MAMMALIA. 689 



day. On making a longitudinal section of the young horn, it is 

 found to be continuous with the os frontis, having its outer sur- 

 face covered with a vascular periosteal membrane derived from 

 the pericranium, which in turn is protected by a fine velvety skin. 

 Moreover, when a growing antler is injected minutely, and its 

 earthy matter removed by means of an acid, vessels derived from 

 the periosteum are found to traverse it in all directions, proving its 

 identity with real bone. As growth goes on, the external carotid 

 arteries, thus called upon rapidly to furnish a prodigious supply of 

 materials, dilate in a remarkable 'manner, and soon the palm and 

 the antlers of the horn have acquired their full dimensions. No 

 sooner is this accomplished, than a prominent ring or burr is 

 formed around the base ; which, projecting outwards, compresses 

 and soon obliterates the vessels that have hitherto supplied the 

 growing defences. The circulation being thus put a stop to, the 

 soft teguments and periosteum peel off in strips ; and the bone, 

 denuded of its covering, becomes a formidable weapon. 



At the close of the breeding season the removal of the horns is 

 speedily effected : the connection between their bases and the os 

 frontis is gradually weakened by interstitial absorption, until at 

 length a slight effort is sufficient to detach the branching honours 

 of the Stag, and they fall off, leaving a broad cicatrix ; this soon 

 skins over, and the succeeding year calls forth a repetition of the 

 process.* 



(791.) The CETACEA form a very remarkable group among 

 the hot-blooded Mammifers, as relates to the external covering of 

 their bodies. No covering of hair or wool would have been effi- 

 cient in retaining the vital heat under the circumstances in which 

 these creatures live ; and, even if such clothing could have been 

 made available, it would have seriously impeded their progress 

 through the water. Another kind of blanket has therefore been 

 adopted : the cuticle is left perfectly smooth and polished, with- 

 out any vestige of hair upon its surface ; but, beneath the skin, 

 fat has been accumulated in prodigious quantities, and, enveloped 

 in this non-conducting material, the Whales are fully prepared to 

 inhabit an aquatic medium, and to maintain their temperature even 

 in the Polar Seas. 



* In a physiological point of view this rapid production of osseous matter is truly 

 wonderful. The horns of the Wapiti Deer, thus annually reproduced, will weigh up- 

 wards of thirty pounds ; and in the fossil Irish Elk the weight of these deciduous 

 defences must have been greater than that of the entire skeleton. 



2 Y 



