STRUCTURE OF CAPILLARIES. clxxix and agminated intestinal glands and adjacent mucous membrane, etc., the small blood-vessels and capillaries commonly receive a coating of connective tissue corpuscles, which are similar to those of the retiform tissue and connected with the fine trabeculae of the network, by which the vessels are Fig. XCVIII.-A SMALL ARTERY A, WITH A CORRESPONDING VEIN B, TREATED WITHI ACETIC ACID, AND MAGNIFIED 350 DIAMETERS (after Kölliker). a, external coat with oblong nuclei ; B, nuclei of the transverse muscular tissue of the middle coat (when seen endwise, as at the sides of the vessel, their outline is circular); 7, nuclei of the epithelium-cells; d, elastic layers of the inner coat. thus supported. On the smallest capillaries the corpuscles are but sparingly distributed, but nevertheless afford a continuous covering to the vessel by their finely reticulating outrunners. This coating is named by His, who has most fully described and figured it, the adventitia capillaris. Vital properties. From the share which the capillaries take in many vital actions, both healthy and diseased, and especially from the part they have been supposed to play in the process of inflammation, much pains has naturally been bestowed to find out whether they are endowed with vital contractility. There is still, however, a difference of opinion on this question; and, while this property evidently exists in vessels, however small, provided with a muscular coat, it has not been shown by equally direct evidence, to belong to the more simply constructed capillaries; and it must be confessed, that the proofs commonly adduced of the existence of vital contractility in these vessels, are ambiguous and inconclusive. These proofs are chiefly the two following: viz., 1st, That stimulants, such as alcohol, oil of turpentine, pepper, and ice or ice-cold water, applied to the frog's foot or mesentery, cause the capillary vessels to shrink in diameter, and that this contraction is speedily followed by their dilatation beyond their natural capacity; the shrinking of the vessels being attributed to the direct operation of the stimuli on their contractility, and their subsequent dilatation to the temporary exhaustion of that property, consequent on its previous undue excitation. 2ndly, That when the vessels are preternaturally dilated, in the way above described, or by the action of ammonia or common salt, they may, after a time, be made to contract to their usual size by the reapplication of stimuli. Termination of arteries.-The only known termination of arteries is in veins, and this takes place by means of capillary vessels of some of the forms above described, unless in the maternal part of the placenta, and in the interior of erectile organs, in which it has been supposed that small arteries open into wide venous cavities, without the intervention of capillaries. Additional modes of termination have, however, been assumed to exist. Thus, it was believed that branches of arteries ended in exhalant vessels, which, in their turn, terminated by open orifices on the skin, on the surface of different internal cavities, or in the areolar tissue; other arterial branches were supposed to be continued into the ducts of secreting glands, and it was, moreover, imagined that, besides the red capillaries, there existed finer vessels, which passed between the arteries and the veins, and from their smallness were able to convey only the colourless part of the blood. The existence of these colourless or serous" vessels, as they were called (vasa serosa, vasa non rubra), was held, by most authorities, to be universal; by others it was assumed as necessary, at least, in the colourless textures; but these views have now been generally abandoned, although they long prevailed almost without question, and were made the basis of not a few influential doctrines in pathology and practical medicine. Of course it is not denied, that in growing parts there may be capillaries in an incomplete state of development, which admit only the plasma of the blood. Erectile, or cavernous tissue.-By this term is understood a peculiar structure, forming the principal part of certain organs which are capable of being rendered turgid, or erected, by distension with blood. It consists of dilated and freely intercommunicating branches of veins, into which arteries pour their blood, occupying the areole of a network formed by fibrous, elastic, and probably contractile bands, named trabeculæ, and inclosed in a distensible fibrous envelope. This peculiar arrangement of the blood-vessels scarcely deserves to be regarded as constituting a distinct texture, though reckoned as such by some writers; it is restricted to a very few parts of the body, and in these is not altogether uniform in character; the details of its structure will, therefore, be considered with the special description of the organs in which it occurs. DEVELOPMENT OF BLOOD-VESSELS. The first vessels which appear are formed within the ovum, in the germinal membrane; and the process subsequently goes on in growing parts of the animal body. New vessels, also, are formed in the healing of wounds and sores, in the organisation of effused lymph, in the restoration of lost parts, and in the production of adventitious growths. The following may serve as an outline of the process. The network of vessels which form the vascular area in the germinal membrane of the egg at an early stage of incubation (see page li.), consists of arteries and veins communicating, without capillaries. These vessels are at first solid cylinders of larger or smaller diameter, made up of formative cells cohering together. By liquefaction of their substance in the interior, these cylinders become tubes, and their central cells thus set free are the primitive blood-corpuscles. The uniformly cellular substance forming the wall of the primitive vessels is then converted into the different coats. It is probable that a similar mode of formation of arteries and veins goes on within the body of the embryo as its organs and members are progressively developed; but arteries and veins may also begin as capillaries, which grow into larger vessels, as will presently be explained. The small vessels and capillaries originate from nucleated cells similar to those which at first constitute the different parts of the embryo. The cell-wall, or envelope, of these cells, shoots out into slender pointed processes, tending in different directions, so that they acquire an irregularly star-shaped or radiated figure. The prolongations from neighbouring cells encounter one another, and join together by their ends, and the irregularly ramified or reticular cavities thus produced are the channels of rudimentary capillaries. In growing parts, such as the tail of batrachian larvæ, where new vessels are formed in the vicinity of those already existing, as represented in the adjoining figure (xcix.) by Kölliker, not only do the processes of the stellate cells join those of neighbouring cells, but some of them meet and join with similar DEVELOPMENT OF BLOOD-VESSELS. Fig. XCIX. clxxxi pointed processes which shoot out from the sides of neighbouring capillary vessels, and in this manner the new vessels are adopted into the existing system. The junctions of the cells with each other or with capillary vessels are, at first, of great tenuity, and contrast strongly with the central and wider parts of the cells; they appear then to be solid, but they afterwards become pervious and gradually widen, blood begins to pass through them, and the capillary network acquires a tolerably uniform calibre. The original vascular network may become closer by the formation of new vessels in its interstices, and this is effected by similarly metamorphosed cells, arising in the areolae and joining at various points with the surrounding vessels, and also simply by pointed offshoots from the existing capillaries stretching across the intervals and meeting from opposite sides, so as when enlarged to form new connecting arches. From observations made on the fœtal membranes of sheep, Mr. Paget has found that the mode of formation of capillaries described by Kölliker in batrachians, holds good also in mammiferous animals. The simple homogeneous coat of the capillaries is thus formed out of the walls of the coalescing cells; the lining epithelium must be a subsequent formation. Whilst the finest capillaries retain this simple structure, those that are larger acquire the additional coats already described; and arteries and veins, as already stated, especially the smaller ones, appear to be formed in the same manner; indeed, it would seem not unreasonable to presume, that the several gradations of structure seen as permanent conditions in vessels of successively larger calibre, may represent the successive steps by which a vessel, having originally the small size and the simple membrane of a fine capillary, increases in width and acquires the complex tunics of a vein or artery. Further observations, however, are required on this point. Kölliker states, that many vessels which eventually attain a medium size, are originally derived from round cells, which unite in single or double rows and form the primitive simple membranous tube of such vessels, by coalescence of their cavities and walls. Fig. XCIX.-CAPILLARY BLOOD VESSELS OF a, capillaries permeable to blood; b, granules, attached to the walls of the vessels and concealing nuclei; c, hollow prolongation of a capillary, ending in a point: d, a branched cell, containing a nucleus and granules, and communicating by three branches with prolongations of capillaries already formed; e, blood corpuscles. The blood-vessels may be said to increase in size and capacity in proportion to the demands made on their service. Thus, as the uterus enlarges in pregnancy, its vessels become enlarged, and when the main artery of a limb is tied, or otherwise permanently obstructed, collateral branches, originally small and insignificant, Supplement to Müller's Physiology, by Baly and Kirkes, 1848, p. 104. augment greatly in size, to afford passage to the increased share of blood which they are required to transmit, and by this admirable adaptation of them to the exigency, the circulation is restored. In such cases, an increase takes place in length, as well as in diameter, and accordingly the vessels very commonly become tortuous. ABSORBENT OR LYMPHATIC SYSTEM, Under this head we include not only the vessels specially called lymphatics, together with the glands belonging to them, but also those named lacteal or chyliferous, which form part of the same system, and differ in no respect from the former, save that they not only carry lymph like the rest, but are also employed to take up the chyle from the intestines during the process of digestion and convey it into the blood. An introductory outline of the absorbent system has already been given at page xlvii. A system of lymphatic vessels is superadded to the sanguiferous in all classes of vertebrated animals, but such is not the case in the invertebrata; in many of these, the sanguiferous vessels convey a colourless or nearly colourless blood, but no additional class of vessels is provided for conveying lymph or chyle, at least none such has hitherto been detected. Distribution.—In man and those animals in which they are present, the lymphatic vessels are found in nearly all the textures and organs which receive blood; the exceptions are few, and with the progress of discovery may yet possibly disappear. In the different regions of the body, and in the several internal viscera, the lymphatics are arranged in a superficial and a deep set. The former run underneath the skin or under the membranous coats immediately enveloping the organs in which they are found; the latter usually accompany the deep-seated blood-vessels. The principal lymphatic vessels of a part exceed the veins in number, but fall short of them in size; they also anastomose or intercommunicate much more frequently than the veins alongside of which they run. Origin.-Lymphatics may arise superficially, i.e. immediately underneath free surfaces, both external and internal, as for example those of the skin and mucous membranes, or deeply, in the substance of organs. Plexiform origin.-When they arise superficially, the lymphatics most generally begin in form of networks or plexuses, out of which single vessels emerge at various points and proceed to enter lymphatic glands or to join larger lymphatic trunks. Such mode of commencement may be termed the plexiform. The plexuses for the most part consist of several strata, becoming finer as they approach the surface, in respect both of the calibre of the vessels and the closeness of their reticulation. This is shown in figure c., which is meant to represent the lymphatic plexuses of the skin, the most superficial and finest network is composed of vessels which are larger than the sanguiferous capillaries. But even The short anastomosing branches of these plexuses are often of very unequal size, even in the same stratum, some being dilated and almost saccular whilst others immediately communicating with these are narrow, so that the network may assume a varicose character. In some situations the plexuses have much the appearance of strata of intercommunicating cellular cavities, and a characteristic example of this appearance is afforded by the intestine of the turtle after its lymphatics have been injected with mercury; these vessels are then seen to emerge from what has all the appearance of a dense stratum of small rounded cells filled with mercury aud lying beneath the surface of the mucous coat. This appearance, however, may be regarded as ORIGIN OF LYMPHATICS. clxxxiii produced by the short distended branches of a very close lymphatic network, and transitions are accordingly met with between this and the more usual and regular forms. But whilst the superficial commencement of lymphatics is generally plexiform, the rule is not without exception. The lacteals of the intestinal villi, for example, although they form networks in the larger and broader villi, arise in others by a single vessel beginning with a blind or closed extremity at the free end of the villus, whence it sinks down to join the general plexus of the intestinal membrane. Lacunar origin. When lymphatics arise deeply, their origin may be hidden from view, Fig. C. La OF THE SKIN OF THE a, superficial, and b, deeper plexus; c, a lymphatic vessel, which proceeded to the axillary glands. and the precise mode in which it takes place Fig. C.-LYMPHATIC VESSELS unknown. There may be cases in which it is still plexiform; but another and doubtless more general mode of origin from the interior of organs, long suspected and often upheld on imperfect evidence, has now been satisfactorily ascertained, which may not inappropriately be termed lacunar. In this case the lymphatic vessels proceed from irregular or shapeless spaces in the internal parts of organs; the spaces, that is, which intervene between the several structures of which the organ is composed. Thus, in a gland, they are the spaces which lie between or surround the blood-vessels, secreting tubes or saccules, partitioning or inclosing membranes, and the like. Though shapeless, or at least of no regular form, these anfractuous cavities are limited and defined by a lining of epithelium, agreeing in character with that of the lymphatic vessels. It may be presumed that their opposite sides are in apposition or in near proximity, as in serous membranes, for the lymph deposited in these recesses is not suffered to accumulate, but is drained off by the lymphatic vessels which lead out of them. The lacunar condition of the lymphatic system at its commencement was shown to exist in the testicle by Ludwig and Tomsa, and has since then been found in the kidney by Ludwig and Zwarykin, in the thymus gland by Frey, in the spleen by Tomsa, in the liver (forming canals which inclose the blood-capillaries) by MacGillivray, and in the salivary glands by Giannuzzi. His has also discovered that the blood-vessels of the brain and spinal cord are surrounded and inclosed by lymph-channels-perivascular canals-which follow their course and eventually terminate in ordinary lymphatic vessels; an arrangement that brings to mind an earlier observation of Rusconi, who found that the aorta and mesenteric arteries of the frog and salamander are inclosed in large lymphatic canals. The spaces which so extensively separate the frog's skin from the subjacent muscles, were recognised by the late Professor Johannes Müller as belonging to the lymphatic system, and von Recklinghausen has shown that the subcutaneous lymph spaces of the frog's leg communicate with lymphatic vessels which envelope the blood-vessels of the foot; also that milk injected into these spaces finds its way into the blood. The lymphatic system of man and the higher animals in being thus partly constituted by lacunæ or interstitial receptacles, so far agrees with the sanguiferous system of crustaceans and insects. It has been sometimes maintained that the lymphatics of glandular organs communicate at their origin with the ducts; but, although it is no uncommon thing for matters artificially injected into the ducts of glands, as, for instance, those of the liver and testicle, to pass into the lymphatics, a careful examination of such cases |