ACTION OF THE HEART. 475 



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 semilunar valves, since they are re- 

 strained by the chordae tendineaa; whence it is, that no sound is produced by 

 their closure. The blood is expelled by the ventricular systole into the Pulmon- 

 ary Artery, which it distends, passing freely through its Semilunar valves ; but 

 as soon as the vis a teryo 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 circulated through the lungs, returns as Arterial blood, 

 by the Pulmonary Veins, to the left Auricle; whence it passes through the 

 Mitral valve into the left Ventricle, and thence into the Aorta through its Semi- 

 lunar valves in the same manner with that on the other side, as just described. 

 504. 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. 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 measurements, 1 taking the average of 

 males from 16 to 79 years. 



BASE. MIDDLE. APEX. 



Left Ventricle 4 lines 5 lines 3| lines. 



Right Ventricle . . lj| lines If 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 that of 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 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 somewhat 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. The average capa- 

 city of the cavities may be estimated, in the full-sized Heart, at about three 

 ounces; that of the Auricles being probably a little less; and that of the Ven- 

 tricles a little greater. It has been shown that the Ventricles receive more blood 

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

 themselves once. There is a well-known anatomical difference between the 

 auriculo- ventricular valves on the two sides, which has giyen rise to the diversity 

 of name. This seems, from the researches of Mr. King, a lo be connected with 

 an important functional difference. The Mitral valve closes much more perfectly 

 than the Tricuspid; and the latter 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, com- 

 pression of the venous system by muscular action, &c., are known to favor 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 para- 

 lysis; 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, 



1 "Mm. de la Soc. Medic. d'Observation de Paris," torn. i. 



2 "Guy's Hospital Reports," vol. ii. 



