128 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



lung-chamber, in close proximity to the kidney, and contained in 

 a pericardial cavity bounded by a firm translucent pericardium. 

 It is oval in form, and made up of an anterior thin-walled auricle, 

 which communicates by a valve with a posterior ventricle. 



Arteries. The ventricle is continued into a large artery, the 

 aorta, which almost immediately gives off an important visceral 

 artery to the visceral hump, then runs forwards, supplying the 

 body-wall, muscles, viscera, &c., finally perforating the ventral 

 nerve-mass, and breaking up into branches for the head. 



Venous System. The smallest arteries form networks (? capil- 

 laries), from which the blood passes into minute spaces (lacunae), 

 which communicate on the other hand with large venous sinuses. 

 Of these the most important are the spacious body-cavity sur- 

 rounding those viscera which are not contained in the visceral 

 hump, two lateral sinuses in the foot, a visceral sinus along the 

 inner edge of the coiled visceral hump, and a pulmonary sinus 

 with which this communicates, running round the floor of the 

 lung-chamber. 



From the pulmonary sinus numerous a/erent pulmonary vessels 

 are given off, which branch in the roof of the lung, and from 

 these branches efferent pulmonary vessels arise, which unite together 

 to form the pulmoruary vein opening into the auricle. A number 

 of the afferent trunks enter the kidney, and form a network 

 within it, from which one large and several smaller renal veins run 

 to the pulmonary vein. 



Course of the Circulation. Blood, oxygenated in the lung, and 

 (part of it) purified in the kidney, enters the auricle during its 

 diastole by the pulmonary vein. It then passes into the ventricle 

 during the auricular systole. The ventricle, owing to its muscular 

 walls, contracts more vigorously, and the blood, prevented by the 

 auriculo-ventricular valve from returning to the auricle, is forced 

 into the arteries. From the fine ramifications of these it passes 

 into the lacunae, and thence into the venous sinuses, ultimately 

 reaching the pulmonary sinus. From this the blood, now carbon- 

 ated and loaded with nitrogenous waste, passes by the afferent 

 pulmonary vessels into the roof of the lung. Here it is oxygen- 

 ated, and passes into the efferent pulmonary vessels from which 

 the pulmonary vein arises. Part of the blood traverses the 

 kidney before entering the pulmonary vein, and thus gets rid of 

 nitrogenous waste, some of which has been previously eliminated 



