COMPARATIVE ANATOMY. 



are united again towards the elbow and 

 knee. 



All birds possess a very remarkable 

 peculiarity in the structure of the heart. 

 The right ventricle, instead of having- a 

 membranous valve (such as are found in 

 both ventricles of mammalia, and also in 

 the left of birds,) is provided witU a strong, 

 tense, and nearly triangular muscle. This 

 singular structure assists in driving the 

 blood with greater force from the right 

 side of the heart into the lungs ; since 

 the expansion of the latter organs by re- 

 spiration, which facilitates the transmis- 

 sion of the carbonated blood in mamma- 

 lia, does not take place in birds, on ac- 

 count of the connection which their lungs 

 have with the numerous air-cells, which 

 will be afterwards described. 



Frogs, lizards, and serpents, have a 

 simple heart, consisting of a single ven- 

 tricle and auricle. 



The structure of this part is very dif- 

 ferent in the turtle, and has given rise to 

 more controversy than that of any order 

 of animals. Their heart possesses two 

 auricles, which are separated by a com- 

 plete septum, like those of warm blooded 

 animals, and receive their blood in the 

 same manner as in those animals, viz. the 

 two venae cavse terminate in the right au- 

 ricle, the pulmonary veins in the left. 

 Each pours its blood into the corres- 

 ponding ventricle, of which cavities there 

 are two : thus the structure of the heart 

 hitherto resembles that of mammalia. 



The characteristic peculiarities which 

 distinguish the heart of these animals con- 

 sist in two circumstances : first, both the 

 ventricles communicate together ; there is 

 a muscular, and as it were tubular valve, 

 going from the left to the right cavity, by 

 means of which the former opens into the 

 latter. Secondly, the large arterial trunks 

 arise altogether from the right ventricle 

 only, (no vessel coming from the left.) 

 The aorta, forming three grand trunks, is 

 situated towards the right side and the 

 upper part ; the pulmonary artery comes 

 as it were from a particular dilatation, 

 which is not situated in the middle of the 

 basis of the heart, but lower ; (it must be 

 understood that we apply these terms 

 according to the horizontal position of the 

 animal.) 



We can now comprehend how this won- 

 derful and anomalous structure, by which 

 all the blood is propelled from the right 

 ventricle only, is accommodated to the pe- 

 culiar way of life of the animal, which sub- 

 jects it frequently to remaining for along 

 time under water. For the greater circu- 



lation is so far independent of that which 

 goes through the lungs, that it can proceed 

 while the animal is under water, and there- 

 by prevented from respiring, although the. 

 latter is impeded. In warm blooded ani- 

 mals, on the contrary, no blood can enter 

 the aorta, which has not previously passed 

 through the lungs into the left ventricle ; 

 and hence an obstruction of respiration 

 most immediately influences the greater 

 circulation. 



The heart in this class of animals is ex- 

 tremely small in proportion to the body. 

 Its structure is very simple, as it consists 

 of a single auricle and ventricle, which 

 correspond with the right side of the heart 

 in warm-blooded animals. The ventricle 

 gives rise to a single arterial trunk,(which 

 is expanded in most fishes into a kind of 

 bulb as it leaves the heart,) going straight 

 forwards to the branchiae, or organs of re- 

 spiration. The blood passes from these 

 into a large artery, analogous to the 

 aorta, which goes along the spine, and 

 supplies the body of the animal. It is 

 then returned by the venae cavx into the 

 auricle. 



It appears that insects possess neither 

 blood-vessels nor absorbents. Cuvier has 

 examined, by means of the microscope, all 

 those organs in this class, which in red- 

 blooded animals are most vascular, with- 

 out discovering the least appearance of a 

 blood-vessel, although extremely minute 

 ramifications of the trachea are obvious in 

 every part. And Lyonet has traced and 

 delineated in the caterpillar, purts infi- 

 nitely smaller than the chief blood-vessels 

 must be, if any such existed. " Anatomic 

 de la Chenille," &c. 



Yet insects, both in their perfect and 

 in their larva state, have a membranous 

 tube running along the back, in which al- 

 ternate dilatations and contractions may 

 be discerned. From this circumstance 

 it has been supposed to be the heart ; but 

 it is closed at both ends, and no vessels 

 can be perceived to originate from it. 



It is obvious from these data, that the 

 functions of nutrition and secretion must 

 be performed, in the animals which we are 

 now considering, in a very different man- 

 ner from that which obtains in the more 

 perfect classes. Cuvier expresses the 

 mode in which he supposes growth and 

 nutrition to be effected, by the term " im- 

 bibition." And he explains, from this cir- 

 cumstance, the peculiar kind of respirati- 

 on which insects enjoy. Since the nutritive 

 fluids have not been exposed to the atmo- 

 sphere, before they arrive at the parts for 

 whose nourishment they are destined, this 



