TISSUES OF THE BODY. 369 



/ 



unchanged. The former, however, may frequently acquire the characters of 

 a reticular elastic tunic, owing to incipient absorption at certain points ; 

 this is the so-called fenestrated membrane ( 127). Condensation may 

 also lead to the formation of an elastic network stretched in the direction 

 of the axis of the vessel. The middle layer consists of several strata 

 of transverse un striped muscle-fibres, laid one over the other. In the 

 outer, finally, the connective-tissue becomes fibrillated, and the corpuscles 

 of the latter unite to form fine elastic fibrous networks. 



We must be permitted here to refer, in a few words, to the umbilical 

 arteries (fig. 362, 1). These are remarkable for the extraordinary 

 development of their muscular middle coat (c). As tunica adventitia wo 

 find (d) a reticular connective substance, already seen in the gelatin of 

 Wharton (p. 191). The arteries of the ovaries likewise have very strong 

 muscular tunics. The latter may attain an enormous pitch of develop- 

 ment in the branches supplying the so-called corpus luteum. 



Trunks of more considerable magnitude, of about 2 mm. in diameter, for 

 instance, show in their internal coat an increase of elastic tissue, in addition 

 to which longitudinally striated layers may occur. There are likewise inter- 

 posed between the greatly thickened laminae of muscle fibres imperfectly 

 developed membranes of elastic nature, with webs of elastic fibres holding 

 an oblique course ; the latter attain also, in the outer tunic, a high pitch of 

 development. In vessels of larger diameter still these elastic networks are 

 developed more and more, especially internally, towards the tunica media. 



Turning now, finally, to the largest arterial trunks of the body (fig. 

 362, 2), we find, in the first place, that the internal layer (b) has increased 

 in thickness by multiplication of its elastic laminae. These latter present 

 themselves, in keeping with the variability of elastic tissue, either in the 

 form of membranes or of membranous networks stretched in the long 

 axis of the vessel, or again as fenestrated coats. More internally, close to 

 the epithelial layer, may be seen laminse, either homogeneous or longitu- 

 dinally striated, in which, as for instance in the ascending aorta, networks 

 of stellate cells exist, lying one over the other, as was discovered by 

 Langhans and confirmed by Ebner. In the middle coat (d, e) the mem- 

 branous character of the obliquely running elastic webs becomes more and 

 more marked. The latter may be very thick, or, again, fine and delicate, 

 and the whole present a fenestrated appearance, owing to absorption of 

 the interstitial connecting substance. As a rule, these membranous elastic 

 layers, whose number may amount to from thirty to fifty and upwards, 

 are interleaved with tolerable regularity with the lamina of the muscular 

 substance (e). The latter presents itself in varying degrees of perfectness, 

 and is frequently but slightly developed, which may depend upon the high 

 degree of development of the elastic intermediate layers : its direction, like- 

 wise, is by no means always the same. In the outer portions of the middle 

 tunic fibrillated connective-tissue is also to be found (Schultze, von Ebner\ 

 In the most external coat (g) the elastic networks are frequently more and 

 more strongly marked at its inner portion (/), so that among the larger 

 mammals, as, for instance,. in the whale, they furnish one of the strongest 

 examples of elastic tissue that can be met with. As an exception, smooth 

 muscle may also make its appearance in the internal coat of human 

 arteries. Corresponding muscle elements to those we have described as 

 occurring in the external layers of the veins appear to be entirely absent 

 in the arteries of the human body. 



Commencing even in the smaller twigs, the blood-vessels are supplied 



