338 



EPITHELIAL AND ENDOTHELIAL TISSUE. 



bundles are separated by broad layers of fibrous connective 

 tissue. A relatively large amount of muscle is found in the 

 middle coat of the veins of the spermatic cord. (See Fig. 145.) 

 The muscle coat is almost completely wanting in the system 

 of the Y. cava descendens, in the veins of the nerve centers, and 

 also in those of the bones. Here the middle coat is composed of 

 delicate fibrous connective tissue, with more or less elastic fibers, 

 running in a circular or oblique direction. The venous adven- 

 titial coat is likewise less developed than the arterial, but contains 

 a comparatively greater number of longitudinal bundles of 

 smooth muscle, so much so that in the V. portae and V. renalis 

 these bundles form an almost continuous layer. In the adven- 

 titial coat of the veins which empty into the heart, near their 

 cardiac orifices, we find a varying amount of striated muscle-fibers, 

 in a reticular arrangement, which are prolongations of the muscle 

 of the heart. 



c 



FIG. 145. BLOOD-VESSELS FROM THE LOOSE ADVENTITIAL CONNECTIVE 

 TISSUE OF THE SPERMATIC CORD OF MAN. 



A, artery, with E, endothelial and covering hyaline layer; M, smooth muscle layer; 0, 

 adventitial layer; V, vein, exhibiting the same layers as the artery; CV capillary blood- 

 vessels, composed of a single endothelial layer, with a surrounding delicate adventitial 

 fibrous connective tissue. Magnified 200 diameters. 



The valves of the veins are reduplications of the inner coat, 

 with a well-defined elastic reticulum, especially on the convex 

 surfaces, and with a complete endothelial investment. 



The cavernous bodies are veins or sinuses of greatly varying 

 calibers, freely anastomosing with each other; they are en- 

 sheathed by a dense connective-tissue capsule, which sends 



