their Secretions. 615 



Pearls, as Mr. Gray justly observes, are merely the internal 

 nacred coat of the shell, which has been forced, by some ex- 

 traneous cause, to assume a spherical form. They are, there- 

 fore, not properly " a distemper in the creature that produces 

 them," and cannot, under any view, be compared with calculi 

 in the kidneys of man (Lister, Hist, An. A/ig., p. 150.) ; for, 

 though accidental formations, and, of course, not always to 

 be found in the shellfish which are known usually to contain 

 them, still they are the products of a regular secretion, ap- 

 plied, however, in an unusual way, either to avert harm or 

 allay irritation. That in many instances they are formed by 

 the oyster, to protect itself against aggression, is evident ; for, 

 with a plug of this nacred and solid material it shuts out 

 worms and other intruders which have perforated the softer 

 shell, and are intent on making prey of the hapless inmate : 

 and it was apparently the knowledge of this fact that sug- 

 gested to Linnaeus his method of producing pearls at pleasure, 

 by puncturing the shell with a pointed wire.* But this ex- 

 planation, it is obvious, accounts only for the origin of such 

 pearls as are attached to the shell ; while we know that the 

 best and the greatest number, and, indeed, the only ones 

 which can be strung, have no such attachment, and are formed 

 in the body of the animal itself. " The small and middling 

 pearls," says Sir Alexander Johnston, " are formed in the 

 thickest part of the flesh of the oyster, near the union of the 

 two shells ; the large pearls almost loose in that part called the 

 beard." (Home's Lect. Comp. Anat.,\o[. v. p. 308.) Now, these 

 may be the effect merely of an excess in the supply of calca- 

 reous matter, of which the oyster wishes to get rid ; or they 

 may be formed by an effusion of pearl, to cover some irritat- 

 ing and extraneous body. The reality of the latter theory is, 

 perhaps, proved by a practice of the modern Chinese, who 

 force the swan muscle (A'nodon cygneus) to make pearls, by 



(Micrographiay p. 209., folio, 1667.) Sir E. Home disproves this explana- 

 tion of Hooke and Brewster, so far as the same is applied to pearls, by the 

 following experiment : — " Upon taking a split pearl, and putting a candle 

 behind the cell, the surface of the pearl became immediately illuminated ; 

 so that the fallacy of my philosophical friend's opinion was made self-evi- 

 dent. . . . The error my friends fell into was, taking for granted that the 

 pearl was a solid body ; and therefore, considering the subject mathemati- 

 cally, the brilliancy must be produced by the reflection from the nacral 

 surface ; but this reasoning was entirely inapplicable when applied to a 

 sphere that is hollow." {Lectures on Comp. Anatomy y vol. v. p. 306., and 

 Phil. Tram. 1826, part iii. p. 339.) 



* Lin. Corresp. by Smith, vol. ii, p. 429. Pearls formed somewhat in this 

 manner, by the freshwater muscle, are preserved in the Hunterian Museum. 

 Home's Lect. Comp. Anat , vol. vi. p. 296, 



R R 4 



