HEART. 



HEART. 



46 



which may be regarded as the most passive of the parts engaged in it, 

 though they are constructed so as to permit many important external 

 agencies to act upon the motion of the blood. While the arteries 

 form a series of branching canals in which the main current is 

 diverted into streams whose number increases as their individual size 

 decreases, the veins are made up of a series in which a vast number 

 of currents gradually unite into others whose number decreases as 

 their size increases (as shown in the annexed figure), and which all 



at last meet in two 

 great trunks the 

 vena; cavse. In addi- 

 tion to other pecu- 

 liarities of structure, 

 the veins have valves 

 (5, 5, 5), very similar 

 to those at the origins 

 of the arteries, which 

 are arranged in pairs, 

 or in threes, at differ- 

 ent distances, in the 

 course of nearly all 

 those veins in which 

 the blood has to rise 

 against the power of 

 gravitation. Their 

 simple use is evident : 

 the blood, returning 

 slowly through the 

 capillaries, and much 

 of the force of the 

 heart being expended 

 in propelling it so far, 

 would be apt to retro- 

 grade, or remain sta- 

 tionary, if the weight 

 of the whole column 

 in the veins bore down 

 upon the arteries, and 

 through them on the 

 heart. As soon how- 

 ever as a portion is 

 raised into a vein, 

 when it tends to re- 

 turn to the arteries it 

 forces down the 

 valves, which close 

 the canal of the vein 

 and support the blood 

 above them, till ano- 

 ther portion rising 

 sends it onwards. 

 Thus, while they per- 

 mit the blood to pan 

 without obstacle to- 

 wards the heart, they 

 entirely prevent it 

 from retrograding ; 



and if it were stationary, the column of blood would be like a stream 

 branching out, and divided by a number of closed locks in which the 

 portion of fluid between any two is prevented from exercising any 

 influence on the portions adjacent to it. 



The veins, like the arteries, are elastic, and this power is occasion- 

 ally exerted in recovering them from too great distension ; they too 

 have a vital contractility whose influence is remarkably shown in their 

 shrinking when cold is applied ; but its effects have been even less 

 calculated than in arteries. 



We have seen that pressure exerted equably on all parts of the 

 walla of an artery would force the blood as much in one direction as 

 the other, so that it could be of no use in accelerating the circulation ; 

 but from the arrangement of the valves equal pressure on the veins 

 has a very different effect, for it will be prevented by them from pro- 

 ducing any retrograde current towards the extremities of the arteries, 

 and thus the greater part of the power exerted will be gained in 

 favour of the flow of blood towards the heart. Such pressure is 

 exercised by the muscles surrounding the veins ; as they contract they 

 compress the veins, and thus force the fluid to flow in the only pos- 

 sible direction, namely, towards the heart. Their influence in this 

 way is shown in the greater tendency to diseased and permanent dila- 

 tation of the veins immediately under the skin and other parts remote 

 from the muscles, than in the deep-seated branches in which this 

 varicose state (as it is called) very rarely occurs, although the number 

 of valves in the former is always much greater ; in the benefit derived 

 by supplying the place of muscles by artificial pressure on the veins 

 , Ac. ; by the increased fullness of the veins, and velo- 

 city of the current commonly seen in bleeding when the bleeding- 

 teff is compressed and moved about in the hand ; by the general 

 acceleration of the circulation by muscular exertion ; and, on the 

 ry, by the tendency to stagnation and swelling of the veins 



1, capillary venous branches; 2, small branches 

 formed by the union of the capillary ; 3, larger 

 branches formed by the union of the smaller and 

 gradually increasing in si/r, to form the great 

 trunk, 4, a portion of which is laid open to show its 

 inner surface and the arrangement of the valves, 5, 

 formed by its inner coat. 



in the indolent, or those whoss muscular systems are greatly 

 debilitated. 



But a still more important influence which is permitted by the 

 presence of the valves is that of atmospheric pressure. It acts prin- 

 cipally in respiration. When the chest is expanded for the purpose 

 of inspiration, it is evident that the atmosphere will press with equal 

 weight on all parts to fill up the vacuum thus produced. From with- 

 out it will pass at once into the most open course through the trachea 

 into the lungs, which it distends ; but at the same time the blood 

 will be forced towards the heart and the great vessels contained in 

 the chest, and will assist in filling up the vacuum to a degree directly 

 proportionate to their volume as compared with that of the lungs. 

 An experimental proof of this influence in the veins (for in the arte- 

 ries its effect is prevented by the valves at their origins) is afforded 

 by introducing a tube into the jugular vein of an animal, and placing 

 its opposite extremity in a vessel full of fluid. At every inspiration 

 the fluid will be seen to rise, and at every expiration to descend a 

 little, indicating first a suction towards the heart, and next a slighter 

 expulsion of fluid from it. It is seen also in cases in which the brain 

 is exposed by removing a portion of the skull ; and in cases of Hernia 

 Cerebri [HEAD, INJURIES OF, ia ARTS AND Sc. Div.], in which, in addi- 

 tion to its slight elevation by the pulsation of the arteries at its base, 

 the brain is seen to enlarge and rise at every expiration, and to become 

 flattened at every inspiration. All these phenomena are still more 

 evident when a strong inspiratory effort is made, as in sighing. 



As inspiration draws the blood into the chest, so expiration acts by 

 compressing all the large vessels to force it out of that cavity away 

 from the heart. Its principal influence must be exerted on the 

 arteries, for the blood would be prevented from passing far along the 

 veins by their valves. In the arteries (and to a less extent in the 

 veins) it is seen to act in the increased velocity of the current that 

 issues from a wound ; in the fullness of the vessels of the face and 

 other parts during a strong expiration, or when holding the breath, 

 coughing, or sneezing, actions which are sometimes the causes of 

 rupture of the smaller vessels, "and produce an evident alteration in, 

 the pulse. 



Atmospheric pressure on the veins must also act to some extent in 

 filling up the vacuum which the sudden contraction of the ventricles 

 must produce in the pericardium. Of course the lungs will expand, 

 and in part effect this by pressing the pericardium towards the heart ; 

 but at the same time the blood will tend to rush towards the auricles 

 and dilate them, so that they may fill up the vacant space. Dr. Barry 

 proved this further, by showing that if a tube be introduced into the 

 sac of the pericardium, without allowing any air to enter with it, a 

 fluid placed in it will 1 e seen to be drawn towards and driven from 

 the sac at each contraction and dilatation of the ventricles. 



Such are the powers concerned in the circulation, and the principal 

 effects which they produce. The influence of each is certain, but 

 what ia its extent, and what are the circumstances under which it is 

 chiefly exercised, cannot be accurately determined. In order of 

 importance, the contraction of the ventricles must undoubtedly be 

 placed far highest; then would come the auricular contraction, the 

 ventricular dilatation, the auricular dilatation ; then inspiration, 

 expiration, and the cavity in the pericardium when the ventricles 

 contract. All these assist at all times in moving the blood ; the 

 elasticity of the arteries tends at all times to equalise the velocity of 

 the current, while their vital contraction and that of the veins, the 

 action of the contents of the capillaries, and the muscular pressure 

 on the veins, would also seem to exercise an important influence. 



One of the most interesting enquiries in connection with the heart 

 is its development, to which we have before alluded, and on which 

 considerable light has been thrown by modern research. The circu- 

 lating system which has been described is not perfected until the 

 moment of birth ; and, as before observed, in its several transitory 

 stages of growth it resembles permanent forms of the circulating 

 apparatus amongst the lower animal*. As the egg of the bird affords 

 the best means of studying these changes, we give an outline of them, 

 from Dr. Carpenter's ' Physiology : ' 



"At an early period of incubation the yolk is found to be 

 enveloped by a germinal membrane, composed of distinct cells, which 

 is divisible into three layers ; and a thickened portion of this is 

 easily distinguishable, at which the embryo will be subsequently 

 evolved. 



" The middle layer gives origin to the circulating system, and is 

 therefore termed the ' vascular ' layer. The thickened portion of this 

 that surrounds the germ soon becomes studded with numerous irregular 

 points and marks of a dark yellow colour ; and as incubation proceeds 

 these points become more apparent, and are gradually elongated into 

 small lines, which are united together, first in small groups, and then 

 into one net-work, so as to form what is called the ' vascular area.' A 

 large dark spot of a similar kind is seen in the situation to be subse- 

 quently occupied by the heart. These dark points and lines are 

 formed by collections of blood-corpuscles, which originate in the 

 transformation of the cells of the embryo and of the germinal mem- 

 brane ; and the rows and masses of blood-discs seem at first to lie in 

 mere channels, the walls of the heart and blood-vessels that subse- 

 quently inclose them being of later formation. 



" From the first however a definite plan is perceptible ; the 



