472 SANGUIFEROUS SYSTEM. 



to the higher classes, and as we ascend through the succes- 

 sive stages of the development of the same species. The 

 blood contains more fibrine and red globules than in the 

 invertebrata, and its colour is always red, being of a much 

 deeper hue in the mammalia and birds than in the cold- 

 blooded classes, especially in the fishes, where it is also less 

 extensively circulated through the body. 



As fishes move and breathe in a dense element, where 

 the air is less abundantly supplied to oxygenate their blood, 

 and as their respiration is still effected by branchiae, as 

 in most of the invertebrated classes, the heart is placed 

 on the branchial and not on the systemic portion of their 

 great aortal artery, and by contributing its impulse to 

 the movement of the branchial blood, the extent of respira- 

 tion is increased beyond that of the aquatic mollusca, where 

 the heart is entirely systemic. The extent and the activity 

 of their muscular system render less necessary, in fishes and 

 in the tadpoles of amphibia, an exclusively systemic heart to 

 propel the blood through the body, that being greatly ef- 

 fected by their active movements ; and the most active period 

 of the amphibia is the tadpole state, when the heart is en- 

 tirely appropriated to the branchial ramifications of the great 

 aortal trunk, as it is to the branchial arches of this vessel in 

 higher embryos. 



The venae cavee of fishes generally unite to form a sinus 

 venosus between the tendinous diaphragm and the auricle, 

 and this sinus is often a capacious muscular pulsating cavity, 

 Like the auricle, as in the rays. Two crescentic valves com- 

 monly defend the entrance of the sinus venosus into the 

 auricle, to prevent the return of the blood during the con- 

 traction of the latter cavity. The heart, like the respiratory 

 organs, is situated far forwards under the head, between the 

 anterior terminations of the branchial arches; it consists of a 

 thin, wide capacious auricle, and a small muscular ventricle, 

 which are enclosed, along with the bulbus arteriosus, in a 

 thin pericardium. The heart is placed farther backwards 

 from the head in the cartilaginous than in the osseous fishes, 

 and still more in amphibia and in higher vertebrata. The 

 two cavities of the heart are not placed in the same continuous 

 straight line as in the gasteropods and other mollusca, and in 

 the earlier conditions of the vertebrated embryo ; the auricle is 



