clxxiv 



BLOOD-VESSELS. 



Other veins present peculiarities of structure, especially in respect of muscularity. 

 1. The striated muscular fibres of the auricles of the heart are prolonged for some way 

 on the adjoining part of the venae cavae and pulmonary veins. 2. The plain muscular 

 tissue is largely developed in the veins of the gravid uterus, and is described as being 

 present in all three coats. 3. On the other hand, muscular tissue is wanting in the 

 following veins, viz., a, those of the maternal part of the placenta ; b, most of the veins 

 of the brain and pia mater ; c, the veins of the retina ; d, the venous sinuses of the 

 dura mater ; e, the canceUar veins of the bones ; /, the venous spaces of the corpora 

 cavernosa. In most of these cases the veins consist merely of an epithelium and a 

 layer or layers of connective tissue more or less developed ; in the corpora cavernosa 

 the epithelium is applied to the trabecular tissue. It may be added that in the 

 thickness of their coats the superficial veins surpass the deep, and the veins of the 

 lower limbs those of the upper. 



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

 in the same manner as those of the arteries. Nerves have not been demon- 

 strated in the coats of veins generally, but small branches have been traced 

 on some of the larger veins. 



Vital properties. Veins, when in a healthy condition, appear to be 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, exhibit quick and decided contractions, somewhat 

 resembling those of the auricles themselves. Mr. Wharton Jones has discovered a 

 rhythmic pulsation in the veins of the bat's wing, the pulsation occurring from ten to 

 twelve times in a minute ; and it is worthy of note that the muscular tissue of these 

 veins appears to be of the plain or unstriped variety. 



Valves. Most of the veins are provided with valves, a mechanical con- 

 trivance beautifully adapted to prevent the reflux of the blood. The valves 



are formed of seiniluuar folds of the 

 lining membrane, strengthened by 

 included counectivetissue, which pro- 

 ject obliquely into the vein. Most 

 commonly two such folds or flaps 

 are placed opposite each other (fig. 

 xcm. A) ; the convex border of each, 

 which, according to Haller, forms a 

 parabolical 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 current of the blood along the 

 vessel, and the two flaps obliquely 

 incline towards each other iu this 

 direction. Moreover, the wall of 

 the vein immediately above (or 

 nearer the heart than) the curved 

 line of attachment of the valves, is 

 dilated into a pouch or sinus on each 

 side (fig. xciii. B a), so that, when 

 distended with blood or by artificial 



Fig. XCIII. 



Fig. XCIII. DIAGRAMS SHOWING VALVES 

 OP VEINS. 



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. 0. Portion of a distended vein, ex- 

 hibiting a swelling in the situation of a pair 

 of valves. 



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. c). From the above description, it is plain that 

 the valves are so directed as to offer no obstacle to the blood in its 



