THE CIRCULATORY SYSTEM. 



c 



posed muscular tissue within the adventitia of other vessels (su- 

 perior mesenteric, splenic, renal, 

 and iliac arteries). 



In passing from medium-sized ar- 

 teries towards smaller vessels, the 

 coats become reduced in thickness, 

 the media being earliest affected. 

 The intima of the smallest ar- 

 terioles consists of an endothelial 

 layer alone, the middle coat in- 

 cludes but a single layer of muscle- 

 cells, while the external tunic is re- 

 duced to a few longitudinal bundles. 

 The vessels intermediate between 

 small arteries and true capillaries 

 no longer possess a complete layer 

 of muscle-cells, the media being 

 represented in such arterioles by 

 scattered groups of circularly placed 

 spindle-cells, forming an imperfect 

 muscular sheet, which partially en- 

 circles the vessel. The nuclei of 

 these circular muscle-cells are trans- 

 versely placed, while those of the 

 endothelial plates are usually longi- 

 tudinal or parallel with the axis of the vessel. 



A, small human artery, in which the coats 

 are reduced each to a single layer of cells ; 

 the media here consists of only one layer of 

 muscle-cells (;), which are seen in optical 

 section : /', intima ; a, adventitia ; e, nuclei 

 of the endothelial plates. B, an arteriole 

 just before becoming a capillary ; the vessel 

 still possesses muscle-cells (m), but these 

 are now arranged as irregular groups. C, 

 true capillary vessel, consisting of only an 

 endothelial coat, the other tunics having 

 disappeared ; the nuclei are those of the 

 endothelial plates. 



THE VEINS. 



The veins possess the same tunics as the arteries, but, in general, 

 are characterized by thinner walls and a preponderance of connective 

 over the muscular and elastic tissues. There is, further, less regu- 

 larity and constancy in the structure of the coats. 



The inner layer of the intima consists of a single layer of endo- 

 thelial cells, rather broader and more polyhedral in form than those 

 lining the arteries, the spindle shape being best marked in the 

 smaller veins. The subendothelial tissue contains numerous con- 

 nective-tissue corpuscles, and, in the larger veins, is arranged in 

 distinct lamellae. An inner elastic membrane is generally present, 

 in some cases taking the form of a fenestrated layer. 



The media consists of circular bundles of muscle-cells, associated 

 with lamellae of fibro-elastic tissue in the larger veins. This coat is 

 best developed in the veins of the inferior extremities, less so in 

 those of the upper limbs. The muscle-tissue of the veins is sub- 

 ject to many variations, both in amount and in arrangement, that 



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