RESPIRATORY PULSE VENOUS PULSE. 341 



near the heart, by another cause entirely distinct from the preceding; 

 namely, the regurgitation of blood from the ventricle into the auricle, and 

 thence into the venae cavae, during the ventricular systole ; and the pul- 

 sation thus occasioned is synchronous, therefore, with that in the arteries 

 (proceeding backwards, however, from the heart), instead of correspond- 

 ing with the respiratory movement. This regurgitation may take place, 

 not from any disease in the valves on the right side of the heart, but 

 simply from over-distension of its cavities, resulting from any obstruc- 

 tion to the circulation of blood through the lungs ; for when this occurs, 

 the tricuspid valve does not completely close, and allows a portion of 

 the blood to escape from the ventricle backwards into the auricle and 

 venae cavae. This want of complete closure, constituting what has been 

 termed the " safety-valve function' r of the tricuspid valve, has been par- 

 ticularly noticed in diving animals, in which the circulation through the 

 lungs is liable to be temporarily suspended. The venous pulsation 

 which is thus produced, may be noticed in almost every case of long- 

 standing dyspnoea ; especially when this is accompanied (as it usually 

 is) by hypertrophy and dilatation of the right ventricle of the heart. 



609. The Venous circulation is much more liable than the Arterial, 

 to be influenced by the force of Gravity ; and this influence is particu- 

 larly noticeable, when the tonicity of the vessels is deficient. The fol- 

 lowing experiments performed by Dr. 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 Congestion. 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 arranged in curves and coils, but without 

 angles and crossings. When the two ends 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 mem- 

 branous 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 inr 

 creased considerably, the amount of fluid discharged by the gut being 

 much diminished ; 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 nigh 

 enough to overflow. When the force of the stroke was increased, the 

 part of the intestine nearest the syringe burst. 



610. From these experiments it is easy to understand, how any defi- 

 ciency 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 aug- 

 mentation in the quantity of blood they contain. All these conditions 

 are peculiarly favourable to the escape of the watery part of the blood 

 from the small vessels ; and this may either infiltrate into the areolar 

 tissue, or it may be poured into some neighbouring serous cavity, pro- 



