154 



MANUAL OF HISTOLOGY. 



but in strata. They were observed in particular vessels of 

 young animals. It seems likely that these cells disappear or 

 shrivel with the growth of the individual, but their sudden reap- 

 pearance in pathological processes leads the author to believe 

 that at least some of them persist through life. Talma ( Vir- 

 cTiow's Arch., Vol. LXXVIL, pp. 242-269) observed similar 

 elements, but thinks they are derived from the ordinary en- 

 dothelia, instead of vice versa. He is also convinced that the 

 latter are merely modified leucocytes ; but this view has been 

 shown to be erroneous by Virchow (ArcTiiv f. path. Anat., 

 Vol. LXXVIL, pp. 380-383). Endothelial desquamation is 

 probably, as already stated, a physiological process of constant 



'<SP 



^ > 



-.^-, ^,^:^,;,, 



- ^ <s^ " ,^ 



e, ^/^^ 1 m,b 



FlG. 68. Transver-e section through small artery and vein : A, artery ; a, intima with bulging en- 

 dothelial cells, the vessel being drawn in a state of contraction ; ft, internal elastic coat, wavy for same 

 reason ; c, media ; d, adventitia. B, vein, same denominations. 



occurrence, and in some respects analogous to the epithelial 

 shedding from the surface of the skin and mucous membranes. 

 The media musculosa, or middle coat, consists of superim- 

 posed layers of smooth muscle-elements disposed in groups. 

 Most of them lie transversely to the course of the vessel. The 

 intervals between neighboring groups are occupied by connec- 

 tive tissue and elastic fibres, arranged in networks. This inter- 

 stitial substance becomes augmented with the increasing calibre 

 of the artery. In the largest trunks it all but replaces the 

 muscle-cells. Here, however, the elastic fibres also reach their 

 maximum development, encroaching upon the connective-tissue 

 elements until the latter become quite inconspicuous. Besides 

 its principal transverse layer, the media also contains fusiform 

 muscle-cells, placed in an oblique or longitudinal direction. 



