MINUTE ANATOMY OF THE BLOODVESSELS. 265 



muscular elements, which are frequently arranged in layers, but 

 are often also irregularly combined into a tunic, that in opposi- 

 tion to the internal cellular membrane may be called the 

 external vascular coat or investing membrane. The thick- 

 ness of this membrane does not increase proportionally to 

 the diameter of the vessel, as there are wide vessels with 

 very thin, and small vessels with comparatively thick coats. 

 Amongst the Invertebrata, as in snails and mussels, even the 

 large lacunar blood spaces which surround the viscera are 

 bounded only by a very delicate cellular membrane, which 

 invests the various organs as an external epithelial tissue, 

 similar to the epithelium of the peritoneum. 



The smaller vessels have thicker walls in comparison with 

 the larger, but the several components of the wall do not parti- 

 cipate to an equal extent in producing this increase of thickness. 

 It is chiefly effected by an augmentation of the muscular tissue, 

 which becomes abundant in proportion to the diminution in 

 the quantity of the elastic and connective tissue. 



The tissues which form the investing tunics are arranged in 

 layers, the thickness of which, as well as the order of their 

 succession, undergo many variations. 



The investing layer is limited internally by an elastic mem- 

 brane termed the internal elastic coat. The external surface 

 of this membrane is covered by a muscular layer composed of 

 smooth muscular fibres, which are partly arranged in a circular 

 and partly in a longitudinal direction. This layer is termed 

 the middle coat in consequence of the position it occupies 

 between the elastic coat on the one hand, and the external coat 

 or tunica adventitia, composed chiefly of connective tissue and 

 elastic fibres, on the other. 



To these tunics must still be added a fourth connective tissue 

 layer the internal tunic or internal longitudinal fibre layer, 

 which lies between the endothelium and the elastic internal 

 coat, and which I shall term the intermediate layer. In the 

 arteries it is only present in the larger vessels, and is gradually 

 lost towards the periphery. In the veins it attains its maxi- 

 mum in some of the peripheral vessels, and diminishes towards 

 the heart, so that it is almost entirely absent in such large 

 vessels as the vena cava. 



