sure that he would not state that which he did not believe to be true, but at the same time we think that in this instance his assertions are stronger than the actual state of the case would fairly warrant. We know that the practice he speaks of does prevail among a certain class of practitioners, but as to the extent to which it prevails, we believe Mr. Musgrave's experience must be exceptional. It is not an uncommon occurrence for several cases of a disease which we know to be rare, as, for instance, abscess of the antrum, to come under the notice of a practitioner within a short period of time, and in all probability a similar coincidence of cases of amalgam fillings in front teeth has impressed Mr. Musgrave with the idea that the practice is becoming more general than it really is.-ED. B. J. D. S.'] To Correspondents. Communications intended for insertion in the ensuing number must be forwarded to the Editor, at the Office, 11, New Burlington Street, London, W. by the 8th and 23rd of the month, or they cannot be published in the ensuing issue; they must also be duly authenticated by the name and address of the writer. ANSWERS TO CORRESPONDENTS. Communications have been received from Messrs. H. B. BOOKS AND PAPERS RECEIVED. 'British Medical Journal.' 'Medical Times and Gazette.' 'Missouri Dental Journal.' 'Dental Jairus.' 'Il Progresso Dental de la Habana.' Mason (Exeter), Hon. Sec. of the · Palmer (South Transactions of the Odonto-Chirurgical Society.' 'Wakefield Herald.' &c. NOTICE. We desire that it may be clearly understood that our pages are open to all for free expression of their views on matters connected with our profession. We only ask for terseness of expression and MODERATION IN TONE. When otherwise unobjectionable, difference of political or other opinion will never be regarded by the Editor as a disqualification for the admission of any communication to the pages of the BRITISH JOURNAL OF DENTAL SCIENCE. British Journal of Dental Science. No. 313. LONDON, FEBRUARY 1, 1881. VOL. XXIV. A COURSE OF LECTURES ON DENTAL ANATOMY AND Delivered at the National Dental College during the Winter By THOMAS GADDES, L.D.S. Eng. Lecturer also on the Elements of Histology; Assistant Dental Surgeon to the National Dental Hospital. LECTURE II. IN vertebrate animals horn is found in the form of teeth in the lamprey and hag-fish (as I mentioned in my previous lecture) and in the Ornithorhynchus or duck-mole; as the representative of teeth in the baleen plates of the whalebone whale, and in the horns of the Ungulata; it is also found as a dense protective covering to the jaws of the amphibious siren or mud-eel, of the tortoise and turtle, the manatee and dugon; and likewise as the bills of birds. Modifications of horn are to be found in the carapace and armour-plate of the tortoise and turtle, the spines of the porcupine and hedgehog, the quills of birds, and the hoofs, claws, and nails of animals. The horns of animals may be classified as-(1) The consolidated horn of the rhinoceros; (2) the hollow horns of the genus Cavicornia-the ox, sheep, and antelope; also (3) the solid horns or "antlers" of deer. The term horn," as applied to the projection from the head of an animal, is restricted to the horny weapon which is supported upon the head, and which may be solid or hollow. In the latter case it is composed of a bony base or "core," covered by a sheath of true horny tissue. The antlers of deer are not, therefore, true horns. The substance horn consists chiefly of consolidated albumen and a small portion of phosphate of lime-from about 1 to 3 or 5 per cent. The horn of the rhinoceros is wholly composed of longitudinal fibres of horny matter which are agglutinated together. These fibres are of epidermic growth, and the horn is not fixed to the bone of the skull. VOL. XXIV. 7 In some of the Cavicornia, or hollow-horned animals, the horny sheath is shed annually. This takes place notably in the prong-buck of North America; but it is not usual for the horn case of the Cavicornia to be so shed. Antlers consist of bone only, being solid throughout. They are anchylosed or fixed to the frontal bone, and they are shed and reproduced annually. During the period of their growth the antlers are covered by a vascular, shorthaired, velvety skin, continuous with the integument of the head. When growth is completed this skin dries up and peels off, leaving the antler quite bare. Around the base, preparatory to the separation of the vascular integument, a ring of osseous tubercles is developed, forming what is termed the "burr." As the vessels of the skin in the region of the burr decrease in size, the ossification of the burr encroaches upon their diminishing area, until, at length, the antler is deprived of all vascularity and nutrition; and it becomes separated from the outer plate of the living cranial bone by the process of absorption, and is shed. FIG. 1. Transverse section of horn of rhinoceros B. The A plate of whalebone consists of a central, coarse, hair-like substance, enclosed within an outer and more compact covering. The central coarse fibres are hollow tubes and contain at their bases vascular developmental papillæ. These fibres are loosely agglutinated together. The tubular fibres of the outer lamina receive from the gum substance around the base of the plate a horny cementing substance. This causes the surface of the plate to be smooth and more brittle than the inner portion. In use this brittle lamina breaks off, and leaves the ends and margins of the plates to fringe out by the separation of the fibres from one another. The horny teeth of the Ornithorhynchus is, in its central portion, distinctly fibrous with a more dense and horny external covering, Bone. Bone is essentially composed of an animal basis impregnated with calcareous salts, and through the substance of the tissue are scattered minute cavities, or Haversian canals, and lacunæ, which latter send out multitudinous ramifications called canaliculi. The organic and inorganic matters, although intimately incorporated, may be separated from one another by the action of dilute hydrochloric acid, which dissolves the earthy constituents and leaves the animal matter behind; the organic basis thus remaining corresponds in form and size to the original bone. If, on the other hand, the animal matter be destroyed by a red heat, the earthy material will retain the original form of the bone; and, in consequence of having been deprived of its organic matter, it will be found to be so brittle that it may be broken down by very slight pressure. So, then, the organic matter of bone, which constitutes about one third, gives tenacity and elasticity, and the inorganic matter, which constitutes about two thirds, gives hardness and rigidity to the osseous tissue. The formation of the organic matter is an operation quite distinct from its impregnation with earthy material, and the former process may occur without the latter. The animal basis may be formed, but if not impregnated with earthy salts it will be destitute of those physical properties for which osseous tissue is required-it will be so soft that it will neither support the weight of the body, nor constitute a firm framework for the attachment of muscles, a condition which exists in rickets. In some situations bone tissue is arranged to form an open texture resembling lattice work, and exhibiting spaces or cancelli, which communicate freely with one another. This form is called cancellated or spongy bone. These spaces are occupied with a little fatty matter, traces of connective tissue, cells, and vessels, and are lined with a delicate membrane-the medullary membrane. In birds, with some exceptions, the spaces in the bones contain air. The bony walls of these spaces are composed of thin plates or spicule of osseous tissue, in which are lacunæ and canaliculi. If the bony walls of the cancelli become very much thickened, so as only to leave room for one vessel in the centre, we have an approach to the other kind of bone tissue, which is called compact tissue. This, however, in its fully formed state, contrasts remarkably in character with the spongy cancellated texture. The compact tissue is so firm and dense that you would not suppose it was traversed by numerous vessels which run in channels or Haversian canals, and which are connected here and there by transverse branches, so that if the whole of the bony matters were removed from the compact tissue we should have left a web or net work of capillary vessels having elongated or longitudinal meshes. The difference in structure between the compact and cancellated forms of bone tissue depends, therefore, merely upon the different amount of solid matter, and the size and number of the spaces in each the cavities being small in the compact tissue, and the solid matter between them abundant; whilst in the cancellated tissue the spaces are large, and the solid matter in smaller quantity. The compact tissue is placed upon the exterior of a bone, and the cancellated tissue is almost always internal just the situations where we saw (Lecture 1) the laws of morphological and physiological development indi cate. I shall now describe the microscopic structure of the compact bony tissue. The Haversian canals contain blood-vessels, and the largest of them also contain marrow. The blood-vessels are supported by processes from the medullary and periosteal membranes, which enter the minute canals along with the vessels, and through the medium of these processes do the medullary membrane and the periosteum become continuous with one another. Besides these processes the Haversian canals are lined with osteoblasts, and the function of these osteoblasts we shall see by-and-bye. The canals are from Toth to th of an inch in diameter. Some are much smaller than this, but they are rarely to be found. The widest Haversian canals are, in long bones, nearest the medullary cavity, and towards the circumference of the bone they are smaller. In longitudinal section they may be seen to run for the most part parallel to the surface, to be quite short, and to be somewhat oblique and crooked at their ends, where they freely open into one another. Arteries and veins usually occupy separate canals, and the veins, which are larger, often present, at irregular intervals, small pouch-like dilatations. In transverse section, surrounding the Haversian canals are a series of concentric lamellæ, one within the other. There are lamellæ also immediately beneath the periosteum and the medullary membrane, which extend uninterruptedly |