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Show 1900.] STRUCTURE O F T H E M U S K - O X . 693 horny substance takes place periodically, as can be seen from the 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 tbe 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 tbe surface of the base of the horn, the base of which thus rests as in a thin basket1 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 grow'th 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 Bichardson as " very rough" and " coarsely columnar " (I. c. p. 67), and by Lydekker2 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. |