ON THE MOTION OF THE BLOOD. 67 



109. These fleshy fibres are supplied with very soft 

 nerves* and an immense number of blood vessels, 

 which arise from the coronary arteries, and are so in- 

 finitely ramified, f that Ruysch described the whole 

 structure of the heart as composed of them.:!: 



110. The heart is loosely contained in the pericar- 

 dium. This is a membraneous sac, arising from the 

 mediastinum, very firm, of the same figure as the heart, 

 and moistened by an exhalation from the arteries of that 

 organ. Its importance is evinced by its existence being, 

 in red blooded animals, as general as that of the heart; 

 and by our having only two instances on record of its 

 absence in the human subject. || 



111. By this structure, the heart is adapted for per- 

 petual and equable motions, which are an alternate 

 systole and disastole, or contraction and relaxation of 

 the auricles and ventricles in succession. 



112. Thus, as often as the auricles contract to impel 

 the blood of the venae cavae and pulmonary veins into 

 the ventricles, these are at the same moment relaxed, 

 to receive the blood: immediately afterwards, when 

 the distended ventricles are contracting to impel the 

 blood into the two great arteries, the auricles relax 

 and receive the fresh venous supply. 



* Scarpa, Tabula Neurologies ad illust. Hist. Anat. cardiac, nervor. 

 Tab. iii. iv. v. vi. 



f* Ruysch, Thesaur.Anat. iv. Tab. in. fig. 1, 2. 



t Brandis has proposed an ingenious hypothesis to explain the use of so 

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



Haller, Elementa Physiol. T. i. tab. i. 



Nicholls, Philos. Trans. Vol. Iii. P. i. p. 272. 



II Littrc, Hist, de r Academic dcs Sc. de Paris. 1782. p. 37. Baillic, 

 Transactions of a Society for the Improvement of Medical and ChinirgicaJ 

 . T. i. p. 91. 



p 2 



