Formation of osseous tissue in union offrac tured bones.—Supposing the subject in which the fractures occur to be young, cartilage similar in every respect to temporary cartilage is produced between the fractured extremities of the injured bone. In the centre of this ossification commences, the process being somewhat similar to ossification of permanent cartilage, or holding an intermediate place between that process and the ossification of epiphyses.
The corpuscles here increase in size but not in number. The ossification commences in the intercellular tissue, and proceeds to the parietes of the cells, thus forming areolm of bone.
The action may also commence in the carti lage in contact with the fractured surface. This I believe to be the process by which repa ration is effected in all cases where union of fractured bones takes place, but my experi ments have been confined to young animals.
I have examined various cases where union has not been effected in consequence of the patient's advanced age, and the fracture being at the neck of the femur or of the shaft of the same bone. In these I found no cartilage, and but a scanty amount of condensed cellular tissue. In this latter, however, traces of an attempt at repair may generally be found in the pre sence of osseous matter in granules or granular masses. In these there is no arrangement of tubes or bone-cells of definite character; indeed these osseous masses are generally small, and sornetimes without material density, the indi vidual granules not having firmly united. In fact they resemble in every point adventitious bone.
If in a young animal the fracture be not kept tolerably quiet, the motion between the fractured bone will prevent the formation of cartilage, which seems necessary to the deve lopement of bone, and here, therefore, osseous masses will alone present themselves. This fact is very interesting in a surgical point of view, and might be treated more at length; but having given a detailed account of the deve lopement of bone and of the distinction to be made between true bone and adventitious osseous tissuez I shall conclude the article.
( J. Tomes.) PACHYDERMATA.—An extensive group of herbivorous quadrupeds, constituting a dis tinct order of the class Mammalia, generally remarkable for their ponderous bulk and un wieldy appearance, and seemingly forming the transition between the gigantic Cetacea, which from their size are only adapted to an aquatic existence, and the vegetable-eating Mammals of strictly terrestrial habits. Even the localities where they are met with would seern to indi cate that they constitute such a connecting link, seeing that their most typical forms are peculiarly adapted to be occupants of the river and the marsh, from the Ilippopotamus, that might almost be considered an aquatic animal, to the Tapirs and the Ilog, which still love to wallow in mud although they approximate in their habits to the ruminating quadrupeds. At the present day the order Pachydermata con tains but few genera, and these for the most part embrace a very limited number of species. But in former periods of the history of our globe they must have existed under much greater variety of form, seeing that the tertiary deposits yield to the geologist, in abundance, the remains of very numerous genera now totally extinct, to the list of which modem researches are adding day by day ; it is indeed more than probable that many of the existing races will speedily perish, for the hand of man is against them, and the bullet and the spear are doing their work of extermination rapidly, so that the Tapir and the Elephant, like the Palceotherium and the Mastodon, may soon be classified with extinct existences.
The order of Mammalia under consideration is usually divided into PROBOSCIDIANS, in cluding such Pachydermata as are provided with a proboscis and tusks, of which the Ele phant is the only existing example, and into