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by distention than those of the veins, the former, in the progress of the growth of the body, must increase much more in density than the latter; and, therefore, the coats of the arteries, in respect of density and resistance, must come, in time, not only to be in balance with those of the veins, but to prevail over them; a fact which is sufficiently proved by the experiments of the above-mentioned ingenious author.
By these means, the proportional quantities of blood in the arteries and veins must change in the course of life. In younger animals, the quantity of blood in the arteries must be proportionally greater than in old ones; but by the increasing density of the arteries, the quantity of blood in them must be continually diminishing, and that in the veins be proportionally increasing, so as at length to be in a proportionally greater quantity than that in the arteries. When this change hap pens in the proportional quantities of the blood in the arteries and veins, it must be evident that the plethoric state of the arteries will be in a great measure taken off; and, therefore, that the arterial ' hæmorrhagy is no longer likely to happen ; but
that, if a general plethoric state afterwards take place in the system, it must especially appear in the veins.
767. The change I have mentioned to happen in the state of the arterial and venous systems, is properly supposed to take place in the human body about the age of thirty-five, when it is manifest that the vigour of the body, which depends so much upon the fulness and tension of the arterial system, no longer increases; and therefore it is, that the same age is the period, after which the arterial hæmorrhagy, hæmoptysis, hardly ever appears. It is true, there are instances of the hæmoptysis happening at a later period; but it is for the reasons given (758), which shew that an hæmorrhagy may happen at any period of life, from accidental causes forming congestions, inde pendent of the state of the balance of the system at that particular period. :..
768. I have said (766), that if, after the age of thirty-five, a general and preternatural plethoric state occur, it must especially appear in the venous system ; and I must now. observe, that this venous plethora may also give occasion to hæmorrhagy.
769. If a plethoric state of the venous system take place, it is to be presumed, that it will especially and in the first place affect the system of the vena portarum, in which the motion of the venous blood is more slow than elsewhere; in which the motion of the blood is little assisted by external compression, and in which, from the want of valves in the veins that form the vena portarum, the motion of the blood is little assisted by the
compression that is applied; while, from the same want of valves in those veins, the blood is more ready to regurgitate in them. Whether any regurgitation of the blood can produce an action in the veins, and which inverted, or directed towards their extremities, can force these, and occasion hæmorrhagy, may perhaps be disputed: but it ap. pears to me that an hæmorrhagy, produced by a plethoric state of the veins, may be explained in another and more probable manner. If the blood be accumulated in the veins, from any interruption of its proper course, that accumulation must resist the free passage of the blood from the arteries into the veins. This again must produce some congestion in the extremities of the red arteries, and therefore some increased action in them, which must be determined with more than usual force, both upon the extremities of the arteries, and upon the exhalants proceeding from them; and this force may occasion an effusion of blood, either by anastomosis or rupture. I
770. In this manner I apprehend the hæmor. rhoidal flux is to be explained, so far as it depends upon the state of the whole system. It appears most commonly to proceed from the extremities of the hæmorrhoidal vessels, which, being the most dependent and distant branches of those veins that form the vena portarum,, are therefore the most - readily affected by every accumulation of blood in
that system of veins, and consequently by any general plethora in the venous system.
1771. It is here to be observed, that I have spoken of this hæmorrhagy as proceeding from the hæ. morrhoidal vessels only, as indeed it most com. monly does; but it will be readily understood, that the same accumulation and resistance to the venous blood may, from various causes, affect many of the extremities of the vena portarum, which lie very superficially upon the internal sur face of the alimentary canal, and give occasion to what has been called the Morbus Niger, or Metæna.
772. Another part in which an unusually plethoric state of the veins may have particular effects, and occasion hæmorrhagy, is the head. In this, the venous system is of a peculiar conformation, and such as seems intended by nature to give there a slower motion to the venous blood. If, there. fore, the plethoric state of the venous system in general, which seems to increase as life advances, should at length increase to a great degree, it may very readily affect the venous vessels of the head, and produce there such a resistance to the arterial blood, as to determine this to be poured out from the nose, or into the cavity of the cranium. The special effect of the latter effusion will be, to produce the disease termed Apoplexy; and which, therefore, is properly named by Dr. Hoffman, Hemorrhagia Cerebri, and the explanation of its cause, which I have now given, explains well why it happens especially to men of large heads and short necks, and to men in the decline of life, when the powers promoting the motion of the blood are much weakened.
773. I have thus attempted to give the history of the plethoric and hæmorrhagic states of the human body, as they occur at the different periods of life; and hope I have thereby explained, not only the nature of hæmorrhagy in general, but also of the particular hæmorrhagies which most commonly appear, and as they occur successively at the different periods of life.
Of the Remote Causes of Hæmorrhagy. 1774. In the explanation hitherto given, I have especially considered the predisposition to hæmorrhagy; but it is proper also, and even necessary, to take notice of the occasional causes, which not only concur with the predisponent, in exciting hæmorrhagy, but may also sometimes be the sole causes of it.