11 



Mammals, like Birds, have a heart composed of two ven- 

 tricles and two auricles, and have warm blood : they breathe 

 quickly ; but inspiration is performed chiefly by the agency 

 of the diaphragm ; and the inspired air acts only on the capil- 

 laries of the pulmonary circulation. 



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

 the Camel-tribe, are circular in form. The right auriculo- 

 ventricular valve is membranous, and the aorta bends over the 

 left bronchial tube. 



The kidneys are relatively smaller and present a more 

 compact figure than in the other vertebrate classes; their 

 parenchyma is divided into a cortical and medullary portion, 

 and the secreting tubuli terminate in a dilatation of the excre- 

 tory duct, called the pelvis : they derive the material of their 

 secretion from the arterial system. Their veins are simple, 

 commencing by minute capillaries in the parenchyma and 

 terminating generally by a single trunk on each side in the 

 abdominal vena cava: they never anastomose with the mesen- 

 teric veins. 



The liver is generally 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, when it exists, always joins the hepatic, and 

 does not enter the duodenum separately. The pancreatic 

 duct is commonly single. 



The mouth is closed by soft flexible muscular lips: the 

 upper jaw is composed of palatine, maxillary and premaxillary 

 bones, and is fixed ; the lower jaw consists of two side-halves, 

 or rami, which are simple or formed by one bony piece, and 

 are articulated by a convex (fig. 3, b) or flat condyle to the 

 base of the zygomatic process, and not to the tympanic ele- 

 ment, of the temporal bone ; the base of the coronoid process 

 (ib. c) generally extends along the space between the condy- 

 loid and the alveolar processes. The jaws of Mammals with 

 few exceptions are provided with teeth, which are arranged 

 in a single row ; they are always lodged in sockets, and never 



