132 STRUCTURE AND PROPERTIES OF THE BLOOD-VESSELS. 



interior traversed by trabeculae or threads. The surface directed toward the 

 blood is covered with endothelium. The investing wall consists of connective 

 tissue, which is often quite firm and tendinous, as in erectile tissue. It not infre- 

 quently contains unstriated muscle-fibers. 



An example of an analogous cavernous formation in arteries is found in the 

 coccygeal gland of man. This mysterious structure, which is richly supplied with 

 sympathetic nerve-fibers, consists of nucleated connective tissue and represents a 

 convolution of ampulliform or spindle-shaped dilatations of the median sacral 

 artery, traversed and surrounded by unstriated muscle-fibers. 



The vasa vasorum do not differ in structure from other vessels of similar caliber. 



Intercellular blood-channels devoid of walls are present in the granulation-tissue 

 of wounds. At first nothing but blood-plasma is found between the constituent 

 cells, and it is not until later that blood-cells are driven through the channels by 

 the blood-current. In the incubated egg the primary basis of the blood-vessels 

 is formed in a manner similar to that of the formative cells of the germinal layer. 

 The blood-vessels without walls in the bone-marrow and in the spleen are con- 

 sidered on p. 43. 



Among the properties of blood-vessels their contractility should be 

 mentioned first, that is, the ability to contract by virtue of the unstriated 

 muscle-fibers contained in their walls. The intensity ' and force with 

 which this contraction takes place are proportional to the degree p of 

 development of the muscle-tissue. 



Heat causes contraction of the blood-vessels (in the mesentery of the frog) . 

 Excised arteries contract when filled with dilute alkaline solutions, digitalin, 

 atropin, and antiarin. The isolated apex of the heart also beats more freely in 

 alkaline solutions. When the vessels are filled with a dilute solution of lactic acid 

 they dilate, and the apex of the heart when immersed in such a solution also 

 beats more rapidly. According to Roy, blood-vessels undergo shortening under 

 the influence of heat, if precautions are taken to prevent evaporation and the 

 load remains the same. 



If blood containing an admixture of certain substances such as amyl 

 nitrite, chloral hydrate, morphin, quinin, and atropin is allowed to flow through 

 the vessels of a recently excised, living organ, dilatation takes place; urea 

 and sodium chlorid have the same effect on the renal vessels; while digitalin 

 and veratrin cause contraction. 



The capillaries also possess the power of dilating and contracting, 

 derived from the protoplasmic granules of the cells of which they are 

 composed. 



The capillaries have been designated "protoplasm in tubular form," and 

 motor phenomena have been observed in them, especially after irritation in the 

 living animal. Strieker observed this chiefly in the capillaries of young frog- 

 spawn. At a later period of the animal's life the reaction of the capillaries to 

 stimuli is much less distinct. Rouget observed the same phenomena also in new- 

 born mammals. Similar observations have been made by Golubew and Tarchanoff. 

 Accordingly, the shape of individual cells varies with the quantity of blood con- 

 tained in the vessels. In greatly distended vessels the cells are flat; but when 

 the vessel is collapsed, the cells are more cylindrical and project into the lumen. 



Among the physical properties of blood-vessels their elasticity 

 should next be mentioned. The elasticity is slight, that is, the vessels 

 offer little resistance to the distending forces, such as pressure or trac- 

 tion; but it is, at the same time, complete, that is, after the distending 

 force has ceased to act, the vessels regain their previous form. 



According to Ed. Weber, Wertheim and A. W. Volkmann, the length of blood- 

 vessels (like that of moist portions of the animal body generally) does not increase 

 in proportion to the weight employed to extend it, but the elongation is considera- 

 bly less with progressive increase in the weight. Hence the extensibility of the 

 dead artery is greatest when it has been slightly distended by intravascular pres- 



