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.
Bichatt asserted that there was a total alp, 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.paucissim'a separatee." 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 maceration, the intervals between the fibres become greater, and as the putrefactive pro cess sets in and advances, the whole substance of the middle tunic takes on the form of a spongy mass, and ultimately the fibres cease to be any longer discernible, having been re duced to the state of a soft pulp, while the cellular structure is rendered more evident.
The following appears to us to be the rationale of the phenomena above : the in crease in thickness which the middle tunic at first undergoes is owing to the cellular tissue interposed between the fibres imbibing the water in which it has been immersed, in virtue of its hygrometric property ; and the spongy appearance observable after the maceration has been continued for a length of time, is the result of the cellular tissue having the property of resisting decomposition by putrefaction much longer than the fibrous tissue.
The internal tunic (intima of Haller) is the thinnest of the three ; it is continuous with the lining membrane of the heart, in extending from which into the arteries it forms a dupli-. cature, contributing to the composition of the semilunar valves : in the larger arteries, when empty, it sometimes forms longitudinal folds ; in some arteries, such as the poplitmal, and the brachial at the bend of the elbow, it presents transverse folds or wrinkles ; it also forms transverse wrinkles in arteries which have re tracted after amputation : its internal surface, which is in contact with the blood in the living body, is smooth, polished, and bedewed with a fine exhalation; its external surface adheres to the internal surface of the middle tunics in the larger trunks of the arteries ; this tunic may be divided into two layers, the internal of which is thin and transparent, while the external is whitish and opaque, having its struc ture blended with that of the middle tunic ; it is the tunica cellulose interior of Haller, and is the seat of the calcareous, steatomatous, and atheromatous deposits, which so frequently occur as morbid appearances in the coats of the arteries. We do not perceive fibres nor any other signs of organization in the inner layer of this tunic in its healthy state ; it is almost completely inelastic and very brittle ; it tears with equal facility in every direction ; compared with other structures it bears the closest resemblance to the arachnoid mem brane of the brain ; the smooth and highly polished condition of the free surface of this tunic is an admirable provision, whereby the effect of friction in diminishing the velocity of the passage of the blood through the arte ries is reduced to the smallest possible amount.