 

 while that in the frog resembles a force-pump (p. 142). 



On either side of the larynx is a soft, vascular, gland-like thyroid body, 

 consisting of two lateral portions connected ventrally by a median 

 bridge. Its function is not thoroughly understood : morphologically it 

 represents a gland developed from the pharynx, but it loses its con- 

 nection with the latter and thus has no duct. The glandular vesicles of 

 which it is composed give rise to an albuminous substance containing 

 iodine, which is passed into the blood and lymph ; if extirpated in 

 the living animal, various functional disturbances result. We are also 

 ignorant of the function of the thymus (p. 507), which is largest in 

 young animals, becoming reduced in size in adults (compare p. 447). 



Organs of Circulation. The heart, as in all Vertebrates, 

 is enclosed in a pericardium consisting of parietal and 

 visceral layers (Fig. 138), between which is a serous peri- 

 cardial fluid. There is a complete separation between the 

 arterial and venous blood in the heart, for in addition to 

 an auricular septum, as in the frog (p. 88), the ventricular 

 portion is divided into right and left chambers by a partition 

 (Fig. 138), the arterial blood from the lungs entering the 

 left auricle and thence passing into the left ventricle to be 

 pumped into the aorta, and the venous blood entering the 

 right auricle and thence into the right ventricle to pass to 

 the lungs through the pulmonary artery. A distinct conus 

 arteriosus and sinus venosus (pp. 79 and 449) can no longer 

 be recognised, the former having become practically absorbed 



