356 OF THE CIRCULATION OF THE BLOOD. 



of the part will undergo compression : and as the blood is prevented, by the 

 valves in the Veins, from being driven back into the small vessels, it is nec- 

 essarily forced on towards the heart. 1 As each set of muscles is relaxed, 

 the Veins compressed by it fill out again, to be again compressed by the re- 

 newal of the force. That the general Muscular movement is an important 

 agent in maintaining the circulation at a point above that at which it would 

 be kept by the action of the heart and arterial system alone, appears from 

 several considerations. The pulsations are diminished in frequency by rest, 

 accelerated by exertion, and very much quickened by violent effort ( 248, 

 d). In all kinds of exercise, and in almost every sort of effort, there is that 

 alternate contraction and relaxation of particular groups of Muscles, which 

 has been just mentioned as affecting the flow of blood through the veins; 

 and there can be little doubt that the increased rapidity of the return of 

 blood through them is of itself sufficient cause for the accelerated move- 

 ments of the heart. When a large number of muscles are put in action 

 after repose, as is the case when we rise up from a recumbent or a sitting 

 posture, the blood is driven to the heart with a very strong impetus ; and if 

 that organ should be diseased, it may arrive there in a quantity larger than 

 can be disposed of; so that sudden death may be the result. Hence the 

 necessity for the avoidance of all sudden and violent movements on the part 

 of those who labor under either functional disorder or structural disease of 

 the centre of the circulation. 



276. The Venous circulation is much more liable than the Arterial, to be 

 influenced by the force of gravity ; and this influence is particularly notice- 

 able, when the tonicity of the vessels is deficient, The following experi- 

 ments performed by Dr. C. J. B. Williams/ to elucidate the influence of 

 deficient firmness in the walls of the vessels, and of gravitation, over the 

 movement of fluids through tubes, throw great light on the causes of venous 

 Cf>'ii</<'f!(i. A tube with two equal arms having been fitted to a syringe, a 

 brass tube two feet long, having several right angles in its course, was adapted 

 to one of them, whilst to the other was tied a portion of a rabbit's intestine 

 four feet long, and of calibre double that of the brass tube, this being ar- 

 ranged in curves and coils, but without angles or crossings. When the two 

 tubes were raised to the same height, the small metal tube discharged from 

 two to five times the quantity of water discharged in a given time by the 

 larger but membranous tube ; the difference being greatest when the strokes 

 of the piston were most forcible and sudden, by which the intestine was 

 much dilated at its syringe end, but conveyed very little more water. When 

 the discharging ends were raised a few inches higher, the difference increased 

 considerably, the amount of fluid discharged by the gut being much dimin- 

 ished ; and when the ends were raised to the height of eight or ten inches, 

 the gut ceased to discharge, each stroke only moving the column of water 

 in it, and this subsiding again, without rising high enough to overflow. 

 When the force of the stroke increased the part of the intestine nearest the 

 syringe burst. From these experiments it is easy to understand how any 

 deficiency of "tone" in the Venous system will tend to prevent the ascent 

 of the blood from the depending parts of the body, and will consequently 

 occasion an increased pressure on the walls of the vessels, and an augmen- 

 tation in the quantity of blood they contain. All these conditions are pe- 

 culiarly favorable to the escape of the watery part of the blood from the 

 small verv-'ds ; and this may either infiltrate into the areolar tissue, or it may 



1 Sadler, Ueber dm Blutstrom in den ruhcnden, vcrkiirzU-n mid ermiideton Mus- 

 koln dcs li-hendes Tliicres, in Ludwig's Arbeiten, 1870, p. 77. Also Sczelkow, in 

 "Winner Sit/.iiiigslicnrhte, Band xlv. 



2 Principles of Medicine, 2d edit., p. 188. 



