COMPARATIVE ANATOMY. 



exposure is effected in the parts them- 

 selves by means of the air-vessels, which 

 ramify most minutely over the whole body. 

 ** En un mot, le sang ne pouvant aller 

 chercher 1'air, c'est Pair, qui va chercher 

 le sang." 



The heart of the Crustacea, according to 

 Cuvier, has no auricle ; and it is what he 

 calls an aortic heart. For it expels the 

 blood into the arteries of the body ; and 

 this fluid passes through the gills previ- 

 ously to reaching the heart again. The 

 different parts of the system are here 

 found under a mode of connection exactly 

 the reverse of what we observe in fishes, 

 where the blood is sent into the gills, and 

 passes subsequently into the aorta. The 

 circulating organ in that class is therefore 

 a pulmonary heart. 



According to Cuvier, the cuttle-fish 

 has three hearts, neither of which pos- 

 sesses an auricle. Two of these organs 

 are placed at the root of the two bran- 

 chiae : they receive the blood from the 

 body, (the vena cava dividing into two 

 branches, one for each lateral heart,) 

 and propel it into the branchiae. The 

 returning veins open into the middle 

 heart ; from which the aorta proceeds. 



The other mollusca have a simple 

 heart, consisting of one auricle and ven- 

 tricle. The vena cava assumes the office 

 of an artery, and carries the returning 

 blood to the gills ; whence it passes to 

 the auricle ; and is subsequently ex- 

 pelled into the aorta. Here therefore, 

 as in the Crustacea, the heart is a pulmo- 

 nary one. 



The vermes of Cuvier have circulat- 

 ing vessels, in which contraction and di- 

 latation are perceptible, without any 

 heart. They can be seen very plainly in 

 the lumbricus marinus. The leech, naias, 

 nereis, aphrodite, &c. are further exam- 

 ples of the same structure. This anato- 

 mist is of opinion that the mollusca, crus- 

 lacea, and vermes, possess no absorbing 

 vessels ; and he thinks that the veins ab- 

 sorb, as he finds them to have communi- 

 cation with the general cavity of the bo- 

 dy, particularly in the cuttle-fish Hence 

 the above mentioned classes will hold an 

 intermediate rank between the vertebral 

 animals, which possess both blood-vessels 

 and absorbents, and the insects, which 

 have neither. 



ABSORBING SYSTEM. 



The chyle of birds is transparent : and 

 there are no mesenteric glands in these 

 animals. 



The lacteals are uncommonly numer- 



ous on the intestines and mesentery of 

 the turtle, in which animal there are no 

 absorbent glands. 



The lymphatics of fishes have neither 

 glands nor valves. 



ORGANS OF RESPIRATION. 



The incessant continuation of the great 

 chemical process, by which oxygen is ex- 

 changed for hydrogen and carbon, is es- 

 sentially necessary to the well being of 

 the greater part of animals. Yet the or- 

 gans and mechanism, by which this won- 

 derful function is carried on, vary very 

 considerably. In the mammalia, after 

 birth ; in birds, when they have left the 

 egg ; and in amphibia, when complete- 

 ly formed ; the chief organ of this func- 

 tion is the lungs : in fish it is perform- 

 ed in the gills ; in most insects in their 

 tracheae ; in the vermes, in analogous, 

 but at the same time very different 

 parts. 



The respiratory organs of birds consti- 

 tute one of the most singular structures 

 in the animal economy, on account of se- 

 veral peculiarities which they possess ; 

 but more particularly in consequence of 

 their connection with the numerous air- 

 cells, which are expanded over the whole 

 body. 



The lungs themselves are compara- 

 tively small, flattened, and adhering above 

 to the chest, where they seem to be 

 placed in the intervals of the ribs; they 

 are only covered by the pleura on their 

 under surface, so that they are in fact on 

 the outside of the cavity of the chest, if 

 we consider that cavity as being defined 

 by the pleura : a great part of the tho- 

 rax, as well as the abdomen, is occupied 

 by the membranous air-cells, into which 

 the lungs open by considerable apertures. 

 Those of the thorax are divided, at least 

 in the larger birds, by membranous trans- 

 verse septa, into smaller portions ; each 

 of which, as well as the abdominal cells, 

 has a particular opening of communica- 

 tion with the air-cells of the lungs, and 

 consequently with the trachea. The 

 membranes of these cells, in the larger 

 birds, are provided here and there with 

 considerable fasciculi of muscular fibres, 

 which have been regarded as a substitute 

 for the diaphragm, which is wanting in 

 this class of animals. They also serve 

 very principally, as we may ascertain by 

 examining large birds in a living state, to 

 drive back again into the lungs the air 

 which they receive in inspiration; whence 

 the repletion and depletion of the thora- 

 cic cells must alternate with those of the 

 abdominal cavities. 



