314 CIRCULATION OF NUTRITIVE FLUID. 



bathed by the circulating fluid. It is perhaps in this part of its course, 

 that it most readily takes up the fresh nutrient materials, which have 

 been prepared by the digestive process, and which would, under such 

 circumstances, find their way with comparative facility from the inner 

 surface of their walls, to the outer. After being thus diffused, in its 

 venous or carbonized state, through the substance of the tissues and 

 through the visceral cavity, it is again collected into distinct trunks ; 

 and these convey it to the respiratory organs. Now although it cannot 

 be doubted, that the impelling power of the heart is the chief cause of 

 the movement of the blood through the systemic vessels, yet it would 

 seem impossible to suppose, that this power can be exerted over the 

 unrestrained currents, in which it is diffused through the body, after 

 passing through the systemic capillaries ; and it can scarcely be doubted, 

 that its passage through the capillaries of the respiratory organs is due 

 to the power which is developed in themselves, under the conditions 

 already alluded to. 



557. There is a very curious phenomenon to be observed in the cir- 

 culation of some of the lowest Molluscs; namely, the continual reversal 

 of the course of the current. The heart, in these animals, is much less 

 perfectly formed, than in the higher tribes ; and seems more like the 

 mere contractile dilatation of the principal trunk, which is the sole 

 representative of that organ in the Echinodermata. The circulating 

 fluid is sometimes transmitted first to the system ; and, after being dis- 

 tributed to its different parts by the ramifications of the main artery, 

 it meanders through the channels excavated in its tissues ; and then 

 flows towards the respiratory surface, after passing over which, it re- 

 turns to the heart. But after a certain duration of its flow in this 

 direction, the current stops, and then recommences in the contrary 

 direction, proceeding first to the respiratory organs, and then to the 

 system in general. It would seem as if in this, one of the lowest forms 

 of animals possessing a distinct Circulation, the central power were not 

 yet sufficiently strong, to determine the course which the fluid is to 

 take : so that it undergoes continual vacillations. In a group of Com- 

 pound Polypes, to which this class of Molluscs has many points of 

 affinity, there is a movement of fluid through the stem and branches, 

 which in like manner continually changes its direction. This move- 

 ment, however, can scarcely be regarded in the light of a proper Cir- 

 culation ; since the tubes in which it occurs are in direct communication 

 with the digestive cavities of the Polypes. But the flow seems altogether 

 independent of any mechanical propulsion ; and takes place most ener- 

 getically and regularly towards parts in which new growth is going on. 



558. We have now to consider the chief forms in which the Circu- 

 lating apparatus presents itself in the Vertebrated classes ; and first in 

 that of Fishes. We have here, as in Molluscs, a heart with two cavi- 

 ties, an auricle and a ventricle ; this heart, however, is not placed at the 

 commencement of the systemic circulation, but at the origin of the 

 respiratory vessels. The blood which it receives and propels, is venous 

 or carbonized ; this is transmitted along a main trunk, which speedily 

 subdivides into lateral branches or' arches ; and these distribute it to 

 the fringes of gills, that hang on the sides of the neck. By the action 



