Motion of the Heart and Blood 79 



trunks of the veins ; the two for there are for the 

 most part two together regard each other, mutually 

 touch, and are so ready to come into contact by their 

 edges, that if anything attempt to pass from the trunks 

 into the branches of the veins, or from the greater 

 vessels into the less, they completely prevent it ; they 

 are farther so arranged, that the horns of those that 

 succeed are opposite the middle of the convexity of 

 those that precede, and so on alternately. 



The discoverer of these valves did not rightly under- 

 their use, nor have succeeding anatomists added any- 

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

 explained 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 upwards ; the valves, 

 in a word, do not invariably look upwards, but always 

 towards the trunks of the veins, invariably towards the 

 seat of the heart. I, and indeed others, have some- 

 times 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 [save at 

 their roots], and that dogs, oxen, &c., 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 divarica- 

 tions 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 



