ACTION OF THE HEART. 369 



a line and a half; whilst that of the latter is only a line. In regard to the 

 relative capacities of the right and left cavities, much difference of opinion has 

 prevailed. The right Auricle is generally allowed to be more capacious than 

 the left; and the same is commonly taught of the right Ventricle. So much 

 fallacy may arise, however, from the peculiar condition of the animal at the 

 moment of death, that this is not easily proved, and is indeed by no means 

 certain. Many eminent Anatomists maintain, that the two cavities are equal. 

 The capacity of each of the cavities may be estimated, in the full-sized Heart, 

 at about two ounces ; that of the Auricles being probably a little less ; and 

 that of the Ventricles a little greater. That the Ventricles receive more blood 

 from the Auricles than the latter could transmit to them by simply emptying 

 themselves once, seems therefore probable ;. and may be accounted for by the 

 fact already stated, regarding the slight intermission in the Ventricular Diastole, 

 during which more blood may enter the Auricle from the veins. 



489. There is a well-known anatomical difference between the Auriculo- 

 Ventricular valves on the two sides, which has given rise to the diversity of 

 name. This seems, from the researches of Mr. King,* to be connected with 

 an important functional difference. The Mitral valve closes much more per- 

 fectly than the Tricuspid ; and the latter is so constructed", as to allow of con- 

 siderable reflux, when the cavities are greatly distended. Many occasional 

 causes tend to produce an accumulation of blood in the venous system, and in 

 the right side of the Heart : thus, any obstruction to the pulmonary circulation, 

 cold, compression of the venous system by muscular action, &c., are known 

 to favour such a condition. This is a state of peculiar danger, from the lia- 

 bility which over-distension of the Ventricular cavity has to produce a state 

 of muscular paralysis ; and in the structure of the Heart itself, there seems to 

 be a provision against it. For, when the Ventricle is thus distended, the 

 Tricuspid valves do not close properly ; and a reflux of blood is permitted, not 

 only into the Auricle, but also (through the imperfect closure of their valves 

 under the same circumstances) into the large veins. This is proved by the 

 fact, several times observed by Dr. J. Reid in his experiments upon Asphyxia, 

 &c., that, when the action of the Right Ventricle had ceased from over-disten- 

 sion, he could frequently re-excite it, not merely by puncturing its walls, but 

 by making an opening in the Jugular vein. This fact evidently affords an 

 indication of great importance in the treatment of Asphyxia ; and it explains 

 the reflux of blood, or venous pulse, which is frequently observed in cases of 

 pulmonary disease, and which, according to Mr. King, always exists, though 

 in a less striking degree. 



490. It is not quite certain whether the Ventricles empty themselves com- 

 pletely at each contraction ; but it seems probable that the blood which they 

 contain is not entirely forced into the arteries. The quantity which is pro- 

 pelled by each Ventricle, at every stroke, may be estimated, therefore, at from 

 1 oz. to 2 oz. If we adopt the lower of these numbers, we shall find that, 

 reckoning 75 pulsations of the Heart to a minute, 112 oz., or. 7 Ibs., of blood 

 pass through each Ventricle in that time ; and, on the higher estimate, 150 

 oz., or 9 Ibs. 6 oz., would pass through in the same period. Now the whole 

 quantity of blood contained in the human body, according to the estimate of 

 Haller (which is considered by Dr. Allen Thomson to be near the truth), is 

 about one-fifth of the weight of the body or 28 pounds in a person weighing 

 140 Ibs.t This quantity would pass through the Heart, therefore, in four 

 minutes, on the lower of the two preceding estimates, or in three minutes on 

 the higher ; and would circulate afresh, fifteen or twenty times in an hour. 



* Guy's Hospital Reports, vol. ii. 



j- Cyclopaedia of Anatomy, Art. Circulation. See also 581. 



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