1900.] STRUCTURE OP THE MUSK-OX. 689 



worn off in any higher degree than that a section from the tip of 

 the horn in which such a horn-core fits fairly well corresponds to 

 the original horn of that age, fig. 1 A is prepared. It is represented 

 in the same plane as the main axis of the horn (as also is the case 

 with figs. 1 B and C). Pig. 1 A should thus represent the horn of 

 a young bull in the second autumn of its life. The longitudinal 

 section of the horn of an adult bull, which is reproduced in fig. 2, 

 shows that the horn has grown straight only until it reached a 

 length of 5 cm. or hardly that, because already at that distance 

 from the tip the originally ventral (later en distal) side of the 

 horny sheath is thickened. Such a thickening of the ventral side 

 of the horn produced by stronger growth of these parts effects, of 

 course, the curving upwards of the tip of the horn. This curving 

 upwards continues the whole of this period of growth, viz., the 

 second summer of the animal's life. At the end of this summer 

 the horns probably have attained a length of 35 cm., more or less, 

 and their tips are at that time more raised above the facial line 

 than at any later time. 



Kg. 2. 



Longitudinal section through a full-grown horn of the Musk-ox, showing 

 lines indicating the growth at different periods. 



I have above used the words " period of growth," for I think 

 that the horns are chiefly enlarged during that time of the year 

 when the animals are able to procure food in sufficient quantities 

 not only to sustain life, but also to add to their bulk. Theoretically 

 such an assumption does not seem too hazardous considering the 

 circumstances under which these animals live. But the prob- 

 ability of this is strengthened by the fact that on the longitudinal 

 section of the horn are seen some lines of demarcation which are 

 most easily interpreted as the limits between such parts as have 

 been added during different periods of growth 1 . At " a" on fig. 2 



1 Similar lines of demarcation are also seen on a longitudinal section of the 

 horns of a common cow, and stand in that case plainly in connection with the 

 rings at the base of the horn which in this country commonly are called " calf- 

 rings," because it is known that a new ring, that is a new layer of horny 



