. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. 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). T
. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. 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 Fig. 352. Heart of dugong, after Macal- lister, showing the double character of the ventricles, b, e; a, d, auricles; c, pulmonary aorta; y, systemic Please note that these images a
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