The last glands to which we would direct attention are two in number, and from their situation may be appropriately termed the anterior lingual glands. They have been re cently described by Blandin, Nuhn*, and Schlemm, as being situate below the apex of the tongue, between the lower longitudinal and transverse muscular fibres, and pouring their secretion during the movements of that organ on to the mucous membrane beneath the tip. They- have only as yet been disco vered in man and the ouran-outan. We have observed them on the inferior surface of the tongue, half an inch behind its anterior border, immediately above the longitudinal muscular fibres, one on either side of the median line, and about two-thirds of an inch long. Broad behind and narrow in front, they are separated from each other in the former direction by an interval of about half an inch, in the latter are almost in mutual contact. Their direction therefore is obliquely from behind forwards and inwards (fig. 305, b). Each glen() is fur a. Bristles in the orifices of the ducts of the left anterior lingual gland. b. The right anterior lin gual gland. c. The raph6 of the tongue.
nished with three or four delicate ducts, given off from its lower surface, and which perforate the mucous membrane obliquely parallel to the long axis of the gland (fig. 30.5, a). These glands are of less consistence than the molar or labial. We have met with one instance in which only one gland was present, running at right angles to the middle line. It was convex in front and concave behind, having a trans verse diameter of one-third of an inch, an antero-posterior one-eighth of an inch. It gave off' three delicate ducts.
The minute structure of the glands in general has been already fully inquired into*, and to the type on which they are formed the salivary glands offer no exception. A simple cmcal membranous prolongation is the model to which they can all be referred, however complex each individual series of glands may appear. This grand generalisation, by which the ex treme simplicity of the operations of nature is remarkably illustrated, has mainly been the result of a minute inquiry into developmental and comparative anatomy. We are particu larly indebted, however, to MUIler and E. H. Weber for the exposition of the evolution and minute structure of the salivary- glands.
Miller thus describes the first appearance of a salivary gland in Mammalia, and his ob servations were taken from the embryo of a sheep, two inches long : — Its form is that of a simple canal with bud-like processes, lying in a gelatinous nidus or blastema, and com municating with the cavity of the mouth. As the development of the gland advances, the canal becomes more and more ramified, in creasing at the expense of the germinal mass or " blastema," in which it is still enclosed. The blastema soon acquires a lobulated form, corresponding to that of the future gland, and is at last wholly absorbed. Valentint remarks that a portion of this blastema, which contains nuclei and cell-formations, and which is not converted into glandular structure, is changed into blood-vessels, nerves, and connecting cellular tissue; and he has, further, accu rately determined that the secondary tubes are formed independent of the primary, at the expense of portions of the blastema, in the vicinity of the main duct, with which, by a centripetal development, they ultimately com municate. Thus, in the first stage of their development, the salivary ducts can be seen to constitute an independent closed system of tubes. Tbe investig,ations of E. Weber t carry us a step further in the inquiry. He found, by a successful injection of the parotid in a human fcetus, that the excretory duct, after having undergone its ultimate state of subdivision, by an extensive ramification of its secondary tubes, terminated in microscopic twigs, each twig having appended to it one or more minute cells or vesicles, forming small group-like lobules or bunches. These cells have not a uniform size, their long diameter, which is more or less in a line with the axis of each of the terminal divisions of the duct with which the cells are structurally conti nuous, is, on the average, almost of a Paris line. Gerber * states these vesicles or
cells are variously shaped, from to 1+5, of a Paris line in diameter, and upon the periphery of the gland appear mutually to compress each other and to become poly hedral in their outline. They are united to gether into small lobules, from four to seven times greater than each individual vesicle, the latter consequently being almost three times, the former about twelve times the dianieter of the capillary blood-vessels which ramify on the surface. They form, in fact, the cmcal terminations of the branches of the excretory tubes, without having of necessity an indi vidual narrow connecting pedicle, as figured by Berres± in the minute anatomy of the parotid.
Such, then, is the essential structure of the salivary glands; and in the full state of or ganisation of each we recognise the elements of a mucous membrane, constituting the in ternal lining of the excretory d,uct and conti nuing throughout the series of its ultimate ramifications as far as the terminal vesicles ; a middle elastic coat, and an external covering of areolar tissue. The mucous membrane consists of an epithelial layer, and a basement membrane. The epithelium is of the co lumnar variety, and maintains this character along the track of the excretory duct as far as its delicate divisions, where it gradually changes its character, so that that lining the interior of the vesicles is of the pavernent type. This transition of columnar into pavement epithelium would appear gradual; so that it is difficult to determine the point at which the one form terminates and the other commences. The basement membrane is continued along the entire track of the tubular ramifications, as far as the vesicles, the form of which it would appear to determine. There can be little doubt that this is the membrane which Berres alludes to as the proper wall of the vesicles, and describes as a small transparent membrane, covered over with molecules, and which also has been represented by Henle as homogeneous, but which he at the same time considers as composed of filaments of cellular tissue solidly united together4 Considerable difFerence of opinion has ex isted as to the nature of the middle coat of the glandular tubes, according as the largest or smallest have been examined. Valentin re marks that in the first case it has been considered fibrous,in the second simply homogeneous. In by far the greater number of the terminal extre mities of the glandular tubes the intermediate membrane appears clear and transparent, and gives neither in the fresh state, nor when re agents are applied, any indication of a fibrous character. In all the large tubes the intermediate coat is formed of distinct flat fibres, together writh the characteristic fibres of cellular tissue, studded at intervals with elongated or rounded nuclei, which present a great analogy with the fibres of organic muscle, if they be not completely identical with it. We are at pre sent, however, in doubt as to whether the intermediate membrane of small glands, and of the terminal portion of the large, be really a simple transparent membrane, or whether it acquire, as the tubes which it envelopes en large, cellular and muscular fibres externally, whilst thb previously transparent membrane disappears or remains as a basement mem brane towards the epithelium ; or whether a separation or splitting of the transparent membrane into fibres takes place. In the opinion of Henle, all the true glands having a vesicular termination, from the smallest to the most complicated, have an intermediate mus cular tunic, with a series of longitudinal fibres situated within, and circular fibres without, the former being much more highly developed than the latter, and entirely absent in the more delicate ramifications of the duct. Miller, admitting the great difficulty of determining by the rnicroscope the muscular character of the intermediate coat, is nevertheless of opinion that such is its nature, and appears inclined to believe that the frequent sudden expulsion of the saliva is attributable to it.