Circulatory System General, 99 



be defined the Lympliatioe of Mammalia are seen to be more 

 numerous, minute, and bigbly organised than in tbe Lower 

 Yertebrata ; and to have tbeir inner tunic folded to form 

 many and efficient valves of the ' semi-lunar ' type, commonly 

 in pairs, rarely single. 



The Heart consists of two auricles and two ventricles ; the 

 auriculo- ventricular chambers are usually quite distinct inter- 

 nally, but the division is seldom indicated externally. The 

 venous and arterial parts of the vascular system have no com- 

 munication beyond the heart, save at the peripheral capillaries. 

 The Heart is invested by a pericardium. In the prone trunk 

 of quadrupeds the pericardium adheres to the sternum, rarely 

 to the diaphragm : from which in many Mammals it is more 

 or less separated by a lobe of the lung. The form of the 

 Heart is in general more rounded than in Man, the Apex is 

 occasionally indented, and, most exceptionally, cleft. In Man 

 the heart is placed obliquely ; in lower Mammalia it is almost 

 always placed more in a line with the axis of the body. The 

 right auricle is less definitely divided into * sinus ' and ' auri- 

 cle ' proper than in Birds. The Foramen ovale, though 

 generally closed in the adult, yet is sometimes found open 

 some time after birth. The Eustachian valve is frequently 

 wanting. The Precavals* (* descending cavae ') sometimes 

 terminate in the auricle by distinct orifices. The right auri- 

 culo- ventricular valve resembles in structure the left, as beings 

 membranous and attached by tendinous threads to muscle ;, 

 it rarely takes on the form of the single muscular valve of 

 the Bird. Occasionally there is found as a normal formation, 

 in the septum ventriculorum, below the origin of the aorta, a 

 cruciform ossification called the bone of the heart. In most 

 other particulars the Heart of other Mammals is the same 

 as that of man. 



. * Owen. 



