MOTION OF THE BLOOD IN THE VESSELS. 375 



497. This is still more evidently the case in regard to the Circulation of 

 nutritious or elaborated sap, which takes place in the under surface of the 

 leaves and in the bark. The object of this movement is not to convey the 

 fluid in a direct line from one point to another (as in the case with the ascend- 

 ing current), but to supply every part with materials for its growth, or for the 

 production of its peculiar secretions. Hence the vessels in which it takes 

 place, form a minutely anastomozing network, instead of consisting of a system 

 of straight and distinct tubes. Through this network, the latex or elaborated 

 sap is seen to move, exactly as does the blood through the capillary vessels of 

 animals. The movement takes place, under favourable circumstances, with 

 considerable rapidity ; it is accelerated by heat, and retarded by cold ; and it is 

 subject to all those minor irregularities (such as the cessation of movement, or 

 change in the direction of the current, in a particular channel), which are so 

 constantly to be noticed by any one who attentively watches the capillary 

 circulation of Animals, and which clearly prove the operation of some causes 

 independent of the heart's action ( 478). The general direction of the ela- 

 borated sap, through this capillary system, is downwards; but that the force 

 of gravity cannot have much to do with the movement, is shown by the fact 

 that, in a dependent branch, it has to ascend towards the stem, which it will 

 do without interruption from this cause. 



498. 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 Polypifera, a 

 similar current may be seen ; and it is curious that, in many species, its direc- 

 tion undergoes a periodical change ; being reversed at intervals of a certain 

 number of seconds. In the Star-fish and Sea-Urchin tribe, a complex circula- 

 tion 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 cen- 

 tral 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 particular 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 struc- 

 tures, like those of the Polypifera, there is the same want of definiteness in 

 the course of the circulation, as has 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. 



