ARTERIES. 233 



coat is cellular in character, composed principally of 

 white fibrous tissues arranged in a reticulated manner ; 

 there are also found some elastic and muscular fibres. 

 The middle coat in the aorta and large vessels is a dense, 

 thick cylinder of yellow elastic tissue. As the vessels 

 become smaller in size the elastic fibres become mixed 

 with the involuntary muscular fibres, and in vessels of 

 the size of the ulnar the muscular tissue nearly entirely 

 replaces the yellow elastic fibres. The muscular fibres 

 encircle the vessel, and are also disposed in longitudinal 

 bundles. The internal or serous coat presents, like any 

 other serous membrane or sac, three layers : First, a free 

 or epithelial ; second, a middle or basement membrane ; 

 third, an outer or fibrous layer. The epithelial consists 

 of a single layer of flat epithelial cells accurately joined 

 together by an intercellular cement-substance resembling 

 gelatin ; here and there, however, a slight fissure be- 

 tween the cells can be seen. Second, a basement-mem- 

 brane consisting of a homegeneous membrane supported 

 by the skeletal, reticulated, connective-tissue cell-layer. 

 Third, a fenestrated membrane composed of white and 

 yellow elastic fibres. v 



As arteries become much reduced in size they lose 

 some of their physical characters, and when they are very 

 small and but two coats are distinguishable they take 

 the name of arterioles. While capillaries are the small- 

 est radicles of the arterial system, measuring on an 

 average about one three-thousandth of an inch in 

 diameter, they present, as a rule, but a single coat, 

 the inner, and frequently of this only the epithelial 

 layer exists. Thus, in the capillaries of the lungs, 

 brain, and liver the only structure presented by the 

 capillary is a single layer of epithelial cells jointed at 

 their edges. 



