we see the imperfect blood, or chyle, carried from the stomach 

 to the lungs ; we see the sap elaborated in the leaves by the 

 air, as the blood is elaborated by the same agent in the lungs ; 

 and we observe the sap, after being rendered, during its pro- 

 gression through the leaves, fit for the support of the vegetable 

 functions, distributed all over the plant, just as the blood, 

 after its ventilation in the lungs, is conveyed to every part of 

 the body. I do not mean to say that vegetables possess a cir- 

 culating apparatus as complete as that of the higher orders of 

 animals. They have no heart, or central organ of circulation ; 

 and the vessels which return the elaborated sap from the 

 leaves are, according to Dutrochet, not continuous tubes, like 

 arteries, but consist of small cells or sacs, called clostresi 

 which are joined together in such a manner as to reiSemble a 

 chain of beads. We have seen, however, that, if the heart 

 r!^> wanting in plants, it is likewise wanting in many of the 

 lower classes of animal beings ; and it is perhaps to the circu- 

 lation of blood in these, that the circulation of the sap ought 

 Co be compared, although, in many respects, the latter is more 

 perfect and exact than the former. I have already observed 

 that there are some animals which possess no organs of circu- 

 lation whatever ; the same deficiency occurs in some plants, 

 as the mosses and ferns. In these, the sap passes from one 

 cell to another, in the same manner as the nutritive fluid per- 

 li^at^s the gelatinous substance of the polj-^i and other 

 animals, in which there exists no vascular system. V[«,i.itiif; '\o 

 Ever since the discovery of the circulation of the bl-oodl 

 physiologists have endeavoured to ascertain the efficient'catrses 

 by which this function is carried on. The mechanical physiold- 

 gists imagine that the heart is the sole agent in the circulation 

 of the blood ; that the bloodvessels are in no way concerned 

 in the operation ; that they are mere tubes, and that the blood 

 '\§ propelled from the arteries into the veins, and from the 

 latter into the right auricle, by the contractile power of the 

 heai't alone. Another set of physiologists consider this opinion 

 as not correct ; and affirm that the circulation of the blo6d' is 

 not effected by the action of the heart alone, but likewise^ ih 

 some degree, by that of the arteries, all of which they suppose 

 to be more or less contractile. They allow that the larger 

 arteries have but little contractility, and the blood which is 

 transmitted through them receives its chief impulse from the 

 heart. The capillary or small arteries, however, they regard 

 as pd^essing a considerable share of contractile power, by the 

 agency of which the blood is projected along their ramifica- 

 tions itlto the extremities of the veins. They consider that 

 the v«ins are little more than elastic tubes; that their action is 

 '"MY^nn&m jfiu.:- :.■_. ... . ..=-,.: ,.^. ,.ao" 



