SANGUIFEROUS SYSTEM. 511 



and at length reunite with them to compose the usual trunks 

 which give off the arteries of the fore-arm and leg. This plexi- 

 form structure exists also in the extremities and tail of the 

 ant-eaters, in the long legs of the tarsius, in the arms of the 

 porpoise and the lamantine, and perhaps in many other mam- 

 malia, and it resembles the rete mirabile of the internal 

 carotids in other tribes, where the plexiform branches however, 

 reunite to compose the trunks of the cerebral arteries. In the 

 two-toed ant-eater, the arterial plexus envelopes the com- 

 mencement of the radial and ulnar arteries, as well as the 

 trunk of the brachial; but in the lamantin, and also in the 

 porpoise, the trunks of these arteries are entirely subdivided 

 into fasciculi of minute vessels, and even the branches which 

 they appear to give off, are only smaller fasciculi of the same 

 minute plexiform arteries. This structure appears to be general 

 in the arms of cetacea, and is probably connected with the 

 limited mobility of these members, as supposed by Bear ; 

 the plexiform condition of the arterial trunks is seen also 

 in the legs of many birds. 



As the parts supplied by the descending aorta are more 

 uniform in their character, than those nourished by the 

 ascending branches, the arteries of the trunk are more con- 

 stant arid less modified than those of the extremities in the 

 different tribes of mammalia, and vary little from the ar- 

 rangement presented by these vessels in the human body. In 

 the thorax, the descending aorta (142. A. q. q.) commonly 

 gives off two or more bronchial arteries, to accompany the 

 ramifications of the bronchi, and nourish the lungs ; several 

 small asophageal branches, to that part of the alimentary 

 canal ; minute posterior mcdiastinal arteries, to the pericar- 

 dium ; and a variable number of symmetrical pairs of inter- 

 cost als (142. A. q. r.), chiefly to the muscular parts of the 

 thorax. In many of the cetacea are observed numerous 

 azygous intercostal arteries, arising from the posterior and 

 median part of the thoracic aorta, which form a compact mass 

 of arterial plexuses, lining the whole dorsal part of the chest, 

 surrounding the vertebral column, penetrating the vertebral 

 canal, enveloping the spinal chord, and even extending to 

 the interior of the cranium. They are enveloped in a loose 

 elastic cellular tissue, accompanied with corresponding plex- 

 uses of veins, and have been supposed connected with the 



