it only an exterior envelope, to retain the vessel in situ. The next coat is the middle, muscular or proper coat, the character of which has been the subject of much discussion. It was, at one time, almost universally believed to be muscular. Such was the opinion of Mr. Hunter.1 Henle2 advances the opinion, that its structure is intermediate between areolar and muscular tissue; its microscopic elements being broad and very flat, slightly granulated fibres or bands, which lie in rings around the internal membrane, and are about 0-003 lines in diameter. These with a system or network of dark streaks constitute the middle coat. In the large arteries, as the aorta and its main branches, nearly the whole thickness of this coat is composed of yel low elastic tissue-the tissu jaune of the French anatomists: few mus cular fibres are perceptible; but in the smaller arteries the proportionate thickness of the elastic coat gradually diminishes; whilst, as a general rule, the muscular fibres increase in number, and form a layer within the elastic coat. Kölliker, indeed, affirms, that the middle tunic of the small arteries is purely muscular, without the slightest admixture of connective tissue and elastic elements. The muscular fibres resemble those of the intestinal tube, being of the nonstriped or nonstriated variety. They are arranged areolarly; are pale and flat, and mingled with filaments of fine elastic tissue.

Nysten, Magendie,' and Müller applied the galvanic stimulus to the middle coat, which is the most sensible test of irritability, but without effect. It is proper, however, to remark, that the heart seems equally unsusceptible of the galvanic stimulus; or at least is not affected by it like the voluntary muscles. In the cases of two executed criminals, which the author had an opportunity of observing, although all degrees of galvanism were applied half an hour after the drop fell, no motion whatever was perceptible; yet the voluntary muscles contracted, and continued to do so for an hour and a half after execution. The same fact is recorded in the galvanic experiments of Dr. Ure, detailed in another part of this work, and is attested by Bichat, Treviranus and others. Humboldt, Pfaff, J. F. Meckel, Wedemeyer, and J. Müller, however, affirm the contrary. The last observer states, that with a single pair of plates he excited contractions not only in a frog's heart, which had ceased to beat, but also in that of a dog, under similar circumstances. Into the subject of the cause of the heart's action, we shall, however, inquire presently. Müller suggests, that in the capability to contract under the influence of cold, as exhibited in the experiments of Schwann, referred to hereafter, the contractile tissue of the arteries resembles that of the dartos,

7

On the Blood, Inflammation and Gunshot Wounds; by Palmer, Amer. edit., p. 156, Philad., 1840.

2 Casper's Wochenschrift, May 23, 1840, cited in Brit. and For. Med. Rev., Oct. 1840, p. 551.

3 Mikroskopische Anatomie, 2ter Bd. s. 507, Leipz., 1854; or his Manual of Histology, Sydenham Society edit., Lond., 1854; or Da Costa's Amer. edit. of the same, p. 679, Philad., 1854.

Recherches de Physiologie, &c., p. 325, Paris, 1811.

5 Précis, 2de édit., ii. 387, Paris, 1825.

• Handbuch der Physiologie, Baly's translation, p. 205, Lond., 1838.

7 Loc. cit.

8 Archiv. fur 1836, in Lond. Med. Gaz., May, 1837.

and that found in many parts of the skin, as about the nipple and follicles, although the physical characters of the latter are so different from elastic tissue. The third or inner coat is smooth and polished, and a continuation of the membrane that lines the ventricles. It has an epithelial lining, resembles the serous membranes, and is lubricated by a form of serous exhalation.'

The arteries receive the constituents that belong to every living part, -arteries, veins, lymphatics, and nerves. These arteries do not proceed from the vessels they nourish, but from adjacent trunks, as we have remarked of the vasa vasorum, to which class they really belong. The nerves proceed from the great sympathetic; form plexuses around the vessels, and accompany them through all their ramifications. By some anatomists, the arteries of the head, neck, thorax, and abdomen, are conceived to be supplied from the great sympathetic, whilst those of the extremities are supplied from the nerves of the spinal marrow. It is probable, however, that more accurate discrimination might trace the dispersion of twigs of the nerves of involuntary motion on all these vessels. The organization of the arteries renders them tough and extremely elastic, both of which qualities are necessary to enable them to withstand the impulse of the blood sent from the heart, and to react upon the fluid so as to influence its course. It is by virtue of this structure, that the parietes retain their form in the dead body,—one of the points that distinguish them from the veins.

The vitality of the arteries is inconsiderable. Hence their diseases are by no means numerous or frequent,-an important fact, seeing that their functions are essential, and their activity incessant.

c. Intermediate, Peripheral or Capillary System.

The capillary or intermediate vessels are of extreme minuteness, and are by some considered to be formed by the terminations of arteries and the commencement of veins; by others to be a distinct set of vessels. This system forms a plexus which is distributed over every part of the body, and constitutes, in the aggregate, what is meant by the capillary system. It admits of two great divisions, one situate at the termination of the branches given off from the aorta, and called the general capillary system; the other at the termination of the branches of the pulmonary artery,-the pulmonic capillary system. Although the capillary system of man does not admit of detection by the unaided sight, its existence is evidenced by the microscope; by injections, which develope it artificially in almost every organ; by the application of excitants, and by inflammation. The parietes frequently cannot be distinguished from the substance of the tissues;--the colour of the blood, or the matter of the injection alone indicating their course. In some parts, as in the white textures, these vessels do not seem to admit the red particles of the blood, whilst others admit them always. This diversity gave rise to a distinction of the capillaries into red and white; but there are probably none of the latter. It is difficult, indeed, to

For some speculations as to the agency of this secretion in the production of the buffy state of the blood, &c., see M. Romain Gérardin, in Journal des Connaissances Medico-Chirurgicales, Mars, 1836.

conceive how the red particles could be arrested at the mouths of the white arteries-if such existed-without their preventing altogether the entrance of blood into them. The true cause of the whiteness appears to be the small quantity of blood they receive; and it is only when the network is very close, and the quantity of blood passing through them great, that a perceptible colour is produced. If a plate of red glass be reduced to a very thin pellicle, and be placed between the eye and light, its colour will be scarcely sensible. To perceive it, several of these pellicles must be placed over each other, and they must be examined not by their transparency, but by causing the light to fall on their surface, or by reflection.

There are certain textures, again, which receive no blood vessels,the corneous and epidermic, for example. They are probably nourished by transudation of nutritive matter from the vessels of the surrounding tissue.

The ancients were of opinion, that arteries and veins are separated by an intermediate substance, consisting of a fluid effused from the blood, which they called, in consequence, parenchyma. The notion is,indeed, still entertained; and is considered to be supported by microscopical observations. In the examination of delicate and transparent tissues, currents of moving globules are seen with many spaces of apparently solid substances, resembling small islets, surrounded by an agitated fluid. If the tissue be irritated by thrusting a fine needle into

it, the motion of the globules becomes more rapid; new currents arise where none were previously perceptible, and the whole becomes a mass of moving particles, the general direction of which tends towards the points of irritation. But although a part of the apparatus of inter

mediate circulation may be arranged, in this manner, there are reasons for the belief, that a more direct communication between the arteries and veins exists also. The substance of an injection passes from one set of vessels into the other, without any evidence of intermediate extravasation. The blood has been seen, too, passing in living animals, directly from the arteries into the veins. Leeuenhoek' and Malpighi, on examining the swim-bladders, gills, and tails of fishes, the mesentery of frogs, &c.-which are transparent,-observed this distinctly; and the fact has been proved by the observations of Cowper, Cheselden, Hales, Spallanzani, Thomson, Cuvier, Configliachi, Rusconi, Döllinger, Carus, and others.

Fig. 103.

Circulation in the Under Surface of the Tongue of the Frog.

x, x. Venous branches uniting to form a principal vein, y. z, z. Follicles into which a small artery enters, which becomes convoluted before issuing from them. A beautiful capillary rete, and some muscular fibres are also seen.

The artery and vein terminate in two different ways;-at times, after the former has become extremely minute, by sending off numerous

1 Select Works, containing his Microscopical Discoveries, by Samuel Hooke, p. 90, Lond., 1778.

2 Epist. de Pulmonibus, 1661, and Haller, Element. Physiol., lib. iii. sect. 3, § 20, Lausann., 1757.

lateral branches, as Haller states he noticed in the swim bladders of fishes; at others, by proceeding parallel to each other, and communicating by a multitude of transverse branches. Fig. 101 exhibits a microscopic view of the membrane between two of the toes of the hind foot of the frog, Rana esculenta, magnified three diameters.

Fig. 102 shows a portion of the web of a frog's foot magnified 45 diameters. The superficial network of capillaries is seen admitting but a single series of blood particles. All the vessels, here figured, are, according to Wagner,' furnished with distinct parietes.

Fig. 103 is a beautiful representation of the circulation in the under surface of the tongue. Along the larger vessels the blood can be seen rushing with excessive velocity. It is proper, however, to state that the more the parts are magnified, the greater will be the apparent velocity. The mean real velocity, Valentin' thinks, is one-eighth less in the capillaries than in the veins and arteries. These larger vessels have distinct coats; but single files of globules are seen proceeding slowly through channels to which the author has not been able to satisfy himself that there were distinct parietes. The tongue of the frog offers by far the most satisfactory opportunity for distinctly witnessing the circulation; a fact for the knowledge of which the author is indebted to M. Donné.1

The capillary vessels have been esteemed by some to belong chiefly to the arteries, the venous radicles not arising almost imperceptibly from the capillary system, as the arteries terminate in it, but having a marked size at the part where they quit this system, which strikingly contrasts with the excessive tenuity of the capillary arterial vessels; whilst between the capillary system and the arteries there is no distinct line of demarcation. The opinion of Bichat was, that this system is entirely independent of both arteries and veins; and Autenrieth imagined, that the minute arteries unite to form trunks, which again divide before communicating with the veins, so as to represent a system analogous to that of the vena porta. The experiments of Dr. Marshall

Elements of Physiology, by R. Willis, Lond., 1842.

2 Lehrbuch der Physiologie des Menschen, i. 467, Braunschweig, 1844.

See also Lebert, Physiologie Pathologique, i. 7, Paris, 1845.

Cours de Microscopie, p. 109, Paris, 1844; and Atlas, planche vi., Paris, 1845. 5 Anatomie Générale, &c., édit. de MM. Blandin et Magendie, ii. 299, Paris, 1832. 6 Physiologie, ii. 138.