336 EPITHELIAL AND ENDOTHELIAL TISSUE. 



broadest diameter at the seat of the nucleus. The separating 

 cement-substance is best demonstrated by the injecting of gela- 

 tine, to which a two per cent, solution of nitrate of silver is added. 

 Upon treatment with distilled water and exposure to daylight, the 

 lines of the cement-substance appear dark brown in color and 

 pierced by numerous tranverse light lines, which (see page 125) 

 correspond to the connecting filaments of living matter (" thorns" 

 or "prickles"). Not infrequently dark brown dots or delicate 

 rings are visible along the brown line of the cement-substance, 

 varying greatly in number in different localities ; these are the 

 so-called stigmata. 



In the smallest arteries, the arterioles, there is an extremely 

 delicate structureless membrane, next to the endothelia. The 

 two together have a fluted contour, and the cement-substance a 

 zig-zag appearance, evidently on account of the contraction of 

 the middle or muscle coat. In larger arteries, the structureless 

 membrane is found to be considerably broader and f enestrated 

 i. e., with numerous, irregularly distributed perforations ; in 

 still larger arteries, the fenestrated membrane is replaced by a 

 dense reticulum of elastic fibrillaB. 



In the largest arteries two more or less distinct layers are 

 found. over the endothelium; the inner being the hyaline or 

 elastic membrane proper, the outer a reticulum of elastic fibrillaB. 



The middle coat is composed of smooth, spindle-shaped 

 muscle-fibers, which in the terminal arterioles are present in one 

 layer only, twined around the vessel. In longitudinal sections 

 of arterioles, these fibers are easily recognized by their transverse 

 sections along the wall outside the endothelium. The nucleus 

 of the fusiform muscle-fiber, which is rod-like in front view, 

 becomes visible in the transverse section only when the middle, 

 broadest portion of the spindle shows its optical section. As the 

 caliber of the artery enlarges, the muscle spindles become aug- 

 mented, being very numerous in the larger arteries. Here, also, 

 a longitudinal layer of muscle spindles becomes apparent, out- 

 side of the circular layer; but the longitudinal layer nowhere 

 reaches a high degree of development. In the middle coat of 

 larger arteries the bundles of smooth muscles are separated by 

 elastic fibrillaB, and in the largest arteries by continuous mem- 

 braneous formations of elastic substance, which attain their 

 highest development in the aorta. In this situation the lamella? 

 usually take an oblique course, and are connected uninterrupt- 

 edly with the elastic reticulum above and below the muscle coat. 



