VEINS OF MAMMALIA. 551 



the probe be now introduced from the extreme towards the more 

 central parts, the valves, like the floodgates of a river, give way, 

 and are most readily pushed aside.' l 



The most conspicuous tissue in these valves is of the white 

 fibrous kind, having a general direction parallel with the free 

 border of the valve and remarkable for their minute and equal 

 undulations in this course. The epithelial lining is feebly indi- 

 cated by scattered nuclei, especially at the extreme margin of the 

 valve. As a general rule the cerebral and myelonal veins, those 

 of the heart, lungs, kidneys, uterus, and liver, both portal and 

 hepatic, have no valves : there are few valves in the external 

 jugular ; none in the internal jugular : valves abound in the veins 

 of the pectoral limb, including the axillary, but are not present in 

 the subclavian or precavals. The postcaval and iliac veins have 

 no valves, but they abound in those of the pelvic limb. The 

 spermatic veins have valves, but not the ovarian veins ; they are 

 few and incomplete where they have been found in the azygos 

 veins. 



The varieties in the disposition of the veins exemplify, in mam- 

 mals, a greater degree of repetition of primary or embryonal steps 

 than do those of the arteries. The cardinal veins, which persist 

 in great proportion in Lizards, fig. 420, unite at v x with the 

 brachio-jugulars to form a short ' precaval ' trunk 2 on each side. 

 That of the left receives the blood from the coronary vein before 

 terminating in the right auricle h. A smaller proportion of blood 

 is returned by the persistent venae cardinales, z, z, in mammals ; 

 but with this exception, the disposition of the great trunks return- 

 ing the blood from the head, trunk, and pectoral limbs, is essen- 

 tially such in Lyencephala and most Lissencephala as is exemplified 

 in Sauriaiis. The blood from the left side of the trunk (intercostal 

 or intervertebral spaces) is carried, in the ascending series of 

 Mammals, by progressively increasing anastomosing channels from 

 the left into the right cardinal vein : and, to such an extent in 

 Man, that the right cardinal vein was noted for its want of sym- 

 metry as the ' vena azygos,' while the remnant of the left cardinal 

 was called ' hemi-azygos.' With this change goes on an enlarge- 

 ment of an anastomosing vein between the right and left precavals 

 at the upper and fore part of the chest, ultimately diverting the 

 blood from the left precaval into the right, as the blood of the left 

 cardinal had been attracted to the right cardinal vein. This is 



o 



accompanied by obliteration of the left precaval trunk, of which a 



1 cciv". 2 Called by some ' cluctus Cuvieri,' 



