TISSUES OF THE BODY. 367 



coat or not. The second layer (d) presents itself in the form of a streaky 

 fibrous tissue envelope with elongated nuclei and connective-tissue cor- 

 puscles. 



If we compare with this an arterial branch (1), we find again the 

 two coats (b and d) just described ; but between the inner membrane 

 and outer tunic of connective-tissue there now appears a layer of trans- 

 versely arranged contractile fibre cells lying side by side (c), whose elongated 

 nuclei present themselves in transverse sections of the vessel as encircling 

 the latter. This tunic is of varying strength. In other arterial twigs it 

 may appear with greater distinctness, either as a single or multiple layer. 

 Fig. 361 represents this in a side view, and fig. 359, d, the transverse 

 section of a small artery with laminated muscular coat, and an adventitia 

 consisting of reticular connective-tissue. The epithelium cells are narrower 

 here than in the veins, hut much more elongated in the direction of the 

 vessel, and fusiform. 



204. 



Thus far is it possible to subject the blood-vessel as a whole to micro 

 scopical analysis. Larger tubes must be examined in their various parts, 

 either by rending the walls, peeling off layers with a forceps, the vessel 

 having been slit up, or by preparing sections of the previously dried or 

 hardened wall. 



The further changes from the next in order to those occurring in the 

 formation of the largest vessels, consist in this, that all the layers, with 

 the exception of that formed of endothelial cells, which remains single, 

 commence to become more and more laminated, especially the internal 

 and middle, thus collectively bringing about an increase in the whole 

 thickness of the vessels. The internal strata preserve in their systems of 

 membranes arranged in laminae one over the other their elastic character, 

 presenting every variety of elastic tissue in longitudinal arrangement. 

 The middle coat is transformed into a system of laminae of smooth muscle 

 fibres, connective and elastic tissue, with a transverse direction. The 

 external tunic, finally, becomes thicker and thicker in its connective sub- 

 stance with an ever increasing development of elastic networks. Tig. 

 362 represents at (1) the umbilical artery of a foetus of eight months old 

 in transverse section, and at (2) a large artery from the adult similarly 

 cut, and gives us for the present an idea of the arrangement of parts in 

 question. The distinction between the different coats becomes, however, 

 less and less apparent at the same time. We must, however, bear in 

 mind that the coats of veins are thinner than those of arteries of corre- 

 sponding calibre, a circumstance which depends upon the minor develop- 

 ment in them of the middle tunic. The endothelial cells of venous 

 vessels preserve everywhere the same short broad figure already men- 

 tioned in the foregoing . 



Small veins, merely higher grades of development of such a vessel as 

 that represented in fig. 360 (2), commence much later to acquire a muscular 

 layer than arteries of the same magnitude. A venous vessel, for instance, 

 of 0'23 mm. in diameter offers for our consideration an internal membrane 

 in which may be observed elastic longitudinal interlacements, a few 

 laminae of muscle-fibres in the middle coat, intermixed with elastic net- 

 works and layers of connective-tissue, and, finally, an external thicker 

 coat, formed of fibrillated connective-tissue and elastic fibres. 



In medium-sized veins the internal coat consists of either one or several 



