MAMMALS. 



369 



The liver, often lobed in a complicated manner, is divided 

 into right and left halves by the ligamentum teres, itself a ves- 

 tige of the earlier umbilical vein. The left half is frequently 

 sub-divided into left and central lobes, while the right also is 

 usually sub-divided, and may have a caudate lobe laterally placed, 

 while a spigelian lobe projects dorsal to the entrance of the 

 portal vein. A gall bladder, which arises as a diverticulum 

 of the hepatic duct, is rarely absent (horse, whales, and some 

 rodents). The pancreas is usually compact, but in some rodents 

 it is diffuse. Its duct, as a rule, unites with the hepatic duct ; 

 but occasionally these may 

 empty into the duodenum 

 at points widely remote 

 from each other. 



As in the birds, the 

 heart is four-chambered ; 

 the divisions occasionally 

 are visible from the outside 

 as in the dugong. Its ma- 

 jor axis is horizontal except 

 in the anthropoids and man. 

 The arch of the aorta bends 

 to the left, a condition ref- 

 erable back to the fact that 

 it is the persistent (fourth) 

 primitive arch of the left 

 side. From the proximal 

 portion of the aorta there are given off, first, the coronary 

 arteries (usually paired), which go to the walls of the heart, and 

 then the subclavians and carotids, the arrangement of which 

 shows many variations ; the most usual condition being first a 

 right arteria anonyma, dividing later into right subclavian and 

 the two carotids, and then the left subclavian. Other arrange- 

 ments can be seen from the diagram. In all cases the right 

 subclavian is in part the persistent right fourth arch of the 

 embryo. The internal carotids enter the cavity of the brain 

 either through the periotic (petrosal) bone, or between it and 

 the base of the skull. 



FIG. 352. Heart of dugong, after Macal- 

 lister, showing the double character of the 

 ventricles, l>, e ; a, d, auricles ; <", pulmonary 

 aorta ; f, systemic aorta. 



