THE MOTION OF THE BLOOD. 165 



lar in their course and division ; In adult age, softer and far less 

 elastic, but still very firm and remarkably expansile. 



" Their coats are so much thinner that the blood appears 

 through them. They are likewise less in number, being solely 

 a cellular external, somewhat resembling the nervous of the 

 arteries ; and a very polished internal, also nearly agreeing with 

 that of the arteries. 



" A muscular coat exists only in the trunks nearest the heart. 



" The interior coat forms, in nearly all veins of more than a 

 line in diameter, very beautiful valves of easy play, resembling 

 bags, generally single, frequently double, and sometimes triple, 

 placed with their fundus towards the origin of the vein, and 

 their edge towards the heart. 



" These valves are not found in some parts : not in the brain, 

 heart, lungs, secundines, nor in the system of the vena portse. 



" The twigs, or, more properly, the radicles, of the veins, 

 unite into branches, and these again into six principal trunks : 

 viz. 



" Into the two cavae, superior and inferior ; 



" And the four trunks of the pulmonary vein (the arteria 

 venosa of the ancients). 



" The vena portse is peculiar in this, that, having entered the 

 liver, it ramifies like an artery, and its extreme twigs pass into 

 the radicles of the inferior cava, thus coalescing into a trunk. 



" That the blood may be properly distributed and circulated 

 through the arteries and veins, nature has provided the heart k , 

 in which the main trunks of all the blood-vessels unite, and which 

 is the grand agent and mover of the whole human machine, 

 supporting this the chief of the vital functions, with a constant 

 and truly wonderful power, from the second or third week after 

 conception to the last moment of existence." 



The heart is essentially a muscular organ, conical, with four 

 cavities, placed in the left half of the chest, not quite vertically, 

 but rather obliquely to the left, and from behind forward. Its 

 size is usually about that of the closed fist of the individual. 



" It is loosely contained in the pericardium l , which is a mem- 

 branous sac," consisting of two layers : the one fibrous and of the 



k " W. Cowper, Myotomia Reformata. (Posth.) Lond. 1724. fol. max. 

 Tab. xxxvi xl." 



1 " Haller, Elemenla Physiol. t. i. tab.i. 

 ' Nicliolls, Pfiilos. Trans, vol. lii. P. i. p. 272." 



