156 Mammalia. 



receives all the Intercostal veins of its own side, and the lower 

 right Intercostal veins also : its lower part is alwaj^s dilated 

 and muscular : the Coronary vein opens into it, as also a series 

 of small veins from the back of the heart, the mouths of which 

 are all provided with valves, while, on the contrary, no valves 

 exist along the continuation upwards of the large venous 

 trunk (azygos), at least in the neighbourhood of the Heart. 

 We find then in the Sheep, as in most large Quadrupeds, a 

 right Yena Cava Superior and a left Azj^gos Venous Trunk, 

 instead of a right Yena Cava Superior and a left Cardiac 

 Yenous Trunk or Coronary Sinus as in Man. The size of the 

 left Azygos is complementary to that of the right : hence we 

 find the right Azygos in the Sheep is insignificant ; whereas 

 in Man, in whom the left Azygos is all but obliterated*, the 

 right Azygos is large. A further correlation of growth may 

 be noted, viz. that whereas the Thebesian valve is present in 

 every instance in which the left venous trunk forms a ' Coron- 

 ary Sinus ' receiving veins from the heart alone (as in Man) ; 

 it is certainly absent in those animals which have a left Azygos 

 or left Superior Cava, being represented in the Sheep merely 

 by a slight ridge of the Auricular Parietes. Situated superi" 

 orly to the dilated portion of the Left Azygos may be seen 

 the cut orifices of the Pulmonary Veins, which return the 

 blood from the Lungs and are without valves. Hence we see 

 that the Auricular portion of the Heart is immediately con- 

 nected with the great Yeins : and the ventricular portion with 

 the great Arterial trunks. 



* The remnant of the obliterated left Azygos trunk of the Foetus exists in the 

 adult, as (i.) an oblique Auricular yein running in a direct course from the back 

 part of the left Auricle to open into the ' Coronary Sinus,' unprotected by any 

 valve at its mouth, (ii. ) lines or streaks continued upwards from this vessel on 

 the wall of the right Auricle, (iii.) a further prolongation in the shape of a small 

 duplicature of the serous layer of the pericardium, passing between the left 

 Pulmonary artery and the subjacent Pidmonary vein, termed by Marshall the 

 Vestigial Fold of the Fericardiiim, 



