374 OF THE CIRCULATION OF BLOOD. 



the vessels of an animal recently dead.* But there are certain " residual 

 phenomena" even in Man, which clearly indicate that this is not the whole 

 truth ; and that forces existing in the Blood-vessels have a considerable influ- 

 ence in producing both local and general modifications of the effects of the 

 Heart's action. There are also indications of the nature of an influence, in 

 which the blood-vessels do not partake, arising from those changes occurring 

 between the Blood and the Tissues, that constitute the processes of Nutrition, 

 Secretion, &c. Such, for instance, would appear to be the interpretation of 

 the fact, that whilst any variations in the action of the Heart affect the whole 

 system alike, there are many variations in the Circulation, which, being very 

 limited in their extent, cannot be attributed to such central disturbances, and 

 must therefore be dependent on causes purely local. Of the nature of these 

 influences, and of the mode of their operation, we shall probably arrive at a 

 more correct knowledge, if we examine the phenomena of the Circulation in 

 those beings in which the moving power is less concentrated than it is in the 

 higher Animals ; for just as we find in the latter, that the development of 

 special absorbent vessels does not exclude the function of absorption from being 

 still performed by the general vascular system ( 463), so may we here be led to 

 perceive that there is a generally diffused force, to which alone the Circulation 

 of the nutritious fluid in the lowest organisms is due, and w r hich is not alto- 

 gether replaced by the special organ of impulsion, that is developed in the 

 centre of the system in the higher. 



496. The ascent of the sap in Vegetables is probably to be regarded as due, 

 in part, to the vis a tergo occasioned by the action of Endosmose at the roots ; 

 and in part, to the demand for fluid, occasioned by the vital processes taking 

 place in the leaves. For if the stem of a Vine, in which the sap is rising, be 

 cut across, the sap will continue to flow for some time from the top of the lower 

 portion ; and its force of ascent may be shown to be very considerable, by 

 tying over the cut surface a piece of 'bladder, which will be speedily burst, 

 or by affixing to it a bent tube, containing a column of mercury, which will 

 be raised to the height of forty inches or more. On the other hand, the attrac- 

 tive force of the leaves is shown by the fact, that if the lower end of the upper 

 division be put into water, it will continue to absorb, as long as the vital actions 

 of the leaves are being performed with vigour ; but, if the branch be carried 

 into a dark room, the exhalation from the leaves is immediately checked, and 

 absorption is checked also. The influence of the actions at the periphery of 

 the circulating system in maintaining the flow of fluid towards the part, is 

 further shown by the fact, that if a shoot of an evergreen species be grafted 

 on a stock of one with deciduous leaves, a continual and unwonted ascent of 

 sap will be kept up in the latter through the winter ; this being evidently due 

 to the demand occasioned at its summit. Again, the recommencement of the 

 annual flow of sap in an ordinary tree has been found to take place, in the 

 first instance, not at its roots, but in the neighbourhood of the buds ; for their 

 expansion, under the influence of the returning warmth, exhausts the fluid 

 from the vessels of their neighbourhood ; this, again, occasions a demand from 

 below; and thus the motion is gradually propagated to the roots. Now it has 

 been experimentally ascertained, that, if a branch of a vine growing in the 

 open air be trained into a hot-house, it may be made to vegetate during the 

 winter, and to draw up fluid through the stems and roots, whose condition has 

 not been changed. It is evident, then, that in Plants, the demand for fluid in 

 the organs to which it is distributed by the vascular system, is one of the 

 chief forces by which the supply is obtained. 



* See Dr. Williams's Principles of Medicine, [Am. ed., by Dr. Clymer, p. 155, note.] 



