HORN AND HOOF. BOTH these structures are composed of the same elementary tissues as nail and epidermis. The method of examination is in every respect the same as that recommended for the study of nail. Portions of hoof and horn, with the soft structure attached, are to be dried, and afterwards cut with a razor, and the sections treated in the usual manner. To render the cells distinct, caustic potass is necessary, and must be employed in the way previously directed. Horn is as nearly as possible identical in structure with nail, as will be seen by making a fine transverse section of a portion of the horn of an ox, and comparing it with the drawings of transverse sections of the human nail (figs. 81, 82, pp. 97, 98). Some of the cells of the deep or mucous layer of horn contain pigment, which is also frequently found in the same part of human nail; in that of the negro it is invariably present. The ridges at the under surface of horn are not so close together nor so well developed as those of the nail; but with these unimportant differences, the resemblance is perfect. Hoof. The structure of hoof, as seen in sections, is tubular, the canals running from above downwards. In transverse sections of hoof, the openings of the numerous canals will be observed surrounded by concentric lines, indicating the layers of compressed epithelial cells. A drop of caustic potass placed at the edge of the covering glass, will, in a few minutes, cause the cells to expand without destroying their concentric arrangement. In the embryo hoof the cells are seen very distinctly surrounding the canals, looking remarkably like the adult hoof after the addition of a drop of caustic potass. The next drawing represents transverse sections of the hoof in its natural state, and after treatment with potass. Transverse section of horse's hoof. A. Natural condition. B. After the addition of potash.-Magnified 200 times. Specimens of nail, horn, or hoof, may be mounted dry, or soaked in turpentine, and afterwards put up in thin Canada balsam. TEETH. THE several structures of which a tooth is composed are arranged in definite order round a central space, which is termed the pulpcavity, containing a soft reddish substance which abounds in nerves and vessels. The bulk of the tooth is formed of ivory, or dentine, over which is reflected a coating of enamel, principally upon the crown of the tooth, and, external to these structures, the cementum, or crusta petrosa, which, in the horse and some other of the herbivora, forms a complete investiture, but in man only covers the fang and a small portion of the crown. The accompanying sections of the human molar will convey an idea of the arrangement of the different structures, and the position of the pulp cavity. Human molar tooth; magnified about 5 times. A. A longitudinal, B. a The best view of the three tissues, ivory, enamel, and crusta petrosa, may be obtained by examining a section of a tooth of a horse. The drawing representing these parts is taken from a very successful preparation of an incisor tooth of that animal. Section of incisor of horse. a. Crusta petrosa. b. Enamel. c. Dentine. Microscopic examination of Tooth.-In the first place sections of softened teeth may be studied, these are prepared by macerating a tooth in dilute acid as directed for bone, and afterwards cutting fine sections with a razor. Sections of fresh teeth will also be necessary, but their preparation is so tedious that they are better purchased ready for examination, at prices varying from 1s. to 5s., according to their quality. A variety of sections will be nccessary, in order to demonstrate the characters belonging to the different structures. Very valuable and instructive preparations may be made by Dr. Beigel's method of using acids. If a tooth be boiled for a few minutes in sulphuric acid, the cementum, cuticle of the enamel and the ivory are all dissolved, leaving the enamel only intact in the form of a small horse-shoe. By arresting the process at the proper moment it is possible to obtain specimens of enamel and ivory together, the cementum only being dissolved out by the acid. Nitric acid, on the other hand, destroys the enamel before it attacks the other structures. Dentine. In a longitudinal section of a tooth the dentine appears to be transparent; under a microscope it is seen to consist of tubes running from the centre to the external surface, and describing two or three spiral curves in their course (primary curves), as seen in fig. 86, A. The tubes are also finely undulated throughout their entire length, having as many as 200 flexures in a line (secondary curves). The dentinal canals are largest at their commencement from the pulp cavity, becoming finer as they proceed outwards, and terminating, in some parts of the tooth, in small irregular cavities like lacunæ. From various parts of the dentinal tubes small branches arise, which ramify and anastomose in all directions. In the drawing all these peculiarities of the structure of dentine are represented. Dentine and cement, from the middle of the fang of an incisor tooth. Other sections are necessary to enable the student to see the tubular character of dentine, as in the subjoined woodcut, which represents a transverse section of the human molar, showing the homogeneous matrix containing the dentinal tubules. |