520 THE RABBIT CHAP. 



perforated, air passes in and equalises the pressure I 

 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 connection 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 extir- 

 pated 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 Verte- 

 brates, is enclosed in a pericardium consisting of parietal 

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

 pericardial 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 practi- 



