500 OF THE CIRCULATION OF THE BLOOD. 



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 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 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 de- 

 ficiency 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 augmentation in the quan- 

 tity of blood they contain. All these conditions are peculiarly favorable 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 neighbor- 

 ing serous cavity, producing dropsy. Thus it happens that such effusions may 

 often be traced to that state of deficient vigor of the system which peculiarly 

 manifests itself in want of tone of the bloodvessels; and that it is relieved by 

 remedies which restore this. In many young females of leucophlegmatic tem- 

 perament, for example, there is a tendency to swelling of the feet, by redema- 

 tous effusion into the areolar tissue, in consequence of the depending position of 

 the limbs; the O3dema disappears during the night, but returns during the day, 

 and is at its maximum in the evening. And the congestion which frequently 

 manifests itself in the posterior parts of the body, towards the close of exhaust- 

 ing diseases, in which the patient has lain much upon his back, is attributable to 

 a similar cause ; of such congestion, effusions into the various serous cavities are 

 frequent results ; and such effusions, taking place during the last hours of life, 

 are often erroneously regarded as the cause of death. To the same cause we 

 are to attribute the varicose state of the veins of the leg, which is so common 

 amongst persons of relaxed fibre, and especially in those whose habits require 

 them to be much in the erect posture ; and this distension occasionally proceeds 

 to complete rupture, the causes of which are fully elucidated by the experiments 

 just cited. 



6. Peculiarities of the Circulation in different Parts. 



533. In several portions of the Human body, there are certain varieties in 

 the distribution and in the functional actions of the bloodvessels, which should 

 not be omitted in a general account of the Circulation. Of these, we have in 

 the first place to notice the apparatus for the Pulmonary circulation ; the chief 

 peculiarity of which is, that venous blood is sent from the heart, through a tube 

 which is arterial in its structure, whilst arterial blood is returned to the heart, 

 through a vessel whose entire character is that of a vein. The movement of 

 the blood through these is considerably affected by the physical state of the 

 lungs themselves ; being retarded by any causes which can occasion pressure on 

 the vessels (such as over-distension of the cells with air, obsk-uction of their 

 cavity by solid or fluid depositions, or by foreign substances injected into them, 

 &c.) ; and proceeding with the greatest energy and regularity, when the respira- 

 tory movements are freely performed. The Portal circulation, again, is pecu- 

 liar, in being a kind of offset from the general or systemic circulation, and also 

 in being destitute of valves ; and it may be surmised with much probability, that 

 the purpose of their absence is, to allow of an unusually free passage of blood 

 from one part of that system to another, during the very varying conditions to 

 which it is subjected ( 490). Another very important modification of the 

 Circulating system is that which presents itself within the Cranium. From 

 the circumstance of the cranium being a closed cavity, which must be always 



