280 ELEMENTARY ANATOMY. [LESS. 



Some Deer have enormous antlers, weighing as much as 

 seventy pounds, and are formed at the rate of one pound a 

 day. 



Great as must be the strain on the system from such a 

 demand, it must yet be exceeded by the effects on Birds of 

 the production of an entire new plumage when moulting. 



The antlers, like the bony cores of the hollow-horned rumi- 

 nants, are rather outgrowths from the skull than skin struc- 

 tures. Yet in the Giraffe we find short bony horns formed 

 from ossifications independent of the skull, with which they 

 unite at a later period only. 



Such are the principal structures which may be described 

 under the head of exo-skeleton. Other appendages of the 

 skin which are not skeletal, but excretory (as the milk 

 glands, scent glands), will be noticed in the Twelfth Lesson 

 as coming within the group of "excretory structures." 



