xiii PHYLUM CHORDATA 557 



passes out, crosses the right central lobe near the anterior border. 

 The postcaval lies in contact with, or embedded in, the right 

 lateral lobe near its anterior border, and given off from this lobe, 

 between the postcaval and the portal fissure, is a small lobe, of 

 varying extent the Spigelian. The term caudate lobe is applied 

 to a process of the right lateral lobe, of considerable extent in most 

 Mammals, having the postcaval vein in intimate relation to it, 

 and often closely applied to the kidney. A gall-bladder is usually 

 present, but is absent in the Cetacea, the Perissodactyle Ungulata, 

 the Hyracoidea, and some Kodents. 



Vascular System. The blood of Mammals is warm, having a 

 temperature always of from 35 to 40 C. The red corpuscles are 

 non-nucleated : in form they are most usually biconcave discs, 

 always circular in outline, except in the Camelidae, in which most 

 of them are elliptical. The lymphatic system of vessels is very 

 highly developed, ramifying richly throughout all parts of the 

 body. In the course of this system occur numerous lymphatic 

 glands. The special part of the lymphatic system of vessels (lacteals) 

 which ramify in the wall of trie intestine and absorb the fatty 

 matters of the food, combine with the lymphatic vessels from the 

 hind-limbs and body to form a receptacle the receptaculum chyli 

 from which a tube, the thoracic duct, which may be double, runs 

 forward to open into the base of one of the great veins of the 

 precaval system by a valvular aperture. 



The general statements which have been given with regard to 

 the heart of the Rabbit (p. 448) hold good for the Mammalia in 

 general. The sinus venosus is never distinct from the right auricle ; 

 of its valves, which are more completely retained in the Edentata 

 than in the other orders, the right gives rise to the Eustachian 

 valve a membranous fold, often fenestrated in the adult, extending 

 from the right wall of the postcaval to the edge of the foramen 

 ovale (annulus ovalis) ; while the left becomes merged in the 

 auricular septum, helping to complete the annulus ovalis behind. 

 Each auricle has an auricular appendix. The right auriculo- 

 ventricular aperture has a three-lobed tricuspid valve, and the left 

 a two-lobed bicuspid, or mitral, with chordae tendineae and musculi 

 papillares. In all Mammals the openings of the pulmonary artery 

 and aorta are provided with three-lobed semilunar valves. 



The single aortic arch, situated in all Mammals on the left side, 

 varies greatly in the way in which it gives off the main arterial 

 trunks. Sometimes a single large trunk passes forward from the 

 arch of the aorta and gives rise to both carotids and both sub- 

 clavians. Sometimes there are two main trunks right and left 

 innominate arteries each giving rise to the carotid and subclavian 

 of its own side. Sometimes there is a right innominate giving 

 off right carotid and right subclavian, the left carotid and left 

 subclavian coming off separately from the arch of the aorta ; or, 



VOL. II MM 



