HEAET 559 the touch; and the induration disappears en the muscle becomes relaxed. That the iduration of the heart really coincides in time ,'ith the contraction of the ventricle and the )mpression of the blood, may be easily shown Dy an experiment first performed by Harvey, lamely, by thrusting a small metallic canula through the substance of the left ventricle into its cavity, upon which the blood is driven out from the external orifice of the canula in inter- rupted jets, each jet corresponding in time with the induration and tension of the substance of the heart. The movement by which the point of the heart strikes the walls of the chest pro- duces a visible and tangible undulation of the integument at the fifth intercostal space. This is called the " impulse." The impulse is a val- uable guide for the physician in many cases, as it is increased to an abnormal degree in cases of hypertrophy of the heart, and also displaced from its natural location when the organ is en- larged or from any cause altered in position. The heart is loosely attached, at its base, by means of the great vessels to the anterior sur- face of the spinal column ; but its body and apex are nearly uncoufined, being covered only by the pericardium, whose exterior is itself a serous membrane, being invested by the free surface of the pleura. Consequently, while the base of the heart is generally fixed, the point of the organ may move in various direc- tions when compressed or encroached upon by neighboring growths. When the heart is sim- ply enlarged, its point is turned further toward the left side. When the pericardium is dis- tended with an accumulation of serum, the point of the organ fails to reach the walls of the chest, and the impulse disappears altogether. When there is an accumulation of fluid in the left pleural cavity, the point of the heart may be turned completely over to the right of the ster- num, so that the impulse may be felt on that side. There are also cases, though rare, in which, from a congenital malformation, the relative position of all the internal organs is reversed, the heart being placed on the right side, the liver on the left, the spleen and great pouch of the stomach on the right, &c. The abnormal position of the heart may be more easily distinguished during life than that of any other internal organ, owing to its very perceptible impulse against the walls of the chest. The action of the heart is accompanied by two peculiar sounds, very audible when the ear is applied to the front part of the chest. These sounds follow each other, in the healthy condition, with perfect regularity, and are both produced at each cardiac contraction. They are known as the first and second sounds of the heart. The first sound occurs at the instant of the ventricular contraction. It is distinct, but slightly prolonged, and is heard most plainly over the anterior and lateral surface id toward the apex of the organ. It is pro- luced by the shutting back and sudden ten- of the ventricular valves, combined, ac- 396 VOL. vin. 36 cording to some authorities, with the muscu- lar action of the heart's walls and the passage of the blood through its cavities. The second sound, which immediately follows the first, is shorter, but sharper and clearer. It is heard most distinctly over the sternum at the level of the third costal cartilage. Doubtless it is due exclusively to the shutting back of the ar- terial valves (aortic and pulmonary) ; since, in the first place, it is heard with remarkable clearness directly over the situation of these valves, and becomes fainter and less marked as the ear is moved further away ; and secondly, it has been found by experiment upon the lar- ger animals that if these arterial valves be hooked back by a curved needle introduced into the base of the great vessels, the second sound immediately disappears, but returns again as soon as the valve is liberated. The charac- ter, regularity, position, and relative intensity of these sounds often become valuable signs to the medical man in determining the nature and progress of affections of the heart. The pulsa- tions of the heart follow each other, in every species of animal, with a certain degree of ra- pidity. Generally speaking, this rapidity is greater in the warm-blooded, less in the cold- blooded animals. In species of the same class, the pulsations are more rapid in the smaller, less so in the larger. In the human subject the average rapidity of the cardiac pulsations, for an adult male, is about 70 a minute. They are more frequent, as a rule, in young children and in the female sex ; and there are variations in this respect, within certain limits, in particular persons, owing to individual peculiarities of or- ganization. Thus it would not necessarily be an abnormal sign to find in any particular per- son the habitual frequency of the heart's ac- tion from 60 to 65, or from 75 to 80 a minute. As a general rule, the heart's action is slower and more powerful in fully developed and mus- cular persons, more rapid and feebler in those of slighter organization. In all cases the heart is temporarily excited to increased frequency of action by unusual exertion or by nervous ex- citement ; and its pulsations also become more rapid in many diseased conditions, particularly those of a febrile character. The heart's action is always purely involuntary, being entirely in- dependent of the will. Its persistency is very remarkable. In the warm-blooded animals the heart's action ceases in a very few minutes af- ter the destruction of the nervous centres, or after the circulation of the blood in its own vessels is arrested by haemorrhage, by ligature, by the stoppage of respiration, or by excision of the heart itself; since, like the other organs in these animals, it requires a constant supply of freshly arterialized blood to preserve its vi- tality. But in the cold-blooded animals it will continue to beat for many hours after it has been emptied of blood, and even after it has been cut out of the chest and separated from all its nervous and vascular connections. The pul- sations of the separated heart have been seen
