MEDICINE IN MODERN TIMES. 723 



to moisten and wipe its eyeball without undue pressure on the 

 optic nerve, is manufactured, if so we may express ourselves, out 

 of the suspensorius muscle, which in other animals has but the 

 simple function of slinging up the eye. The scarcely less complex 

 and beautiful arrangement of the bird's levator humeri is the result 

 of a modification of a suiclavius muscle. (See ' Trans. Linn. Soc' 

 vol. xxvi. 1868, Article XI.) 



Secondly, where, by availing herself of the inorganic forces 

 always at work and available in the circumambient medium, what- 

 ever that medium may be, or where, by the employment, as in what 

 is called ' Histological Substitution,' of a lowly organised or vitalised 

 tissue, such as elastic tissue, she can spare herself the manufacture 

 of such expensive structures as muscle, there Nature adopts a line 

 of practice which we call a Law of Parsimony. Where a suspen- 

 sory muscle for the eye can be dispensed with altogether, as where 

 there is a more or less closed bony orbit, as in ourselves, and an 

 air-tight cavity formed by it together with the soft tissues lining 

 it, there atmospheric pressure is trusted to steady the eye in the 

 socket, as it refixes the tooth loosened by inflammation, and holds 

 the head of the femur in the acetabulum. The eye of the burrowing 

 mole, on the other hand, loses its recti and ohliqui before it verges 

 itself into total extinction ; but this very susjoensorkis it retains 

 after the wreck of its other property, as its guardian in the un- 

 divided undifferentiated tempore- orbital fossa. 



Thirdly, where matter that would otherwise be wholly refuse, 

 and to be rejected, can be utilised, there Nature exemplifies this 

 law by her ' utilisation of waste substances.' The transverse colon, 

 with its various contents, aids and ekes out the elastic recoil of the 

 lungs in expiration ; and by its near approximation to the stomach 

 has, as Duverney long ago pointed out, the shock of the ingestion 

 of fresh food propagated directly to it as a warning against slug- 

 gishness in the discharge of its own function. The air we use in 

 speech, as Mr. Paget has pointed out, we could not use for 

 breathing. 



Many other instances of the ' Law of Parsimony ' might be 

 given ; but I know not of any which cannot be reduced under one 

 or other of these three heads ; I know of none, that is, which can 

 be in any way held to negative the tenability of a law of ' Plurality 

 of Causes.' 



3 Aa 



