PHYSIOLOGY. 161 



the conglobate glands, and at length terminate either in the re- 

 ceptacle of the chyle, in the thoracic duct, or in the left sub- 

 clavian vein ; and, in this way, return the absorbed fluid into 

 the course of the ordinary circulation. 



CLIV. There are absorbent vessels, not only in the several 

 cavities, but also on the external surface of the body, by which 

 many extraneous matters may be introduced into it. 



CLV. Most of the fluids secreted from the circulating mass, 

 and poured into cavities, may be absorbed from these, and re- 

 turned again by the lymphatics, as in CL I II., to the course 

 of the circulation. But the same secreted fluids seem often to 

 be returned also into the course of the circulation, by a regur- 

 gitation, or retrograde motion, in the excretory and secretory 

 vessels. 



CHAP. II. OF THE POWERS MOVING THE BLOOD. 



CLVI. The chief power by which the blood is moved, and 

 the circulation carried on, is the action of the heart, or its re- 

 peated contractions, occurring, as mentioned (CXLIX.). For 

 this purpose the heart is a muscular part, the action of which 

 may depend upon an inherent power stimulated by the dilata- 

 tion of its cavities ; but this inherent power requires the con- 

 stant support of the nervous and animal powers, and is often 

 actuated by these. 



CLVI I. The contraction and relaxation" of the heart, or, as 

 these are called, its systole and diastole, are necessarily alter- 

 nate by the general law (CVIIL) affecting all muscles; and by 

 the stimulus from the influx of venous blood being alternately 

 applied and removed. 



CLVIII. If we may be allowed to estimate the vigour of 

 muscles by the number of their fibres, we must suppose the 

 force of the heart to be very considerable ; but it is very diffi- 

 cult to obtain any exact estimate of its absolute force. It is 

 perhaps the relative force only that we are concerned to know. 



CLIX. Do the arteries, by their contraction, contribute to 

 promote the motion of the blood begun by the heart ? They 

 cannot do it by the elasticity of the simple solid in their struc- 

 ture, and can do it only by their being indued with a muscular 

 VOL. i. L 



