174 



BLOOD-VESSELS. 



Fig. 114. — Diagrams showing 

 OF Veins. 



Valves 



The coats of the veins are supplied -with nutrient vessels, vasa 

 vasorum, in the same manner as those of the arteries. Nerves are 

 distributed to them in the same manner as to the arteries, but in far 

 less abundance. 



Vital properties. — Veins, when in a healthy condition, appear, like arteries, to 

 he almost devoid of sensibility. They possess vital contractility, which shows itself 

 in the same manner as that of the arteries, but is greatly inferior in degree, and 

 much less manifest. The muscular parts of the great veins, near the auricles of 

 the heart, on being stimulated, in recently-killed quadrupeds, exliibit quick and 



decided contractions, somewhat resemb- 

 Fi;;. 114. Ihig those of the auricles themselves. 



"Wliarton Jones discovered a rhythmic 

 pulsation in the veins of the bafs 'wing, 

 the pulsation occurring from ten to 

 twelve times in a minute ; and it is 

 worthy of note that the muscular tissue 

 of these vessels is non-striated as in 

 other veins. 



Valves. — Most of the veins are 

 provided with valves, a mechanical 

 contrivance beautifully adapted to 

 prevent the reflu.x of the blood. 

 The valves are formed of semi- 

 lunar folds of the internal coat, 

 strengthened by included connec- 

 tive tissue, which project oblique- 

 ly into the vein. Most connnonly 

 two such folds or flaps are placed 

 opposite each other (fig. 114, a); 

 the convex border of each (which, 

 according to Huller, forms a para- 

 bolic curve) is connected with the 

 side of the vein ; the other edge 

 is free, and points towards the heart, or at least in the natural 

 direction of the ciuTcnt of the blood along the vessel, and the two 

 flaps obliquely incline towards each other in this direction. More- 

 over the wall of the vein immediately above (or nearer the heart than) 

 the curved line of attachment of the valves, is dilated into a ]iouch or 

 sinus on each side (fig. 114, b), so that, when distended with blood or 

 by artificial injection, the vessel bulges out on each side, and thus gives 

 rise to the appearance of a knot or swelling wherever a valve is placed 

 (as in fig. 114, c). From the above description, it is plain that the valves 

 are so directed as to offer no obstacle to the blood in its onward flow, 

 but that, when from pressure or any other cause it is driven backwards, 

 the refluent blood, getting between the dilated wall of the vein, and the 

 flaps of the valve, will press them inwards until their edges meet in the 

 middle of the channel and close it up. 



The valvular folds are usually placed in pairs as above described : in the veins 

 of the horse and other large quadrapeds three are often found ranged round the 

 inside of the vessel ; but this rarely occurs in the human body. On the other 

 hand, the valves are placed singly in some of the smaller veins, and in large 

 veins single valves are not unfrequently placed over the openings of smaller 

 entering branches ; also in the right auricular sinus of the heart there is a single 



A. Part of a vein laid open and spread 

 out, with two pairs of valves. B. Longitu- 

 dinal section of a vein, showing the apposi- 

 tion of the edges of the valves in their closed 

 state. C. Portion of a distended vein, ex- 

 hibiting a swelling in the situation of a pair 

 of valves. 



