Hall' on the batrachia, which were performed with signal care, led him to the following conclusions, which agree with those of Bichat, so far as regards the independent existence of a capillary system. The minute vessels, he says, may be considered arterial, so long as they continue to divide and subdivide into smaller and smaller branches. The minute veins are the vessels that gradually enlarge from the successive addition of small roots. The true capillary vessels are distinct from these. They do not become smaller by subdivision, or larger by conjunction, but are characterized by continual and successive union and division or anastomoses, whilst they retain a nearly uniform diameter. The last branches of the arterial system, and the first root of the venous, Dr. Hall remarks, may be denominated minute, but the term "capillary" must be reserved for, and appropriated to, vessels of a distinct character and order, and of an intermediate station, carrying red globules, and perfectly visible by means of the microscope.

Of late, M. Bourgery has maintained, that besides the intermediate vessels, which form the direct communication between the arteries and veins, there is a special capillary arrangement in every tissue by which the functions of nutrition and secretion are accomplished. The diameter of these capillaries, according to M. Bourgery, is not more than one-half, one-third, or even one-fourth of that of the blood corpuscles; and they can, consequently, convey only liquor sanguinis. But the existence of these vessels is not considered to be demonstrated; whilst their absence in tissues-as cartilage-which they were formerly supposed to penetrate, has been established.3

The capillary arteries are distinct in structure as they are in office from the larger arteries. All the coats diminish in thickness and strength, as the tubes lessen in size; but this is more especially the case with the middle coat, which, according to Wedemeyer, may still be distinguished by its colour in the transverse section of any vessel whose calibre is not less than the tenth of a line; but entirely disappears in vessels too small to receive the wave of blood in a manifest jet. While the coats diminish, the nervous filaments, distributed to them, increase; the smaller and thinner the capillary, the greater the proportionate quantity of its nervous matter. The coats of the capillaries become successively thinner and thinner, and at length disappear altogether; and the vessels-many of them at least-seem to terminate in membraneless canals or interstitial passages, formed in the substance of the tissues. The blood is contained-according to Wedemeyer, Gruithuisen, Döllinger, Carus, and others-in the different tissues in channels, which it forms in them: even under the microscope, the stream is seen to work out for itself, easily and rapidly, a new passage in the tissues, and it is esteemed certain, that in the figura venosa of the egg, the blood is not surrounded by vascular parietes. Most histologists of the day are disposed, however, to believe, that the capillaries are provided with distinct coats. Such, as has been seen, appeared to Wagner to be the case in the frog's foot,

A Critical and Experimental Essay on the Circulation, &o., Lond., 1830; Amer. edit., Philad., 1835.

* Comptes Rendus, &c., 1848, and Gazette Médicale, No. 37, 1848.

3 British and Foreign Médico-Chirurgical Review, p. 527, Oct., 1848.

when magnified 45 diameters; and it has even been announced, that they are composed of a fibrous structure, analogous to the muscular.

1. Deep venous trunk, composed of three principal branches, 2, 2, 2; and covered with a rete of smaller vessels.

Fig. 106, from Wagner, exhibits the vascular rete and circulation of the web of the hind foot of a frog-Rana temporaria-magnified 110 times: here the parietes are very distinct. In another figure in Wagner, which represents a portion of a live newt, magnified 150 diameters, the capillaries are exceedingly delicate, and their walls by no means as distinct. The arterial and venous trunks and the capillaries that form the medium of communication between them are well seen, as well as the islets of the substance of the lung, in which a granular or areolar texture is indistinctly perceptible. Dr. Carpenter' is of opinion, that the mode of origin of the capillaries refutes the supposition, that they are mere passages channeled out of the tissues through which they convey the blood. He thinks there can be no doubt, that they are produced, in any newly forming tissue, not by the retirement of the cells, one from the other, so as to leave passages between them, but by the formation of communications among certain cells, whose cavities become connected with each other, so as to constitute a plexus of tubes, of which the original cell-walls become the parietes.

Of the minute capillaries, the diameter of which, in parts finely injected, varies from the 10th to the 6th, and the 6th of an inch and even more, some, according to Wedemeyer, communicate

1 Human Physiology, § 477, Lond., 1842.

with veins; in others, there are no visible openings or pores in the sides or ends, by which the blood can be extravasated preparatory to its being imbibed by the veins. There is nowhere apparent a sudden passage of the arterial into the venous stream; no abrupt boundary between the division of the two systems. The arterial streamlet winds through long routes before it assumes the nature, and takes the direction, of a venous streamlet. The ultimate capillary rarely passes from a large arterial into a large venous branch. Many speculations have been indulged regarding the mode in which the vascular extremities of the capillary system are arranged. Bichat regarded it as a vast reservoir, whence originate, besides veins, vessels of a particular order, whose office it is to pour out, by their free extremity, the materials of nutrition,-vessels, which had been previously imagined by Boerhaave, and are commonly known under the appellation of exhalants. Mascagni supposed that the final arterial terminations are pierced, towards their point of junction with the veins, by lateral pores, through which the secreted matters transude;-but these points will farther engage attention under Nutrition and Secretion.

d. Veins.

The origin of the veins, like that of all capillary vessels, is imperceptible. By some they are regarded as continuous with the capillary arteries; Malpighi and Leeuenhoek3 state this as the result of their microscopic observations on living animals; and it has been inferred, from the facility with which an injection passes from the arteries into the veins. According to others, cells exist between the arterial and the venous capillaries, in which the former deposit their fluid contents, and whence the latter obtain it. Others, again, substitute a spongy tissue for the cells. It has also been asked,-whether there may not be more delicate vessels communicating with their radicles, similar to the exhalants which are presumed to exist at the extremities of the arteries, and which are regarded as the agents of exhalation. All this is, however, conjectural. It has already been observed, that the mesenteric veins have been supposed by some to terminate by open mouths in the villi of the intestines; and the same arrangement has been conceived to prevail with regard to other veins; but there is no evidence of this. M. Ribes concludes, from the results of injecting the veins, that some of the venous capillaries are immediately continuous with the minute arteries, whilst others open into the cells of the areolar tissue, and into the substance of different organs.

When the veins become visible, they appear as an infinite number of extremely small tubes communicating very freely with each other; so as to form a very fine network. These vessels gradually become larger and less numerous, but still preserve their reticular arrangement; until, ultimately, all the veins of the body empty themselves into the heart by three trunks-the vena cava inferior, vena cava superior, and coronary vein. The first of these receives the veins from the lower part of the

1 Vasor. Lymph. Corpor. Human. Histor., Sen. 1817; and Prodromo della Grande Anatomie, Firenz., 1819.

2 Secunda Epistola de Pulmonibus, Opera, Lond., 1687.

9 Epistol. 59, Opera, Lugd. Bat., 1722.

body, and extends from the fourth lumbar vertebra to the right auricle; the second receives all those of the upper part of the body. It extends

Fig. 107.

from the cartilage of the first rib to the right auricle. The coronary vein belongs to the heart exclusively; between the superior and inferior cava a communication is formed by means of the vena azygos.

Certain organs, as the spleen, appear to be almost wholly composed of venous radicles. Fig. 107 represents the ramifications of the splenic vein, in the substance of that organ; and if we consider, that the splenic artery has corresponding ramifications, the viscus would seem to be almost wholly formed of blood vessels. The same may be said of the corpus cavernosum of the penis and clitoris, nipple, urethra, glans penis, &c. If an injection be thrown into one of the veins that issue from these different tissues, they are filled by the injection; this rarely occurs, if the injection be forced into the artery. M. Magendie' affirms, that the communication of the cavernous tissue of the penis with the veins occurs through apertures two or three millimètres-in. 0.117in diameter.

cations.

1. Trunk of the vein. 2. Gastric branch of this vein coming from the stomach. 3. Branches Small mesenteric vein cut off. 5. Branches coming from external coat of the spleen. 6. Branches

coming from the substance of the spleen. 4.

of lymphatic vessels of spleen.

In their course towards the heart, particularly in the extremities, the veins are divided into two planes;-one subcutaneous or superficial; the other deep-seated, and accompanying the deep-seated arteries. Numerous anastomoses occur between these, especially when the veins become small, or are more distant from the heart. We find, that their disposition differs according to the organ. In the brain, they constitute, in great part, the pia mater; and enter the ventricles, where they contribute to the formation of the plexus choroides and tela choroidea. On leaving the organ we find them situate between the laminæ of the dura mater; when they take the name of sinuses. In the spermatic cord, they are extremely tortuous; anastomose repeatedly, and form the corpus pampiniforme; around the vagina, they constitute the corpus retiforme; in the uterus, the uterine sinuses. They have three coats in superposition, according to most anatomists; but many modern anatomists are disposed to assign them six. The outer coat is areolar; dense, and very difficult to rupture. The middle coat has been termed the proper membrane of the veins. The generality of anatomists describe it as composed of longitudinal fibres, which are more distinct in the vena cava inferior than in the vena cava superior; in the superficial veins than in the deep-seated; in the branches than

Précis, &c., ii. 238.

2

in the trunks. M. Magendie' states, that he has never been able to observe the fibres of the middle coat; but has always seen a multitude of filaments interlacing in all directions; and assuming the appearance of longitudinal fibres, when the vein is folded or wrinkled longitudinally, which is frequently the case in the large veins. It exhibited to him no signs of muscularity; even when the galvanic stimulus was applied; yet M. Magendie suspected its chemical nature to be fibrinous. It was remarked, in an early part of this volume, that the bases of the areolar and muscular tissue are, respectively, gelatin, and fibrin; and that the various resisting solids may all be brought to one or other of those tissues. The middle coat of the veins doubtless belongs essentially to the former, and is a variety of the tissu jaune of the French anatomists. M. Magendie merely states its fibrinous nature to be a suspicion; and, like numerous suspicions, this may be devoid of foundation. Yet we have reason to believe, that it is contractile; and, of late,3 it has been described as formed of one or two or even more layers between the external and internal coats; these layers consisting of fibres, which agree, in all respects, with the white areolar tissue; and are either quite pure, or mixed in one or other of the layers with a greater or less amount of fibres, resembling those of the middle coat of the arteries in having the anatomical characters of the nonstriated or unstriped muscular fibres. The muscular fibre-cells are, however, much fewer in number, and are sometimes wanting. Kölliker says they do not exist in the uterine portion of the placenta, the veins of the cerebral substance and pia mater; the sinuses of the dura mater; the veins of the bones; the venous sinuses of the corpora cavernosa of the male and female; and probably in those of the spleen. M. Broussais affirms, that the contraction of the middle coat is one of the principal causes of the return of the blood to the heart. He conceives, that the alternate movements of contraction and relaxation are altogether similar to those of the heart; but that they are so slight as not to have been rendered perceptible in the majority of the veins, although they are very visible in the vena cava of frogs, where it joins the right auricle. In some experiments by M. Sarlandière on the circulation, he observed these movements to be independent of those of the heart. After the organ was removed, and even after blood had ceased to flow," the contraction and relaxation of the vein continued for many minutes in the cut extremity; and it has been elsewhere remarked, that Mr. Wharton Jones had discovered in the veins of the bat's wing a regular rhythmical contraction and dilatation.

The inner coat is extremely thin and smooth at its inner surface, and has an epithelial lining. It is very extensible, and yet presents considerable resistance; bearing a very tight ligature without being ruptured. In many of the veins, parabolic folds of the inner coat exist,

1 Op. cit., ii. 242. See on the researches of histologists, Mr. Paget, Brit. and For. Med. Review, July, 1842, ii. 242.

2 Page 59.

Quain's Human Anatomy, by Quain and Sharpey, Amer. edit., by Leidy, i. 518, Philad., 1849.

♦ Manual of Human Histology, Amer. edit., by Dr. Da Costa, p. 689, Philad., 1854. Op. citat., American translation, p. 391.

See, on this subject, the remarks on the Circulation in the Veins.