514 SANGUTFEROUS SYSTEM. 



The internal iliacs alone arise from the aorta, in the cetacea, 

 there being no legs to necessitate external iliacs, and the 

 epigastrics take their origin from the internal iliacs. The 

 prolonged aorta, or caudal artery, of these animals, continues 

 its trunk, rapidly diminishing, under the coccygeal vertebrae 

 to beyond the first half of the tail, where it is at length 

 entirely lost in the numerous plexiform branches, which it 

 gives off in its whole course backwards from the abdomen. 

 The trunk of the artery is thus far continued amidst its 

 plexiform branches, as in the arms of the bradypus ; but in 

 the last portion of the tail, the artery is entirely reduced to 

 plexiform branches, as in the extremities of the loris, the 

 tarsius, and the myrmecophagee. This plexiform condi- 

 tion is seen also in the caudal artery of the ant-eaters, and 

 in the internal iliac arteries of the three-toed sloth. In the 

 marsupial quadrupeds, the great development of the external 

 iliac and the caudal arteries, and especially of the epigastrics 

 which supply the mammae and the pouch, and the small size 

 of the internal iliacs, and their uterine branches, render these 

 animals incapable of maturing a foetus in utero, and necessi- 

 tate an early abortion, as a normal character in that remark- 

 able order of mammalia. 



The capillaries into which the systemic arteries ultimately 

 divide, after constituting plexiform ramifications in the tissue 

 of every organ of the body, and transuding through their 

 parietes the various materials of their nutriment, unite to 

 form the branches of the returning veins. The veins convey 

 the blood to the right side of the heart, from the anterior 

 parts of the body, by one or two superior venae cavae ; from 

 the parts below the diaphragm, by one inferior vena cava ; 

 and from the heart itself, by one or more coronary veins. In 

 the muscular and moving parts, as the extremities and the 

 lungs, the return of the blood is aided by the development of 

 numerous small semilunar valves in the interior of the veins, 

 which are not required in those of the more tranquil internal 

 viscera, as the brain and the liver. The venous blood col- 

 lected from the brain and cranial cavity, is transmitted from 

 the sinuses of the dura mater, through the posterior foramen 

 lacerum on each side, into the two internal jugular veins, 

 \vhich, in descending along the sides of the neck, receive 

 also the lingual, the pharyngeal* the occipital, the facial, and 

 the superior thyroid veins, before they terminate in the great 

 trunks of the subclavian veins (142. A. a. b.) The external 



