| OCR Text |
Show 690 DR. E. LONNBERG ON THE [June 19, is such a mark, which is very pronounced : and I think that it may be regarded as the place where the distal tip of the horn-core was situated at the end of the first period of growth. Such a supposition corresponds also very well with the dimensions of tbe horn-core, as they have been given in Bichardson's figure, compared with those of the horn. During the second period of growth (third summer) the upper side of the horn is thickened by more rapid growth than that which takes place on the under side. Through this a pressure is effected by the horny sheath ou the upper side of the horn-core, and this causes a reabsorption on the upper side of the same. On the lower side, on the contrary, the pressure is diminished and, therefore, the horn-core is thickened below by apposition. In such a manner tbe direction of the main axis of the horn is lowered. At the same time the length of the horny sheath is increased by basal growth, and it is driven out from the head in the direction of the axis of the horn-core on which it glides. In connection with this, new layers of horny substance fill up the end of the inner cavity of the horny sheath, and thus form the horny plug which can be seen on fig. 2 extending between " a " and " b." Next period (fourth summer) the growth is continued mostly in the same way. The horny sheath is prolonged, and by stronger growth on the upper side the main axis of the horn-core causes reabsorption on its upper and apposition on its lower sidel. During this period the plug between " b " and " c " is formed. substance, is added after each act of parturition. The cause of this is, of course, the following : as long as the cow is gravid all superfluous nourishment is consumed for the growth of the uterus with its contents, and thus only little can be spared for the enlargement of the horns, but as soon as the calf is born there is material enough also for that purpose. 1 That pressure can effect a transformation of bone through reabsorption and apposition may be proved by a great number of examples, but I shall here only recall a few. The enlargement of the brain-case of the mammals takes place in such a way caused by the pressure from within produced by the physiological growth of the brain itself. In this case reabsorption and apposition caused by the pressure are moments of the normal growth. When the dentist wants to straighten a set of teeth the members of which are placed obliquely or irregularly, he applies a pressure from the palatal side, and so the teeth which protrude behind the row move forward into the same rank as the others (provided there is space enough). This takes place in such a way that through reabsorption of tiie pressed parts of the alveolar walls new room is procured for the respective teeth to move forward, and by apposition from behind they become fixed in the new socket. It is in this case a transformation of bone produced entirely by artificial, mechanical power. But also pathological pressure can alter the shape of the bone. W h e n by pressure of a parasite, as in the cases of pentastomids described by Hensel, mis-shaping skulls of Mustclids, prominences of the cranial wall are produced, or when a tumour is encased in bony tissue, these are also results of reabsorption and apposition. From these examples it may thus be concluded that a pressure on a bone which is not especially adapted to endure pressure, as the bones of the articulations and some others, has as a residt that the bono gives way by reabsorption, but for the maintenance of the equilibrium compensating material is deposited by apposition. This rule is also applied for the core in the development of the horn in the Musk-ox. |
