294 OF THE CIRCULATION OF THE BLOOD. 



tration is increasingly observed, until the whole movement is subordinated 

 to the action of a principal propelling organ, the Heart. 1 



232. The elaborate dissections of Dr. J. B. Pettigrew 2 have shown that 

 the walls of the ventricles of the Heart are composed of a series of seven 

 layers or strata of muscular fibres, of which three are external, the fourth is 

 central, and the remaining three are internal. The direction of the fibres 

 constituting these layers gradually changes from a nearly vertical to a hori- 

 zontal or transverse one, which is the course pursued by the fourth layer, 

 and from this back again to a nearly vertical one. The fibres composing 

 corresponding external and internal layers, such as layers one and seven, 

 two and six, etc., are continuous in the left ventricle at the left apex, and in 

 the right ventricle in the track for the anterior coronary artery, the fibres 

 of both ventricles being for the most part continuous likewise at the base^ 

 forming, therefore, a series of figure-of-eight loops. The outermost fibres of 

 both ventricles, on reaching the apex of the heart, present two bundles, 

 which, curving round in the form of a vortex, or whorl, become continuous 

 with the fibres of the carnese columns and musculi papillares. Only the 

 first and seventh layers are inserted into the auriculo-ventricular rings, to 

 which they pass in a direction almost at right angles to their planes. The 

 object of these special arrangements appears to be, as Dr. Hensley 3 has 

 pointed out, to enable the muscular walls of the cavities of the heart to expel 

 by their contraction the whole of their contents, whilst the shortening of the 

 several fibres is both uniform in amount and moderate in extent, 



233. 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 follow- 

 ing are the comparative results of M. Bizot's measurements, 1 taking the aver- 

 age of Males from 17 to 79 years : 



Base. Middle. Apex. 



Left Ventricle, . . . 41 lines, 5i lines, 3| lines. 



Right Ventricle, . . 4f " If " l-fa 



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 in- 

 creasing during all periods of life, from youth to advanced age ; whilst that 

 of the latter is nearly stationary. During pregnancy there is an hypertrophy 

 of the left Ventricle to the extent of one-fourth or even one-third of the 

 original thickness of its walls. 5 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. 6 And so much fallacy may arise from the peculiar condition 



1 See Princ. of Cotnp. Phys., clmp. v. A very full and comprehensive account of 

 the circulation and the apparatus by which it is effected in plants, in the lower ani- 

 mals, arid in Man, will be found in Dr. J. Bell Pettigrew's work on the Physiology 

 of the Circulation, 1874. 



2 Phil. Trans., part iii, 18G4, p. 445. 



3 Jourti. of A uat. and Physiology, vol. iv, 1870, p. 83. 



4 Mem. de la Soc. Me"dic. d 'Observation do Paris, torn. i. 



5 Larcher, Comptes Rendus; Henle and Meissncr's Bericht, 1857, p. 467. 



6 "VVinslow, Senae, Haller and Lieutaud, Boyer, Bizot, Legallois maintain that the 

 disparity in the si/.e of the Auricles and Ventricles is considerable; Laenncc, Cru- 

 veilhier, Gordon, Bouillaud, Meckel, and Portal that it is trifling; Lower, Sabatier, 

 "Weiss, Andral, and more recently Pettigrew, that there is no difference whatever. 



