122 OF THE ANIMAL STRUCTURE, &C. 



its place. No one can doubt of the use and propriety of 

 this difference ; of the " analogy" being thus " interrupted ;" 

 of the rule, which belongs to the conformation of the bones, 

 stopping where it does stop ; for, had so exquisitely sensi- 

 ble a membrane as the periosteum, invested the teeth, as it 

 invests every other bone of the body, their action, necessa- 

 ry exposure, and irritation, would have subjected the ani- 

 mal to continual pain. General as it is, it was not the sort 

 of integument which suited the teeth. What they stood in 

 need of, was a strong, hard, insensible, defensive coat ; and 

 exactly such a covering is given to them, in the ivory en- 

 amel which adheres to their surface. 



2. The scarf-skin, which clothes all the rest of the body, 

 gives way, at the extremities of the toes and fingers, to nails. 

 A man has only to look at his hand, to observe with what 

 nicety and precision, that covering, which extends over 

 every other part, is here superseded by a different sub- 

 stance, and a different texture. Now, if either the rule 

 had been necessary, or the deviation from it accidental, 

 this effect would not be seen. When I speak of the rule 

 being necessary, I mean the formation of the skin upon the 

 surface being produced by a set of causes constituted with- 

 out design, and acting, as all ignorant causes must act, by a 

 general operation. Were this the case, no account could 

 be given of the operation being suspended at the finger's 

 ends, or on the back part of the fingers, and not on the 

 •fore part. On the other hand ; if the deviation were acci- 

 dental, an error, an anomalism ; were it any thing else than 

 settled by intention ; we should meet with nails upon other 

 parts of the body. They would be scattered over the sur- 

 face, like warts or pimples. 



3. All the great cavities of the body are enclosed by 

 membranes except the skull. Why should not the brain be 

 content with the same covering as that which serves for the 

 other principal organs of the body? The heart, the lungs, 

 the liver, the stomach, the bowels, have all soft integuments, 

 and nothing else. The muscular coats are all soft and 

 membranous. I can see a reason for this distinction in the 

 final cause, but in no other. The importance of the brain 

 to life, (which experience proves to be immediate,) and the 

 extreme tenderness of its substance, make a solid case 

 more necessary for it, than for any other part ; and such a 

 case the hardness of the skull supplies. When the small- 

 est portion of this natural casquet is lost, how carefully, yet 

 how imperfectly is it replaced by a plate of metal ? If an 



