HEART. 



HEART. 



42 



the diameter of an artery may be taken a measure of the force of the 

 current of blood in it. It does not appear moreover that the direction 

 in which a branch is given off from the trunk has any appreciable 

 influence on the velocity of the current in it most frequently the 

 branches of arteries are given off BO as to form an acute angle 

 with the continued trunk as 2, from 1, 2, but often they separate at 

 right angles, and leas frequently so as to form obtuse angles. Neither 

 can the effect of the tortuosity of an artery be calculated, though 

 there can be little doubt that, cajteris paribus, the current in it 

 will be slower. An important point in the arrangement of arteries is 

 the frequent union, or anastomosis, of branches with each other. 

 The purpose of this is evidently to prevent any part being cut off 

 from its supply of blood, by the compression or obliteration of one of 

 its arteries. Hence it is that even when the main artery of a litnb has 

 been tied, the nutrition has still been amply effected by the current of 

 blood being diverted into the collateral channels, which subsequently 

 become enlarged. [ANEURISM, in ARTS AND So. Div.] 



The chief property of the arteries by which they affect the circula- 

 tion is their extreme elasticity. It is by this that when dilated they 

 contract on the blood that had been forced into them, and propel it 

 in every direction and that when elongated they again shorten, and 

 that when empty they remain open and tubular. The chief effect of 

 the action of this elasticity is the gradual conversion of the jetting 

 pulsatile motion which the blood receives from the forcible and succes- 

 sive contractions of the ventricle into an even and steady current. 

 The elastic contraction of any part of an artery after its distension 

 can have no general influence to accelerate the blood, for it would 

 press that fluid equally in all directions, and thus would retard the 

 current coming from the heart to any part to exactly the same 

 degree that it accelerated that passing from that part towards the 

 veins the one influence exactly nullifying the other. 



To convert the pulsatile motion however into a uniform one no 

 more admirable property could have been imagined than this elasticity, 

 which by continually acting to contract the arteries (which are always 

 during health in a state of slight distension) maintains a propelling 

 force upon the blood during the intervals in which the ventricles are 

 at rest. If the elastic power were not exercised, we should see, on 

 opening an artery, a jet of blood, and then a pause, then another jet, 

 followed by a second stoppage of the current, just as when by the 

 successive strokes of a piston we force water through a rigid tube 

 the jets of blood being coincident with the contractions of the ven- 

 tricles, and the pauses with the intervals of their dilatation and rest. 

 But the arteries being elastic, when the impulse of the ventricles, 

 which at the same time distends them and forces a jet of blood through 

 an orifice in them, ceases, their elasticity, making them contract, 

 forces some more blood out of the orifice, and thus makes the stream 

 from it continuous. In the arteries near the heart indeed the force 

 of the ventricular contraction is so great that it predominates over 

 that of the elastic contraction ; and the current, though continuous, 

 is irn-gular, presenting a succession of jets but as we recede from 

 the heart, this predominance becomes less, the succession of jets 

 become! less distinguishable, and hi the veins we find a perfectly 

 equable stream. An artificial contrivance for effecting the same 

 object, namely, the conversion of a series of motions of a fluid into a 

 continuous one, may be seen in an organ. The air is forced into the 

 wind-chest by a series of strokes of the bellows, and if the walls of 

 that chest were fixed it would issue from it into the organ-pipes in 

 the same intermitting manner; but the top of the chest is muvr.-ililr, 

 and is loaded with weights (proportioned to the size of the instru- 

 ment), so that it maintains a constant and steady pressure on the air 

 below it, which therefore, though driven in by a succession of small 

 forces, passes out into the organ-pipes under the influence of the 

 single force from the top of the chest in a continuous steady current. 

 A similar arrangement is employed in fire-engines, where the elastic 

 power is the air which partly occupies the chest into which the water 

 is forced. [FIRK-KNGIXK, in ARTS AND Sc. Div.] 



We have eaid that the arteries are dilated slightly by each column 

 of blood poured out by the ventricles : from this arises their pulse, 

 which may be said to be owing to the dilatation of the arteries pro- 

 duced by the wave which is propagated along the column of blood 

 contained in them. One may form an idea of what is here called the 

 wave by observing a ripple in a running stream. There are in that 

 case two different motions of the particles of the fluid : they move all 

 together, with an absolute change of place, and this constitutes the 

 stream ; but again, they move separately ; one particle or series of 

 particles rises, and the next falls, and as the first falls again, the 

 econd rises this constitutes the ripple or wave. In it the particles 

 undergo only a relative and temporary change of position, for they all 

 return to rest in their former places, as is shown by any light body 

 that may be floating in them, which merely rises to the top of the 

 wave, and again descends to precisely the same place as it had before 

 the wave arrived. So in the circulation there is a constant stream 

 running through the vessels, and at every contraction of the ventricle 

 an impulse is given to that part which is next the heart, producing a 

 wave which is propagated with incalculable rapidity through all the 

 rteries, and causes at each part of them a slight dilatation as it 

 pastes within them. The repeated contractions produce a series of 

 ueh impulse*, waves, and dilatations, and when a finger slightly com- 



presses an artery, it feels the tendency to dilatations, in what is called 

 the pulse. The degree of dilatation is so slight that its existence was 

 much disputed ; but M. 1'oiseuille has proved that iii the larger 

 aiteries it is equal to about 1-1 1th of their average diameter. In 

 feeling the pulse however we perceive a greater impulse than so slight 

 a dilatation could produce, because the finger flattens the artery, and 

 therefore we feel the force of the wave over a large part of its circum- 

 ference, and we increase its velocity by diminishing the size of the 

 channel. From this description it will be evident that the characters 

 of the pulse by which the conditions of health or disease in any person 

 are decided will depend on two circumstances the state of the heart, 

 and that of the artery itself. The frequency of the pulse will depend 

 entirely on the number of contractions of the left ventricle in a 

 certain time, and hence the varieties of frequent, slow, irregular, and 

 intermitting pulses are entirely referrible to the heart. The size and 

 degree of contraction of the artery will produce the fullness or 

 smallness, the hardness or softness, and all the other characters which 

 are determined by the touch rather than by mere counting. 



But the arteries have another power besides that of their elasticity, 

 by which they influence the circulation, though the mode and extent 

 of its operation are less clear. This is the power of contraction 

 which they possess during life, and which is sometimes, but erro- 

 neously, called muscular. The vital contraction of the arteries 

 differs from the muscular contraction, in being slow and gradual, in 

 not being at all times excitable by any of the stimuli that excite the 

 muscles to contract, as mechanical irritation, electricity, &c., but 

 following generally some peculiar influence, as that of cold, or some 

 particular local excitement, as inflammation, &c. From elastic con- 

 traction it is distinguished by being exercised only during life, and 

 then tending to reduce the .artery often to a smaller calibre than its 

 elasticity would, so that on the complete cessation of life the artery 

 dilates to the size which its elasticity, the consequence of its 

 mechanical structure, would in any circumstances tend to give it. 

 It must be regarded as an instance of that vital contractility which 

 is observed in many tissues quite distinct from the muscular, as in 

 the skin, the dartos, &c. It is best seen by exposing an artery din-ing 

 life, and cutting it directly across; the orifice will then contract 

 slowly, but powerfully, till it is completely closed, either at its very 

 extremity or at a short distance within the tube ; if the animal be 

 killed the artery will remain closed for some time, but will then 

 again gradually dilate under the influence of its elasticity. 



It does not appear certain that this vital contractility of the arteries 

 exercises any constant influence on the circulation ; but, on the 

 contrary, the immediate cessation of the flow of blood from a vein 

 and in the capillaries, when the influence of the heart is cut off by 

 tying the main artery (after separating the collateral vessels), or by 

 removing the heart, and the possibility of exactly regulating the 

 velocity of the current in the vein of a part by permitting or 

 preventing the supply from the heart, seem sufficient to prove the 

 reverse. There are many occasions however in which it evidently 

 acts, and in none more importantly than in cases of wounded arteries, 

 in which it is among the most effectual means of arresting haemorrhage. 

 [H.GMORRHAOE, in ARTS AND Sc. Div.] It is shown also in the paleness 

 and shrinking of a part when exposed to cold, and in the smalluess 

 of the pulsations then felt in the main artery ; and Schwann has 

 seen the diminution in size of the arteries in the mesentery of a frog, 

 when cold was applied. It is probably the principal cause of the emp'.i- 

 ness of the arteries after death ; for as they will contract long after the 

 heart has ceased to act and to force blood into them, they will pour 

 much of that which they contained into the veins, in which it will be 

 retained by the valves. Hales found that this power of contraction 

 resisted the passage of stimulants during life through the arteries ; 

 for a much larger quantity of water could be driven with a given 

 force through the vessels ol> a part than of brandy, though the latter 

 passed most easily after death. For some time too after the heart 

 has ceased to act, this power is sufficient to resist the injection of 

 fluids into the vessels, and hence in making preparations it is advisable 

 to wait a few hours after death, that the fluids may run with more 

 ease along the arteries. 



It is probable that the vital contractility of the arteries is prin- 

 cipally useful by regulating, according to their need, the supply of 

 blood to certain organs, for it seems to exist in the greatest degree 

 in arteries which run most tortuously and which are distributed in 

 parts requiring occasionally increased supplies or velocity of blood, 

 as the spleen, &c. ; perhaps too it is exerted in maintaining the 

 portal circulation, which can receive but little assistance from the 

 heart, whose influence must be nearly expended in forcing the blood 

 through the capillaries of the digestive apparatus. However, it must 

 be acknowledged that at present we only know that such a power 

 exists ; the amount and manner of its action are quite problematical. 



Much is undoubtedly due to the influence of the capillaries on the 

 circulation. They form a dense network of extremely minute tubes, 

 in which the arteries seem to terminate and the veins to arise ; for 

 their delicacy prevents the possibility of discovering any such struc- 

 ture as could decide to which set of vessels they belong, and indeed 

 it is only by observing that the currents of blood-globules pass in 

 regular directions, that we can prove that they are cauals with defi- 

 nite membranous walls. [13LOOD.J When the circulation is examined 



