ORGANS OF CIRCULATION.—The researches of the anatomist on the circulating system of the Cetaceans have not hitherto been extended to many species. In its essential parts it is similar to that in other Mammalia. But the peculiar nature of Cetaceans, and the great modifications of their organs o,f movement, have necessarily produced in this system, not only modifications analogous to those of these organs, but vascular developments exclusively characteristic of these animals.
It is not known whether the Manatee pre sents anything particular in regard to the organs of circulation, but the heart of the Dugong (fig. 264) and of the Rytina is cloven by the deep separation of the two ventricles, a cir cumstance which adds an important link of affinity to those already subsisting between these animals.
IN the heart of the Dugong, the ventricles, as Sir Stamford R,afiles has correctly described them,* are not completely detached from one another. The auricles are of equal size and of a rounded form. In the right auricle (a), which receives a single superior cava, the coronary vein, and the inferior cava, there is on the auricular side of the orifice of the latter vein a fleshy Eustachian valve, of the size and form which, in such cases, is com monly seen in the human subject. The valve of the foramen ovale has a reticulate surface at the upper margin, but is entire and im perforate. The right ventricle (b), in the Dn.
gong previously mentioned, which was six feet in length, was three inches and a half long and three inches broad at the base ; the thick ness 'of its parietes one line and a half; the carnem columnm are few, and resemble those in man. The tricuspid and mitral valves are of the usual form and structure, but the latter are broader than in man, measurina each one inch three lines across the base. The diameter of the orifice of the pulmonary artery (c) is one inch and a half. The capacity of this vessel is very great, according with the impediments to the transmission of blood through the lungs which must arise from the submarine habits of this animal. In the left auricle (d) the trans verse pectinated muscular bands are equally if not more developed than in the right. The trace of the foramen ovale is more evident on this side the septum auriculare than in the right auricle ; it appeared as an oblique slit directed upwards, about three lines broad, but was com pletely closed.
The parietes of the left ventricle ( e) are half an inch in thickness ; there is nothing unusual in the mitral valve or the curlew columnw connected with it ; the inner surface of the ventricle was as usual smooth below the origin of the aorta (j). The breadth of the semilunar valves here was ten lines, the dia meter of the orifice being one-third less than that of the pulmonary artery. The ductus ar teriosus was completely obliterated.] The heart in the Dolphins and Whales does not appear to have undergone any remarkable modifications; but their arterial system pre sents a very important one in the infinite circumvolutions of arteries, and the vast ple xuses of vessels, filled with oxygenated blood, which are found particularly under the pleum and between the ribs, on each side of the spine.
[Of this remarkable struCture, which was discovered by Hunter, we here subjoin the original description.
" The general structure of the arteries re sembles that of other animals; and where parts are nearly similar, the distribution is likewise similar. The aorta forms its usual curve, and sends off the carotid and subclavian ar teries.
" Animals of this (the Whale) tribe, as has been observed. have °Tenter iironnrtinn nf mood than any other knovvn, and there are many arteries apparently intended as reservoirs, where a larger quantity of arterial blood seemed to be required in a part, and vascularity could not be the only object. Thus we find, that the intercostal arteries divide into a vast number of branches, which run in a serpentine course between the pleura, ribs, and their muscles, making a thick substance somewhat similar to that formed by the spermatic artery in the Bull. Those vessels, every where lining the sides of the thorax, pass in between the ribs near their articulation, and also behind the ligamentous attachment of the ribs, and anastomose with each other. The medulla spinalis is surrounded with a net-work of arteries in the same man ner, more especially where it comes out from the brain, where a thick substance is formed by their ramifications and convolutions ; and these vessels most probably anastomose with those of the thorax. .