OF THE VALVES OF THE VASCULAR SYSTEM IX VEHTEBRATA. 803 



auriculo-ventricular valves of the bird and mammal, that the spiral action of 

 the segments becomes most conspicuous ; the nature of the action being unavoid- 

 ably determined, by the unmistakably spiral arrangement of the muscular fibres 

 composing the ventricles, and by the spiral nature of the musculi papillares and 

 ventricular cavities. As, however, the action of these valves, has been already 

 explained at great length, further allusion to them at this stage is unnecessary. 



The valves of the vascular system of the vertebrata, as will be perceived 

 from this summing up, form a progressive and gradually ascending series ; the 

 valves in the veins exhibiting a lower type than those in the arteries ; the valves 

 in the arteries, being less fully developed than the valves occurring in the 

 bulbus arteriosus and in the auriculo-ventricular orifice of the fish ; the valves 

 in the fish, being less highly differentiated than the valves in the reptile and 

 bird ; these again falling short both in complexity and adaptive power to those 

 met with in the mammal. In the mammal the valvular arrangements may be 

 said to culminate. 



Descri2ytion of the Plates. 



Plate XXVIII. 

 Figs. 1 and 2. External Jugular Veins of Horse inverted. Show valves, consisting of two (d e), 



three (a b c), and four {/g h) segments. (See pp. 763, 764.) 

 Fig. 3. Section of External Jugular Vein of Horse. Shows valve, consisting of two segments (ah), 



with dilatations [g), corresponding to the sinuses of Valsalva, in the arteries. (See pp. 763, 



764, 767.) 

 Fig. 4. External Jugular Vein of Horse opened. To show the relations of the segments (a b) 



above (r e). (See p. 763.) 



Fig. 5, Portion of Femoral Vein distended with plaster of Paris. Shows dilatations (hg) in the 

 course of the vessel corresponding to the position of the valve. (See p. 765.) 



Fig. 6. Shows Venous Valve, consisting of two segments (ab), in action. (See p. 767.) 



Fig. 7. The same, not in action. (See p. 763.) 



Fig. 8. Venous Valve from External Jugular of Horse, consisting of three segments. (See p. 764.) 



Fig. 9. Venous Valve, consisting of one segment, situated at the entrance of a smaller into a larger 

 vein. (See p. 763.) 



Fig. 10. Venous Sinus from Auricle of Heart of Sturgeon. (See p. 782.) 



Fig. 11. Femoral Vein distended with plaster of Paris. Shows venous valves in action, where a 

 smaller vessel enters the larger one (a 6), and in tlie main trunk (a' 6'). (See pp. 767, 768.) 



Fig. 12. Vertical Section of Vein distended with plaster of Paris, Shows the nature of the union 

 between the segments (e). (See p. 767.) 



Fig. 13. The same, the section being carried between (e) instead of across or through the segments. 

 (See p. 767.) 



Figs. 14, 15, and 16. Show the Structure of the Venous Valves. (See p. 766.) 



Fig. 17. Section carried through Pulmonary Artery and Right Ventricle of Human Heart, between 

 the segments of the semilunar valves (5). Shows the variation in the thickness of the 

 vessel (a 6), and how it bifurcates (rr') at its origin. (See p. 770.) 



Fig. 18. A simihir section, carried through the middle of one of the segments (5). Shows how the 

 Pulmonary Artery (a 6) behind the segments diminishes in thickness in a direction from 

 above downwards (i). See p. 770.) 



