PROPERTIES OF THE BLOOD-VESSELS. 95 



with the large trunks that open into it, the upper part of the inferior cava. Of course, in these 

 <ases the media is very thin. In the smallest veins the media is formed of fine connective- 

 tissue, with very few muscular fibres scattered in the inner part. 



(3) The t. adventitia is thicker than that of the corresponding arteries ; it contains much 

 connective-tissue, usually arranged longitudinally, and not much elastic tissue. Longitudinally 

 arranged muscular fibres occur in some veins (renal, portal, inferior cava near the liver, veins 

 of the lower extremities). The valves consist of fine fibrillar connective-tissue with branched 

 cells. An elastic network exists on their convex surface, and both surfaces are covered by 

 endothelium. The valves contain many muscular fibres (fig. 70). [Ranvier has shown that 

 the shape of the epithelial cells on the side over which the blood passes, are more elongated 

 than on the cardiac side of the valve, where the long axes of the cells are placed transversely.] 



The sinuses of the dura mater are spaces covered with endothelium. The spaces are either 

 duplicatures of the membrane, or channels in the substance of the tissue itself. 



Cavernous spaces we may imagine to arise by numerous divisions and anastomoses of tolerably 

 large veins of unequal calibre. The vascular wall appears to be much perforated and like a 

 sponge, the internal space being traversed by threads and strands of tissue, which are covered 

 with endothelium on their surfaces, that are in contact with the blood. The surrounding wall 

 consists of connective-tissue, which is often very tough, as in the corpus cavernosum, and it 

 not unfrequently contains non-striped muscle. 



Cavernous formations of an analogous nature on arteries are the carotid gland of the frog, 

 and a similar structure on the pulmonary arteries and aorta of the turtle, and the coccygeal gland 

 of man. The last structure is richly supplied with sympathetic nerve-fibres, and is a convoluted 

 mass of ampullated or fusiform dilatations of the middle sacral artery, surrounded and per- 

 meated by non -striped muscle. 



Vasa Vasorum. [These are small vessels which nourish the coats of the arteries and veins. 

 They arise from one part of a vessel and enter the walls of the same, or another vessel at a 

 lower level. They break up chiefly in the outer coat, and none enter the inner coat.] In 

 structure they resemble other small blood-vessels. The blood circulating in the arterial or 

 venous wall is returned by small veins. 



[Lymphatics. There are no lymphatics on the inner surface of the muscular coat, or under 

 the intima in large arteries. They are numerous in a gelatinous layer immediately outside the 

 muscular coat, and the same relation obtains in large muscular veins and lymphatic trunks 

 {Eoggan).] 



Intercellular Blood-Channels. Intercellular blood-channels of narrow calibre, and without 

 walls, occur in the granulation tissue of healing wounds. At first blood-plasma alone is found 

 between the formative cells, but afterwards the blood -current forces blood-corpuscles through 

 the channels. The first blood-vessels in the developing chick are formed in a similar way from 

 the formative cells of the mesoblast. 



Properties of the Blood- Vessels. The larger blood-vessels are cylindrical tubes 

 composed of several layers of various tissues, more especially elastic tissue and smooth 

 muscular fibres, and the whole is lined by a smooth polished layer of endothelium. One 

 of the most important properties is the conti actility of the vascular wall, in virtue of 

 which the calibre of the vessel can be varied, and therefore the supply of blood to 

 a part is altered. The contractility is due to the plain muscular fibres, which are, 

 for the most part, arranged circularly. It is most marked in the small arteries, and 

 of course is absent where no muscular tissue occurs. The amount and intensity of 

 the contraction depend upon the development of the muscular tissue ; in fact, the 

 two go hand in hand. [If an artery be exposed in the living body it soon contracts 

 under the stimulus of the atmosphere acting upon the muscular fibres.] 



[Action of Drugs on the Vascular System. Gaskell finds that a very dilute solution of lactic 

 acid (1 : 10,000 parts of saline solution), passed through the blood-vessels of a frog, always 

 enlarges the calibre of the blood-vessels, while an alkaline solution (1 part sodium hydrate to 

 10,000 saline solution) always diminishes their size, usually to absolute closure, and indeed the 

 artificial constriction of the blood-vessels may be almost complete. These fluids are antagonistic 

 to each other as far as regards their action on the calibre of the arteries. Dilute alkaline 

 solutions act on the heart in the same way. After a series of beats the ventricle stops beating, 

 the standstill being in a state of contraction. Very dilute lactic acid causes the ventricle to 

 stand still in the phase of complete relaxation. The acid and alkaline saline solutions are 

 antagonistic in their action on the ventricle. Cash and Brunton find that dilute acids have 

 a tendency to increase the transudation through the vessels and produce oedema of the surround- 

 ing tissues. They also observed that barium, calcium, strontium, copper, iron, and tin produce 

 contraction of the blood-vessels when solutions of their salts are driven through them, while 

 the same effect is produced by very dilute solutions of potassium. Nicotin, atropin, and 



