APPENDICES OF THE SKIN. 99 



although very remarkable exertions are frequently made by 

 the system to repair the loss *. 



4. Scales. These vary remarkably in their form, struc- 

 ture, mode of adhesion, and situation in different animals. 

 In general they are flat plates, variously marked. In some 

 cases, each scale consists of several plates, the lowest of 

 which are largest; so that the upper surface becomes some- 

 what imbricated. Some scales adhere by the whole of their 

 central surface ; while others resemble the human nail, in 

 having the distal extremity free. 



The composition of scales is similar to that of the cuticle, 

 with the addition of some earthy salts. They appear to be 

 inserted in that layer of the skin, and to resemble it in many 

 of their properties. When rubbed off, they are easily re- 

 newed, and frequently experience the same periodical reno- 

 vations as the cuticle. In some cases, the cuticle may be 

 observed extended over them, as in the armadillos. In ge- 

 neral, however, it only surrounds their base. They derive 

 their colour from the mucous web on which they are placed. 



In quadrupeds, scales occur, covering the whole body 

 like a coat of mail, in the armadillos ; or certain parts only, 

 as the tail of the beaver and rat. In birds, the scales are 

 found only on the feet. In reptiles, they occur on every 

 part of the body, and are placed laterally in some ; while, 

 in others, they are imbricated like the slates of a house. In 

 fishes, the scales are usually imbricated, with the distal edge 

 free, and the epidermis enveloping their base. In some in- 



* TULPIUS notices some curious cases of this kind, which fell under his 

 own observation : " Uugues in digitorum apicibus, scmcl dcpcrditos, ite- 

 rum renasci novum non est, sed raro id conspicitur fieri, in sccundo aut ter- 

 tio articulo, prioribus amputatis, in quibus tamcn non bcmel eosdcm vidimu> 

 non secus progerminare debitamquc acquirere formani, ac si in digitorum 

 consisterent apicibus, deponente nunquam solicitudincm suam ofticiosa na 

 tura." 'Observationes Medica, lib. iv. cap. 56. 



G 2 



