CIRCULATION OF THE BLOOD 125 



The discoverer of these valves did not rightly under- 

 stand their use, nor have succeeding anatomists added any- 

 thing to our knowledge: for their office is by no means ex- 

 plained when we are told that it is to hinder the blood, by 

 its weight, from all flowing into inferior parts; for the edges 

 of the valves in the jugular veins hang downwards, and are 

 so contrived that they prevent the blood from rising up- 

 wards; the valves, in a word, do not invariably look up- 

 wards, but always toward the trunks of the veins, invariably 

 towards the seat of the heart. I, and indeed others, have 

 sometimes found valves in the emulgent veins, and in those 

 of the mesentery, the edges of which were directed towards 

 the vena cava and vena portae. Let it be added that there 

 are no valves in the arteries, and that dogs, oxen, etc., have 

 invariably valves at the divisions of their crural veins, in 

 the veins that meet towards the top of the os sacrum, and 

 in those branches which come from the haunches, in which 

 no such effect of gravity from the erect position was to be 

 apprehended. Neither are there valves in the jugular veins 

 for the purpose of guarding against apoplexy, as some have 

 said ; because in sleep the head is more apt to be influenced 

 by the contents of the carotid arteries. Neither are the 

 valves present, in order that the blood may be retained in 

 the divarications or smaller trunks and minuter branches, 

 and not be suffered to flow entirely into the more open and 

 capacious channels ; for they occur where there are no divari- 

 cations; although it must be owned that they are most fre- 

 quent at the points where branches join. Neither do they 

 exist for the purpose of rendering the current of blood more 

 slow from the centre of the body; for it seems likely that 

 the blood would be disposed to flow with sufficient slowness 

 of its own accord, as it would have to pass from larger 

 into continually smaller vessels, being separated from the 

 mass and fountain head, and attaining from warmer into 

 colder places. 



But the valves arc solely made and instituted lest the 

 blood should pass from the greater into the lesser veins, and 

 cither rupture them or cause them to become varicose ; lest, 

 instead of advancing from the extreme to the central parts 

 of the body, the blood should rather proceed along the veins 



