THE BLOOD-VESSELS. 289 



into the right portion of the septum. The heart, consequently, 

 might be described as composed of two muscular sacs; the 

 thinner of which is common to the whole ; the other and 

 thicker belonging only to the left division, and to some extent 

 being interposed between the layers of the former. To the latter 

 would belong the entire septum, and the middle muscular sub- 

 stance of the left ventricle ; to the former, the superficial layers 

 with their continuations in the innermost muscular strata, and 

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



2.— OF THE BLOOD-VESSELS. 



§214. 



As regards their structure, the blood-vessels are divided into 

 arteries, capillaries, and veins; but these three divisions are by 

 no means separated by definite limits, inasmuch as the capil- 

 laries are continuous with the veins on the one hand, as imper- 

 ceptibly 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 conformity 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 the tunica 

 intima, t. media, and t. externa s. adventiiia. 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 con- 

 nective tissue, and even the transversely striped muscular tissue 

 are also represented in them; besides which, there exist epithelia, 

 peculiar homogeneous membranes, vessels, and even nerves; so 

 that we have presented in them a complexity of structure 

 which renders a general description almost impossible, and 

 which can be made clear only by an accurate examination of 

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



ii. 19 



