the really hard substance comes, which corresponds to the epidermis.

Nevertheless-to discuss the subject at once, seeing that I shall not again have occasion to mention it-the structure of the nails has been difficult to make out, because they were conceived to be a simple formation. Nearly all the discussions, therefore, which have taken place, have turned upon the question where the matrix of the nail was, and whether the growth of the latter took place from the whole surface or from the little fold into which it is received behind. If we consider the nail with respect to its proper firm substance, its compact body (Nagelblatt), this only grows from behind, and is pushed forward over the surface of the so-called bed of the nail (Nagelbett), but this in its turn also produces a definite quantity of cellular elements, which are to be regarded as the equivalents of an epidermic layer. On making a section through the middle of a nail, we come, most externally, to the layer of nail which has grown from behind, next to the substance which has been secreted by the bed of the nail, then to the rete Malpighii, and lastly to the ridges upon which the nail rests.

Thus the nail lies in a certain measure loose, and can easily move forwards, pushing itself over a moveable substratum, whilst it is kept in place by the ridges with which its bed is beset. When a section is made transversely through a nail, we see, as already mentioned, essentially the same appearance presented as that offered by the skin, only that a long ridge corresponds to every single papilla seen in ordinary sections of the skin; the undermost part of the nail has slight indentations corresponding to these ridges, so that, while gliding along over them, it can execute lateral movements only within certain limits. In this manner, the body of the nail which grows from behind moves forward over a cushion of loose epidermic substance (Fig. 17, a) in grooves which are provided by the ridges and

furrows of the bed of the nail. The uppermost part of the nail, if examined when fresh, is composed of so dense a substance that it is scarcely possible to distinguish individual cells in it without applying reagents, and at many points an appearance is presented like that which we see in cartilage. But by treating it with potash, we can convince ourselves that this substance is composed of nothing but epidermiscells. From this mode of development you will see how easily intelligible distinctions may be drawn between the different diseases of the nails.

FIG. 17.

a

There are diseases of the bed of the nail which do not affect the growth of its body, but may give rise to changes in its position. When there is a very abundant development of cells in the bed of the nail, the body may be pushed upwards (Fig. 17, 6); nay, it sometimes happens that the nail, instead of growing horizontally, shoots perpendicularly upwards, the space underneath being filled with a thick accumulation of the loose cushiony substance (Polstermasse) (Fig. 17, c). Thus suppuration may take place in the bed of the nail without the development of its body being thereby impeded. The most singular

changes occur in small pox. When a pock forms upon the

Fig. 17. Diagrammatic representation of a longitudinal section of a nail. a. The normal condition; a gently curved, horizontal nail, implanted in its fold, and separated from its bed by a thin cushion. b. A more markedly curved and somewhat thicker nail, with great thickening of the cushion, and much increased curvation of the bed, the fold being shorter and wider. c. Onychogryphosis; the nail, short and thick, reared up at a considerable angle, the fold short and wide, the bed furrowed on its surface, the cushion very thick and composed of layers of loose cells, piled up one above the other.

bed of the nail, there is nothing to be seen but a yellowish, somewhat uneven, spot; but if, on the other hand, it is developed upon the fold, then its traces are left in the shape of a circularly depressed, and, as it were, excavated spot in the body of the nail as it gradually advances, a proof of a loss of substance precisely similar to that which takes place in the epidermis.

I will not to-day, gentlemen, enter more particularly into the special history of the formation of epidermis and epithelium, although it is of great importance for the right comprehension of many pathological processes, but content myself with calling your attention to the fact, that, under particular circumstances epithelial cells may undergo a series of transformations, through which they become extremely unlike what they originally were, and gradually assume appearances which render it impossible for those who are unacquainted with the history of their development to realize their original epidermic nature. The greatest abnormity of the kind is met with in the crystalline lens of the eye, which is originally a mere accumulation of epidermis. It has its origin, as is well known, in a saccular involution of the external skin. At first its connection with the external parts continues to be maintained by means of a delicate membrane, the membrana capsulopupillaris; afterwards this atrophies and leaves the lens isolated in the interior of the eye. The fibres of the lens are therefore, as C. Vogt has shown, nothing more than epidermoidal cells which have been developed in a peculiar manner, and their regeneration, after the extraction of a cataract for example, is only possible as long as there still remains epithelium in the capsule to undertake the new formation, and to represent, as it were, a thin layer of rete Malpighii. This reproduces the lens in the same way that the ordinary rete Malpighii of the external surface does the cuticle. Amongst the other changes of epithelial struc

CRYSTALLINE LENS. PIGMENT. GLAND-CELLS.

39

tures we shall in due time revert to the peculiar pigment-cells that are produced in the most different parts by the direct transformation of epidermic cells, the contents of which either become coloured by imbibition, or have pigment engendered in them by a (metabolic) transposition of their elements.

With the history of epithelial elements properly so called is immediately connected that of a peculiar class of structures which play a very important part in the accomplishment of the functions of an animal, namely the glands. The really active elements of these organs are essentially of an epithelial nature. One of Remak's greatest merits consists in his having shown that in the normal development of the embryo the outer and inner of the well-known three layers of the germinal membrane chiefly produce epithelial structures, from a gradual proliferation of the elements of which glandular structures arise. Other observers, for example, Kölliker, had indeed before him made similar observations, but by Remak was first established the law that the formation of glands in general must be regarded as consequent upon a direct process of proliferation on the part of epithelial structures. Previously large quantities of cytoblastema had been conceived to exist, in which, spontaneously, glandular substance took its rise; but, with the exception of the lymphatic glands, and perhaps those belonging to the sexual organs, their mode of origin is everywhere this-that at a certain. point, in a manner very similar to that which I described to you in the foregoing lecture, when speaking of the excrescences of plants, an epithelial cell begins to divide, and goes on dividing again and again, until by degrees a little process composed of cells grows inwards, and, spreading out laterally, gives rise to the development of a gland, which thus straightway constitutes a body continuous with layers of cells originally external. Thus arise the glands of the surface of the body (the sudoriferous and sebaceous glands of the skin and the mammary gland), and thus also arise

the internal glands of the digestive tract (the stomachglands and liver). The most simple forms which glands can present do not occur at all in man. In inferior animals, however, uni-cellular glands have recently been discovered. The glands of the human body are invariably made up of a number of elements, which can, however, ultimately be traced back to a nearly simple type. Besides, in our own glands, in consequence of their size and complicated structure, other necessary constituents generally

A

FIG. 18.

e

TO

enter into their composition, so that, regarded as organs, they certainly do not consist of gland-cells only. But all parties are now pretty well agreed that the gland-cells are the really essential elements, just as the primitive bundles are in muscle, and that the specific action of a gland is dependent upon the properties and peculiar arrangement of these elements.

Generally speaking, therefore, glands consist of accumu

Fig. 18. A. Development of sweat-glands by means of the proliferation of the cells of the rete Malpighii in an inward direction. e. Epidermis. r. Rete Malpighii. gg. Solid process, constituting the first rudiments of a gland. After Kölliker.

B. Portion of the duct of a sweat-gland in a state of complete development. tt. Tunica propria. e e. Layers of epithelium.