542 ZOOLOGY SECT. 



umbilical vein. Usually each of these main divisions is divided 

 by a fissure into two parts, so that right lateral (rl.) and ri-ght 

 central (re.), and left lateral (II.) and left central (Ic.) lobes are 

 distinguishable. When a gall-bladder is present, as is the case 

 in the majority of Mammals, it is attached to, or embedded in, 

 the right central lobe. A fissure, the portal, through which 

 the portal vein and hepatic artery pass into the substance of 

 the liver, and the hepatic vein passes out, crosses the right cen- 

 tral lobe near the anterior border. The post-caval lies in contact 

 with, or embedded in, the right lateral lobe near its anterior 

 border, and, given off from this lobe between the post-caval 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 post- 

 caval 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 Rodents. 



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 CamelidaB, 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 

 ( /"deals), which ramify in the wall of the 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 recepta- 

 culum chyli from which a tube, the thoracic duct, which may 

 be double, runs forwards to open into the base of one of the great 

 veins of the pre-caval system by a valvular aperture. 



The general statements which have been given with regard to 

 the heart of the Rabbit (p. 433) 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 post-caval 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 

 tendineaB and musculi papillares. In all, the openings of the 

 pulmonary artery and aorta are provided with three-lobed semi- 

 lunar valves. 



