690 DR. E. LoXXBERG ON TI1E [Julie 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 horu-core was 

 situated at the end of the first period of growth. Such a sup- 

 position corresponds also very well with the dimensious of the 

 horn-core, as they have been given in Richardson's figure, compared 

 with those of the born. 



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 on 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 the 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 side '. 

 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 ouly 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 patho- 

 logical pressure can alter the shape of the bone. When by pressure of a 

 parasite, as in the cases of pentastomids described by Hensel, mis-shaping skulls 

 of Mustelids, 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 result that the bone 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. 



