552 OF THE CIRCULATION OF BLOOD. 



cially, when the tubes are distended ; so that a small amount of reflux usually 

 takes place, and this is much increased when there is any obstruction to the 

 pulmonary circulation. Whilst the right Ventricle is contracting upon the 

 blood that has entered it, the carnese columns?, which contract simultaneously 

 with its proper walls, put the chordse tendinese upon the stretch; and these 

 draw the flaps of the Tricuspid valve into the auriculo-ventricular axis. The 

 blood then getting behind them, and being compressed by the contraction of 

 the ventricle, forces the flaps together in such a manner as to close the orifice ; 

 but they do not fall suddenly against each other, as is the case with the semi- 

 lunar valves, since they are restrained by the r.hordse tendinese ; whence it is, 

 that no sound is produced by their closure. The blood is expelled by the 

 ventricular systole into the Pulmonary Artery, which it distends, passing 

 freely through the Semilunar valves; but as soon as the vis a tergo ceases, 

 and reflux might take place by the contraction of the arterial walls, the valves 

 are filled out by the backward tendency of the blood, and completely check 

 the return of any portion of it into the ventricle. The blood, after having cir- 

 culated through the lungs, returns as Arterial blood, by the Pulmonary Veins, 

 to the left Auricle; whence it passes through the mitral valves into the left 

 Ventricle, and thence into the Aorta, in the same manner with that on the 

 other side, as just described. 



723. There are, however, some important differences in the structure and 

 functional actions of the two divisions of the Heart, which should be here 

 adverted to. 



a. The walls of the left Ventricle are considerably thicker than those of the right; and 

 its force of contraction is much greater. The following are the comparative results of M. 

 Bizot's recent measurements, taking the average of males from 16 to 89 years. 



BASE. MIDDLE. APEX. 



Left Ventricle 4g lines 5^ lines 3J lines 



Right Ventricle lj| lines 1| lines I 3 ' ff lines 



In the female, the average thickness is somewhat less. It will be seen that the point of 

 greatest thickness in the left Ventricle is near its middle ; while in the right, it is nearer the 

 base. The thickness of the former goes on increasing during all periods of life, from youth 

 to advanced age ; whilst thatof the right is nearly stationary. The left Auricle is somewhat 

 thicker than the right; the average thickness of the former being, according to Bouillaud, 

 a line and a half; whilst thatof the latter is only a line. In regard to the relative capaci- 

 ties of the right and left cavities, much difference of opinion has prevailed. The right Au- 

 ricle 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 condi- 

 tion of the animal at the moment of death, that this is not easily proved, and is, indeed, by 

 no means certain. 



b. 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. 



c. There is a well-known anatomical difference between the Amiculo- Ventricular valves, 

 on the two sides, which has given rise to the diversity of name. This seems, from the re- 

 searchesof Mr. King,* to be connected with an important functional difference. The .Mitral 

 valve closes much more perfectly than the Tricuspid : and tln> hitter is so constructed, as to 

 allow of considerable 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 liability 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 



* Guy's Hospital Reports, vol. ii. 



