1900.] STRUCTURE OF THE MUSK-OX. 693 



horny substance takes place periodically, as can be seen from tbe 

 distinct lines of demarcation which are visible on the section (cf. 

 fig. 2, p. 689, and fig. 4, p. 692). These lines are wavy, thus showing 

 the continuation of the warts mentioned above, and on account of 

 the same origin parallel layers can also be distinguished. The 

 reabsorption of the bony mass continues until the horny substance 

 has reached down nearly to the fronto-parietal surface, but on the 

 median and partly posterior side there remains a vertical lamella, 

 thin as a leaf and perforated (as can be seen in fig. 3, p. 691). 

 Anteriorly it extends horizontally under the horn. This thin crest 

 lies close to the surface of the base of the horn, the base of which 

 thus rests as in a thin basket 1 of bone constituting the remains 

 of the former exostoses. These facts, especially the presence of 

 the thin median lamella, which hardly could have been produced 

 in auy other way, proves that the formation of the basal parts of 

 the horns has taken place exactly in the manner described above, 

 and that thus the greatest exostoses belong to comparatively young 

 though just full-grown animals, but that in old bulls the exostoses 

 are more or less completely reabsorbed. 



The last modification of the outer appearance and of the 

 direction of the horns happens during this last phase of development 

 just described. It has been said that during the last periods of 

 growth it is mainly the layers of the upper side at the base of the 

 horn that are thickened. These new layers, which are added from 

 the interior, partly take the place of the reabsorbed bony mass, but 

 grow of course upwards from the matrix, and exercise thus a great 

 pressure from within on the outer layers which have been formed 

 before. This pressure is so strong that the outer layers are broken 

 and cracked, and through this the bases of the horns receive their 

 peculiar aspect, described by Richardson as " very rough " and 

 " coarsely columnar " (?. c. p. 67), and by Lydekker 2 as " marked by 

 coarse longitudinal groovings." In old horns finally transverse 

 cracks across the basal ridges add still more to the roughness of 

 these parts. 



It has been said above that the basal growth also influences the 

 direction of the horn. This depends upon the fact that when 

 the horny sheath is strongly thickened at its base on the upper 

 side, such an addition of substance tends to protrude the sheath ; 

 but when this cannot be done because the base clings to the head 

 in the manner described above, and when moreover there is no 

 corresponding growth on the under side, the effect produced is a 

 pressure on the originally upper, now outer or distal, side of the 

 core, which accordingly is reabsorbed. Simultaneously the core 

 is strengthened by apposition on its first lower, now inner or 

 proximal, side. 



Such views as these cannot be proved without histological 



1 This thin, perforated basket of bone can also be seen on figure 5 (p. 697) 

 of a longitudinally sectioned skull seen from the inner (mesial) side. 



2 ' Wild Oxen, Sheep, and Goats of all Lands,' p. 780. 



