i 



LECTURE V. 235 



the surface of the body are beset with nu- 

 merous valves, or flood-gates, which pre- 

 vent the blood from moving in a retro- 

 grade course, and oblige it to go onwards 

 towards the heart. The blood sometimes 

 returns to the heart, chiefly by the deep- 

 seated veins which accompany correspond- 

 ing contiguous arteries, and at others more 

 abundantly through those on the surface. 

 Mr. Hunter believed that veins possessed 

 vital activity, but it must be of the kind 

 allotted to the muscles of tardigrade ani- 

 mals, and such as various parts of our bodies 

 are also endowed w^ith. Indeed the large 

 veins near the heart are said to evince an 

 irritability of the more common kind. The 

 nature of the irritability of veins may en- 

 able them to adapt their calibre to their 

 contents. Mr. Hunter argues that, as the 

 vena portae in the liver performs the func- 

 tion of an artery, it probably is irritable. 

 We know that it fails to secrete, or secretes 

 with profusion ; that sometimes it prepares 

 healthy, and at others faulty, bile, which va- 

 riation of function can scarcely be supposed 

 to take place in a perfectly passive vessel. 



