222 



ARTERY. 



The parietes of arteries are divisible into three 

 tunics, known by the names of external, mid- 

 dle, and internal. 



The external tunic, called the cellular coat, 

 (tunica ccllulosa propria of Haller,) is of a 

 whitish colour, thin, dense, and firm : it is 

 formed of condensed cellular tissue, containing 

 fibres closely interwoven and crossing each 

 other at obtuse angles to the length of the 

 vessels. The structure of this tunic is loose 

 on its external surface, and connected by deli- 

 cate laminae with the arterial sheath : its internal 

 surface is very closely attached to the external 

 surface of the middle tunic. 



The middle tunic of the arteries (the tunica 

 musculosa of Haller) is dense, firm, of a red- 

 dish yellow colour, and composed of fibres, 

 which, on a superficial view, seem to run 

 transversely : when this tunic is submitted to 

 a closer examination, we find that none of its 

 fibres are sufficiently long to form perfect rings 

 encircling the whole of the circumference of 

 the vessels; they are all short and straight, 

 with a slight degree of obliquity in their direc- 

 tion, and their extremities are lost among the 

 neighbouring fibres. The middle tunic may 

 be divided into several layers by the knife of 

 the anatomist, and these are found to increase 

 in density from the external to the internal 

 surface. There are no longitudinal fibres in 

 this structure. 



As Haller has remarked, the middle tunic 

 of the arteries is not continuous with the mus- 

 cular substance of the heart. For the descrip- 

 tion of the manner in which the middle tunic 

 of the arteries is connected with the heart, and 

 of the fibrous structure interposed between 

 the muscular texture of that organ and the 

 middle tunic of the arteries, we refer to the 

 article AORTA. The continuity of the middle 

 tunic through all parts of the arterial system 

 is uninterrupted. Although the absolute thick- 

 ness of this tunic is greatest in the aorta and 

 larger trunks, its thickness in proportion to 

 the area of the vessels manifestly increases as 

 these diminish in size ; wherever an artery is 

 curved, it is thicker on the convex than on the 

 concave side, and in all the angles formed by 

 the divisions of arteries its thickness is more 

 considerable than in other situations. The 

 colour of the middle tunic is yellower in the 

 larger trunks and more of a reddish tint in the 

 smaller branches. The middle tunic of the 

 arteries has a degree of firmness sufficient to 

 preserve the circular form of the artery even 

 in its empty state, and after the other tunics 

 have been removed. This tunic possesses a 

 slight degree of strength and elasticity in the 

 longitudinal direction; in the circular direc- 

 tion it exhibits both these properties in a more 

 marked degree. The strength and elasticity 

 of this tunic diminish progressively from the 

 larger to the smaller arteries. There is so 

 close a resemblance between the substance of 

 this tunic and the yellow elastic fibrous tissue 

 of the ligamenta subflava connecting the crura 

 of the vertebrae, as well in its yellow colour 

 and the firmness of its fibres, as in its elastic 

 property, that many anatomists regard both these 

 structures as being nearly if aot perfectly 



identical. Mr. Hunter instituted a variety of 

 experiments to prove that this tunic possessed 

 a power of contraction similar to that of mus- 

 cular structure in addition to its elasticity ; 

 but, notwithstanding the results of the re- 

 searches of this great anatomist and physio- 

 logist, by which he showed, in the clearest 

 manner, that the arteries were endowed with 

 a power of contraction totally distinct from 

 their property of elasticity, he never demon- 

 strated, in a positive and unequivocal manner, 

 the presence of muscular fibres in it, nor has 

 any other anatomist, who, since his time, may 

 have investigated the subject of the structure 

 of this tunic, been more successful in dis- 

 covering in it any decided trace of muscular 

 fibres. Beclard* considers it to be a pecu- 

 liar elastic tissue having an intermediate charac- 

 ter between muscular and ligamentous fibre. 



From carefully examining this structure, it 

 appears to differ both from the yellow elastic 

 fibrous tissue and from the muscular tissue ; 

 possessed of the elasticity of the former, but 

 differing from it in being composed of fibres 

 of a softer consistence and more easily torn ; 

 from the latter it differs not only in the colour 

 and consistence of its fibres, but moreover in 

 the slow and gradual mode of its contraction 

 under the influence of mechanical or chemical 

 stimuli ; unlike the muscular fibre, it retains its 

 power of resistance as perfectly in the dead as 

 in the living body. 



Bichatf asserted that there was a total ab- 

 sence of cellular tissue in the structure of the 

 middle tunic of arteries. Meckel, who ranks 

 higher as an authority for matters of fact in 

 anatomy, has admitted this assertion as if it 

 were an established fact: neither of these 

 authors, however, has advanced a single valid 

 argument or brought forward a well-founded 

 proof in support of the correctness of this 

 statement; wherefore we feel the less reluc- 

 tance in registering our dissent from such high 

 authorities on this point, which we found on the 

 consideration of the following circumstances :- 

 First, there is no analogous instance of an 

 organized structure receiving bloodvessels and 

 nerves into which cellular tissue does not also 

 enter as a component part. 



Secondly, we have the authority of the 

 accurate and learned Haller, in testimony of 

 the fact of the fibres of the middle tunic of 

 the arteries having cellular tissue interposed 

 between them, being, as he expresses himself, 

 " cellulositate.paucissima separatae." Beclard 

 entertains a similar opinion founded on the 

 circumstance that when a portion of an artery 

 is stripped of its external tunic, granulations 

 will shoot up from the exposed surface of the 

 middle tunic. 



Thirdly, we have frequently observed that, 

 when a portion of an artery stripped of its 

 external tunic, is divided longitudinally and 

 macerated in water for several days, the mid- 

 dle tunic increases in thickness, and its fibres 

 become more distinct and are more easily 

 separated from each other ; by continuing the 



* Anatomic Generate, p. 325. 

 t Anatomic Generale, torn. iii. 



