324 CIRCULATION OF NUTRITIVE FLUID. 



gous to the first, and sometimes prolonged over the whole interval of 

 repose, indicates the reflux of the blood into the ventricles. When the 

 semilunar valves are thickened by a morbid deposit, their surface rough- 

 ened, and their opening narrowed, the 'first sound becomes harsher and 

 sharper ; and the second sound acquires the same character, the back- 

 ward as well as the forward flow of the blood being affected by this 

 cause. 



575. The natural movements of the mitral and tricuspid valves, 

 appear to be accomplished with perfect freedom from sound ; for the 

 size of the orifices which they guard prevents any considerable friction 

 of the blood, in its flow from one cavity to the other ; and their closure 

 when the ventricular systole begins, does not take place with the 

 rapidity and suddenness of that of the semilunar valves. But when 

 their structure is changed by disease, their action is not so noiseless ; 

 and they give rise to various morbid sounds, which are heard in addi- 

 tion to the ordinary sounds, and which may even obscure them alto- 

 gether. In the^ same manner, the ordinary movements of the heart do 

 not produce any audible friction-sound, between the two surfaces of 

 the pericardium, that which covers the heart, and that which lines the 

 pericardial sac. These surfaces are kept moist, in health, by the serous 

 fluid constantly, exhaling from them ; and they are extremely smooth, 

 so that they glide over one another noiselessly. But if they become 

 dry, as in the first stage of inflammation, a slight creaking is heard, 

 accompanying both the ordinary sounds of the heart, and somewhat 

 resembling the rustling of paper. And if they are roughened by the 

 deposit of inflammatory exudations, this "to and fro" sound becomes of 

 a harsher character. 



576. The walls of the left ventricle are considerably thicker than 

 those of the right; and the contractile power is greater. This difference 

 is obviously required, by the difference in length between the systemic 

 and the pulmonary vessels ; the amount of force necessary to drive the 

 blood through the latter, being far inferior to that which is requisite 

 to propel it through the former. The average thickness of the walls 

 of the left Ventricle is about 4J lines ; being somewhat greater than 

 this at the middle of the heart, and less at its apex. The average 

 thickness of the walls of the right ventricle is not more than 1J line; 

 being a little greater than this at the base, and less at the apex of the 

 heart. The left auricle is somewhat thicker than the right. The 

 capacities of all the four cavities are nearly equal ; each of them, in 

 the full-sized heart, holding about two ounces of fluid. The Ventricles 

 are, perhaps, a little larger than their respective Auricles ; but there is 

 no very positive difference in capacity, between the Ventricles and 

 Auricles of the two sides. 



577. The quantity of blood which is propelled at each Ventricular 

 systole, cannot, therefore, exceed two ounces; and it is probably some- 

 what less, as the ventricles do not seem to empty themselves completely 

 at each contraction. Now the whole quantity of the blood seems to be 

 about one-fifth of the entire weight of the body ; so that it will amount 

 to about 28 Ibs. in an individual of 140 Ibs. weight. Allowing 75 pul- 

 sations to a minute, 150 oz. (or 9 Ibs. 6 oz.) of blood would pass through 



