32 PHILOSOPHICAL TBANSACTIONS. [aNNO 1791. 



their contents, are filled with hair mixed with a curdled substance, or hair with- 

 out any admixture whatever, and have a similar kind of hair growing on their 

 internal surface, which is likewise covered with a cuticle. These cuticular in- 

 cysted tumours were, he believes, first accurately examined by Mr. Hunter, to 

 whom we are also indebted tor an explanation of the mode in which the parts 

 acquire this particular structure. 



Mr. Hunter considers the internal surface of the cyst to be so circumstanced 

 respecting the body, as to lose the stimulus of being an internal part, and receive 

 the same impression from its contents, either from tlieir nature, or the length 

 of application, as the surface of the skin does from its external situation. It 

 therefore takes on actions suited to such stimuli, undergoes a change in its 

 structure, and acquires a disposition similar to the cutis, and is consequently 

 possessed of the power of producing cuticle and hair. What the mode of action 

 is, by which this change is brought about, is not easily determined; but from 

 the indolence of these complaints, it most probably requires a considerable length 

 of time to produce it. That the lining of tlie cyst really does possess powers 

 similar to cutis, is proved by the following circumstances: that it has a power of 

 forming a succession of cuticles like the common skin; and what is thrown off 

 in this way is found in the cavity of the cyst. It has a similar power respecting 

 hair, and sometimes the cavity is filled with it, so great a quantity has been shed 

 by the internal surface. Besides these circumstances, the hair found in the cyst 

 corresponds in appearance with that which grows on the body of the animal; and 

 when incysted tumors of this kind form in sheep, they contain wool. What 

 is still more curious, when such cysts are laid open, the internal surface under- 

 goes no change from exposure, the cut edges cicatrize, and the bottom of the 

 bag remains ever after an external surface. Different specimens, illustrative of 

 the above mentioned circumstances, are preserved in Mr. Hunter's collection of 

 diseases. 



The cysts that produce horny excrescences, which are only another modification 

 of cuticle, are very improperly considered as giving rise to horns; for if we ex- 

 amine the mode in which this substance grows, we shall find it the same with the 

 human nails, coming directly out from the surface of the cutis. It differs from 

 the nails in not being set on the skin by a thin edge, but by a surface of some 

 breadth, with a hollow in the middle, exactly in the same manner as the horn of 

 the rhinoceros*; at least this is evidently the case in the specimen preserved in 

 the British Museinn, and in one which grew out from the tip of a sheep's ear; 

 they are also solid, or nearly so, in their substance. This mode of growth is 

 very different from that of horns, which are all formed on a core, cither of 



• The horn of the rhinoceros is a cuticuhir appendage to the skin, similar to nails and other cuti- 

 cular excrescences, being in no respect allied to horns but in tlie external appearance.— Orig. 



