ON THE MOTION OF THE BLOOD. 63 



In the present state of our knowledge, the umbilical 

 vessels are to be regarded as the only exception to the 

 termination of arteries in veins. We shall shew that 

 they are connected with the uterine vessels by the in- 

 tervention of a spongy substance, called parenchyma. 



92. Another description of vessels arise universally 

 from the arteries and are called colourless, from not con- 

 taining pure blood, either on account of their minute- 

 ness, or of their specific irritability which causes them 

 to reject that fluid. These are the nutrient and other 

 secretory vessels : of which hereafter. 



93. The blood conveyed from the heart by the arte- 

 ries is carried back by the veins. 



These are very different in function and structure 

 from the arteries, excepting however the minutest of 

 both systems, which are indistinguishable. 



94. The veins, excepting the pulmonary, are upon 

 the whole more capacious than the arteries; are more 

 ramified ; much more irregular in their course and divi- 

 sion; in adult age, softer and more elastic, but still 

 very firm and remarkably expansile. 



95. Their coats are so much thinner that the blood 

 appears through them. They are likewise less in num- 

 ber, being solely a cellular external, somewhat resem- 

 bling the nervous of the arteries ; and a very polished 

 internal, also nearly agreeing with that of the arteries. 



A muscular coat exists in the largest trunks only. 



96. The interior coat forms, in most veins of more 

 than a line in diameter, very beautiful valves, of easy 

 play, resembling bags, generally single, frequently dou- 

 ble, and sometimes treble, so placed, that the fundus 

 lies towards the origin of the vein, the Minims towards 

 the heart. 



