568 OF THE CIRCULATION OF BLOOD. 



overflow. When the force of the stroke increased, the part of the intestine nearest the 

 syringe burst. 



b. 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 augmentation 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, producing dropsy. Thus it happens, that such effusions may often be traced 

 to that state of deficient vigour of the system, which peculiarly manifests itself in want of 

 tone of the blood-vessels ; and that it is relieved by remedies which restore this. In many 

 young females of leuco-phlegmatic temperament, for example, there is a tendency to swell- 

 ing of the feet, by cedematpus effusion into the areolar tissue, in consequence of the depend- 

 ing position of the limbs; the cedema disappears during the night, but returns during the clay, 

 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 exhausting diseases, in which 

 the patient has lain much upon his back, is attributable to a similar cause; of such conges- 

 tion, 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. 



5. Peculiarities of the Circulation in different Parts. 



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

 the distribution and in the functional actions of the Blood-Vessels, 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 re- 

 turned 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, obstruction of their cavity by solid or fluid depositions, or by foreign sub- 

 stances injected into them, &c.); and proceeding with the greatest energy and 

 regularity, when the respiratory movements are freely performed. The Por- 

 tal circulation, again, is peculiar, 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 ( 685). 



"747. 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 filled with the same 

 total amount of contents, the flow of blood through its vessels is attended with 

 some peculiarities. The pressure of the atmosphere is here exerted, rather 

 to keep the blood in the head, than to force it out; and it might accordingly 

 be inferred that, whilst the quantity of cerebral matter remains the same, the 

 amount of blood in the cranial vessels must also be invariable. This infer- 

 ence appeared to derive support from the experiments of Dr. Kellie.* On 

 bleeding animals to death, he found that, whilst the remainder of the body was 

 completely exsanguine, the usual quantity of blood remained in the arteries 

 and veins of the cranium ; but that, if an opening was made in the skull, these 

 vessels were then as completely emptied as the rest. It is not to be hence 



* Edinburgh Medico-Chirurgical Transactions, vol. i. 



