THE MOTION OF THE BLOOD. 171 



and mitral valves ; the second detached in their centre, and fixed in 

 the substance of the heart by their two extremities only ; and 

 the third fixed altogether in the substance of the organ. b 



" The fleshy fibres are supplied with very delicate nerves c , 

 and an immense number of blood-vessels, which arise from 

 the coronary arteries, and are so infinitely ramified d , that Ruysch 

 described the whole structure of the heart as composed of them." 6 



" By this structure the heart is adapted for its perpetual and 

 equable motions, which are an alternate systole and diastole, or 

 contraction and relaxation, of the auricles and ventricles in suc- 

 cession. 



" The systole of the ventricles is performed in such a way that 

 their external portions are drawn towards their septum, and the 

 apex of the heart towards the base. f This at first sight seems 

 disproved by the circumstance of the apex striking against 

 the left nipple, and, consequently, appearing elongated, a 

 circumstance, however, to be attributed to the double impetus of 

 the blood flowing into the auricles and expelled from the ven- 

 tricles, by which double impetus the heart must be driven against 

 that part of the ribs." 



Dr.W. Hunter accounted for it thus in 1746: 



" The systole and diastole of the heart, simply, could not pro- 

 duce such an effect ; nor could it have been produced, if it had 

 thrown the blood into a straight tube, in the direction of the axis 

 of the left ventricle, as is the case with fish, and some other 

 classes of animals : but by throwing the blood into a curved tube, 

 viz. the aorta, that artery, at its curve, endeavours to throw itself 

 into a straight line, to increase its capacity ; but the aorta being 

 the fixed point against the back, and the heart in some degree 

 loose and pendulous, the influence of its own action is thrown 

 upon itself, and it is tilted forwards against the inside of the 

 chest." s 



b Manuel d'Anatomie descriptive. Par Jules Cloquet. Paris, 1825. 



c " Scarpa, Tabulae Neurologicce ad ittust. Hist. Anat. cardiac, nervor. tab. iii. 

 iv. v. vi." 



d Ruysch, Thesaur. Anat. iv. tab. iii. fig. 1, 2." 



c " Brandis has proposed an ingenious hypothesis to explain the use of so great 

 an apparatus of coronary vessels. Versuch iiber die Lebenskraft, p. 84." 



f " Consult Ant. Portal, Memoires sur la Nature $ le Traitement 

 Maladies, t. ii. 1800. p. 281." 



8 Treatise on the Blood, &c., by John Hunter, p. 146. Note. 



Dr. Barclay has the following passage on this point : 



