674 SPECIAL HISTOLOGY. 



mon to the whole ; the other and thicker belonging only to the left di- 

 vision, and to some extent being interposed between the layers of the 

 former. To the latter would belong the entire septum, and the mid- 

 dle muscular substance of the left ventricle ; to the former, the superfi- 

 cial layers with their continuations in the innermost muscular strata, 

 and especially the whole of the free portion of the right ventricle. 



2. OF THE BLOODVESSELS. 



214. As regards their structure, the bloodvessels are divided into 

 arteries, capillaries, and veins ; but these three divisions are by no means 

 separated by definite limits, inasmuch as the capillaries are continuous 

 with the veins on the one hand, as imperceptibly as they commence from 

 the arteries on the other. At the same time it is true that both kinds 

 of larger vessels, although in their rudiments presenting a general con- 

 formity of structure, are still sharply and definitely distinguished in 

 many respects. 



Concerning the tissues which enter into the composition of the vessels, 

 and the mode of their arrangement, the following general remarks may 

 be made. Whilst the true capillaries possess only a single perfectly 

 structureless coat, in the larger vessels, with few exceptions, the number 

 of tunics is increased to three, which may most suitably be described as 

 tunica intima, t. media, and t. externa s. adventitia. In these tunics 

 there are found, of the fibrous tissues of the body, in the first place, the 

 elastic and smooth muscular tissues, but the connective tissue, and even 

 the transversely striped muscular tissue are also represented in them ; 

 besides which, there exists epitlielia, peculiar homogeneous membranes, 

 vessels, and even nerves ; so that we have presented in them a com- 

 plexity of structure which renders a general description almost impossi- 

 ble and which can be made clear only by an accurate examination of each 

 particular element ; and the rather so, because the more extensively 

 distributed tissues assume very different forms. With respect to the 

 arrangement and subdivision of these tissues, they may be said to exhibit 

 a very strong tendency to lamination and, in tLe different layers, to the 

 assumption of a constant direction in the course of their constituent 

 elements. The former of these dispositions, however, rarely extends to 

 the actual isolation of the individual layers ; and to the latter, though 

 more rarely, there are also exceptions. The tunica intima is the 

 thinnest of the membranes of the vessels, and always consists of a cel- 

 lular layer, the epithelium ; most usually also of an elastic membrane 

 in which a longitudinal direction of the fibres predominates ; to which 

 again may be superadded other layers of one kind or another, which 

 also almost invariably retain the longitudinal direction. The t. media 

 is for the most part a thick layer, and is especially the seat of the trans- 



