MOTION OF THE BLOOD IN THE VESSELS. 545 



through the whole capillary net-work, untili t is altogether exhausted of its nutritive matter. 

 The source of the movement is thus entirely to be looked for in the changes which take place 

 in the act of growth; and the influence of heat, cold, and other agents, upon the movement 

 is exercised through their power of accelerating or retarding those changes. 



714. The fluid which thus descends through the stem and roots, seems to 

 be at last almost entirely exhausted ; a portion of it appears to find its way 

 into the ascending current, and to be mingled with it ; but all the rest seems 

 to have been entirely appropriated by the different tissues, through which it 

 has circulated. Thus there is no need of any general receptacle, into which 

 it may be collected, and from which it may take a fresh departure; such as 

 is afforded by the heart of the higher animals. And as the purpose of this 

 circulation is only to supply the nutritive materials, and not to convey oxy- 

 gen, this element being but little required in the vegetative processes, and 

 being supplied by other means, the same energy and rapidity are not re- 

 quired in it, as need to be provided for in the higher animals. 



715. In the lowest Animals, the movement of the circulating fluid seems 

 as independent of any central organ of impulsion, as it has been shown to be 

 in Plants. Thus, in the living Sponge, a current of water is continually 

 flowing through the tubes and channels, by which its substance is traversed, 

 the fluid being taken in by the small orifices, and ejected in powerful streams 

 from the large ones; and yet the most attentive examination has not revealed 

 any mechanical cause for this movement. In some of the compound Polypi- 

 fera, a similar current may be seen ; and it is curious that, in many species, 

 its direction undergoes a periodical change ; being reversed at intervals of a 

 certain number of seconds. In the Star-Fish and Sea-Urchin tribe, a com- 

 plex circulation of blood takes place, through regular vessels ; and here we 

 find some indication of a contractile cavity, by the power of which it may be. 

 in some degree, kept up ; but its feeble pulsations can scarcely be regarded, 

 as having any great share in the movement of the fluid which passes through 

 it. In the Articulated series, there is, with a few exceptions, an absence of 

 any central organ of impulsion, possessed of power sufficient to carry the 

 blood through the vascular system, by its contractions alone. In many of the 

 aquatic worms and larvae, the movement of the blood, and the pulsations of 

 the dorsal vessel, may be distinctly seen: and the thinness of the walls of the 

 latter, and the character of its movements, seem clearly to show, that these 

 can scarcely be regarded as propulsive, but that they merely result from the 

 variations in the current which passes through it, the sides flapping together 

 when there is an outward flow, and bulging out when there is an influx. It 

 is in these Articulata, in which there is a provision for respiration throughout 

 the whole structure, as is especially the case in Insects, that the absence of 

 any central impulsive power is most remarkable. In the Crustacea, and in 

 the Mollusca in general, the respiration is aquatic, and is restricted to a par- 

 ticular organ ; and in these, the heart is found to be more muscular, and the 

 circulation to be more under its control. It is curious to remark, however, 

 that, in some of the lower Mollusca, which exhibit a tendency to aggregation 

 into compound structures, like those of the Polypifera, there is the same want 

 of definiteness in the course of the circulation, as lias been just stated to exist 

 in the latter group; the flow of blood, through their complex apparatus of 

 nutritive organs, being arrested at regular intervals, and then recommencing 

 in the reverse direction. 



716. Even in Vertebrated animals, we find indications of the same defi- 

 ciency of central power, over the peripheral circulation. When we look at 

 the simple, thin-walled heart of Fishes, for example, it seems impossible that, 

 it should have much power over the current of blood flowing back to it by 

 the veins ; for of this blood, a considerable portion has to pass through three 



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