Section IV, (Part 5)

traosudatiou like that of the bile through the gall bladder ? Or, again, does it merely consist of a small ([uiintity of &crum that remains in the vessels nAor the expulsion of the bluod? A careful vxumination of this) tunic, both in its hcaltliy and morbid states, has con-/ vinced me that it ia very analogous to the serous mort'I branes. In its natural condition it has a glistening/ and shining appearance, which I believe is never pre^n sented by any surface that is not habitually moistened by B secreted fluid. Innumerable caxes are related by- writers on surgery, which prove that the lining coat isi very liable, like the serouii membranes, to the adhtsivei form of inflammation ; tlius when tbe arteries, veins, or IjinpbaticK are inflamed, fibrin is &e<|uently poured out/ on the inner surface, and the tubes become cIosed,/itiff|ff in fetal cases, flakes of the efliised matter are observeAl in the vessels. Tbe same effect is also produced wheat arttfiw are secured with the ligature, or are woundedi by accident. "4 The external surface is uniM '^^ tbe next coat, from<i which, however, it may be separated without difficulty,! particularly in the arteries. ui o-The internal tunic assists in the circulation, by ftir-jt ntshing a polished surface, along which the blood an(l> lymph readily flow ; ami, by its density, it prevents the escape of the contained fluids out of their proper chtm-^ neU. It also forms, in many parts of its extent, par--! ticularly in the veins and lymphatics, semi-lunar folds, which are distinguished by tbe name of valves. A 368 MIDOLI AND tXTB&NAL COATS. The second or middle coat u very evident in the arteries and in tbt; krge veins, but it is not distinct in the smaller veins, and in the absorbing vessels it c«d- not be perceived. This tunic is composed of a peculiar fibre, vhich is considered by many anatomists to be muKcular, while others regard it as being formed of a peculiar substance, which is called by the French writers, the yellow or tltusHe tiasuc. The fibrous membrane is opake, firm, elastic, and brittle ; it is of a yellowish colour in the arteries, and reddi.sh brown in the veins. It has few properties in common in the tvo sj-stems, but in both it is endowed witli a contractile power, which enables it actively to assist in the circulation of the blood. '"•The external coat, also called ceUular, eUuHc, &c., exists, under *arious modifications, in most parts of the vascular system. It consists of condensed and fibrous cellular substance, which has a yellowish white colour ; it is remarkably elastic, and by its firmness it givM to the vessels the principal part of their strength in th« longitudinal direction. Scarpa contends that this cover> ing ought not to be considered amongst the component parts of the vessel ; lie says that it is merely an adven- titious sheath, furnished by the common cellular mem* brane, which keeps the vascular organs in their situa* tion, and connects them with the surrounding parts.* This opinion is incorrect, for a careful inspection of an arterial trunk, shews that the clastic coat is distinct from the loose cellular tissue bv which it is united to tlie neighbouring organs, although, as elsewhere, there is a general connexion between the two parts. Hie * TmtJK OQ Atiruriim, p. 99- VASA VASORUM AND NIRVES. 989 internal surface of the elastic tunic is dense, and so closely Joined to the middle coat, that it is very difficult to separate one from the other ; it is indeed supposed by Bichat, tliat in the arteries the fibres of the latter coat are inserted into the former.* ., .The blood-vessels and lymphatics, like all other organs of the body, arc provided with proper vessels, which are called the tfosa vatonm. The small nutrient arteries arise from the neighbouring branches, and rarely, if ever, from the vcmd itself to which tbey are distributed. They may be distinctly seen ramifying in the loose cellular stubslance that covers the larger vessels ; they aflerwards penetrate to the fibrous coat in which th«y principally appear to terminat«, although there is no doubt that the most delicate branches extend to and supply the internal membrane. The presence of lym- phatics is assumed rather from what happens in disease, than from any actual demonstration of their existence. It is said that they have been observed with the micro- scope, and also that they may be injected. In many morbid states of the vascular system, their action is ren- dered very apparent ; thus the intemiJ tunic is occa- sionally destroyed by tdcerution ; and again, when an artory has been tied, the ligature is detached by a sirni* lar process affecting the external coat. The nerves of the vessels are numerous, although, 6rom their smalt size, they cannot be traced, except on the large branches. It it generally supposed that the ner- vous s\ipply is derived principally, or entirely, from the ganglions of the great sympathetic ; but it !s certain that ,i^c nemis vagus furnishes filaments to the pulmonar)- * AasLOco. t. i. |>p. 371 tad 286. a 3 1 a6o OIVBLOPUKNT OF v«i<i«la, and it appears that in the limbs the nerves arc dwiveti, at lenst in pari, from the ccrebrcspinal sjBlc-m. /S shall conclude these observations by stating, in tbs words of a most intelligent and judicious writtir, that " the arteries (and also the vems mid hpnphaiies,) arc supplied not only with small arteries and veins, but al&u with abKorbents and nervcK, and hare, in these respects, a simitar organization to the other soft part£ of the body. This structure makes them susceptible of every change to which living parts are subjected in cummoQ ; enables them to inflame when injured, and to pour out a coagu- laling lymph, by which the injury is repaired, or the tube is permanently closed.* The properties and functions of the vessels will be tepamtely considered in the articles on the arterial, venous, and I)'mphatic sy»tem5.