CHARACTERS OF MAMMALIA. 



267 



140 



The blood-discs are smaller than in Reptiles, and, save in the 

 Camel-tribe, are circular. The right auriculo-ventricular valve 

 is membranous, at least never entirely fleshy ; and the aorta bends 

 over the left bronchial tube. The abdominal aorta terminates by 

 dividing beyond the kidneys into the iliac arteries, from which 

 spring both the femoral and ischiadic branches : if continued 

 beyond, it is as a caudal or sacro-median artery. 



The kidneys, ib. k, are relatively smaller and present a more 

 compact figure than in the other Vertebrate classes ; their paren- 

 chyme is divided into a cortical and medullary portion, and the 

 secreting tubuli terminate in a dilatation of the excretory duct, 

 called the pelvis. They derive their secretion exclusively from 

 the arterial system. Their veins, commencing by minute capil- 

 laries in the renal parenchyme, terminate generally by a single 

 trunk on each side in the 

 abdominal vena cava : they 

 never anastomose with the 

 intestinal veins. 



The liver, ib. /, is gene- 

 rally divided into a greater 

 number of lobes than in 

 Birds. The portal system 

 is formed by veins derived 

 exclusively from the spleen 

 and chylopoietic viscera. 

 The cystic duct, wdien it 

 exists, always joins the he- 

 patic, and does not enter 

 the duodenum separately. 

 The pancreatic duct is com- 

 monly single. 



The mouth is closed by 

 soft flexible muscular lips : 

 the upper jaw is composed 



of palatine, fig. 140, 20, maxillary, 21, and premaxillary, 22, bones, 

 and is fixed ; the lower jaw consists of two rami, formed each by 

 one bony piece, ib. 30-32, and articulated by a convex or flat con- 

 dyle, ib. 29, to the squamosal, ib. 27, not to the tympanic, ib. 28. 



The jaws of Mammals, with few exceptions, are provided with 

 teeth. These are limited to the premaxillary, maxillary, and 

 mandibular bones, and are there arranged in a single row ; they 

 are lodged in sockets, not anchyloscd with the substance of the 

 jaw. Only in the present class are teeth implanted by two or more 



Skull of Mammal, Deer. 



