THE VEINS. 



smallest arteries) traces of an epithelial lining, and of circular transverse fibres, 

 are to be seen. 



The Veins are composed of three coats, internal, middle, and external, as the 

 arteries are; and these coats are, with the necessary modifications, analogous 



Fig. 38. 



Fig. 39. 



Finest vessels on the arterial side. 1. Smallest artery. 2. 

 Transition vessel. 3. Coarser capillaries. 4. Finer capillaries. 

 a. Structureless membrane still with some nuclei, representative 

 of the tunica ailventitia ; b, nuclei of the muscular fibre-cells ; 

 c, nuclei within the small artery, perhaps appertaining to nn 

 epithelium ; d, nuclei in the transition vessels. From the 

 human brain. (Magnified 300 times.) 



An artery, .01'", and Z>, a vein, .015", 

 from the mesentery of a child (mag- 

 nified 350 times and treated with 

 acetic acid). The letters as in Fig. 

 37. i. The tunica media of the vein, 

 consisting of nucleated connective 

 tissue. 



to the coats of the arteries the internal being the epithelial, the middle the 

 fibrous, and the external the connective or areolar. The main difference be- 

 tween the veins and the arteries is in the comparative weakness of the middle 

 coat of the former; and to this it is due that the veins do not stand open when 

 divided, as the arteries do ; and that they are passive rather than active organs 

 of the circulation. 



In the veins immediately above the capillaries, the three coats are hardly to 

 be distinguished. The epithelium is supported on an outer membrane of nu- 

 cleated connective tissue, separable into two layers, the outer of which is the 

 thicker, the fibres of both being longitudinal. The interior thinner layer of 

 nucleated tissue is regarded by Kolliker as the analogue of the middle coat. 

 In the veins next above these in size (one fifth of a line, according to Kolliker) 

 a muscular layer and a layer of circular fibres can be traced, forming the middle 

 coat, while the elastic and connective elements of the outer coat become more 

 distinctly perceptible. 



In the middle-sized veins, the typical structure of these vessels becomes 

 clear. The epithelium is of the same character as in the arteries, but its cells 

 are more oval, less fusiform. It is supported by one or more layers of nucle- 

 ated fibrous tissue, arranged longitudinally, and external to this is a layer of 

 elastic fibrous tissue. This constitutes the internal coat. The middle coat is 



