6^ OF MECHANICAL ARRANGEMENT 



flat, and these flat surfaces laid one against the other, and 

 bound tight by sinews ; in the serpent the bones play one 

 within another, like a ball and socket,* so that tliey have 

 a free motion upon one another in every direction ; that is 

 to say, in men and quadrupeds firmness is more consulted — 

 in serpents, pliancy. 



Yet even pliancy is not obtained at the expense of safety. 

 The backbone of a serpent, for coherence and flexibility, 

 is one of the most curious pieces of animal mechanism, with 

 which we are acquainted. The chain of a watch (I mean the 

 chain which passes between the spring barrel and the fu- 

 see,) which aims at the same properties, is but a bungling 

 piece of workmanship in comparison with that of which 

 we speak. t 



IV. The reciprocal enlargement and contraction of the 

 chest to allow for the play of the lungs, depends upon a 

 simple yet beautiful mechanical contrivance, referrible to 

 the structure of the bones which enclose it. (PI. X. fig. 1.) 

 The ribs are articulated to the back bone, or rather to its side 

 projection, ohliquely : that is, in their natural position, they 

 bend or slope from the place of articulation downwards. 

 But the basis upon which they rest at this end being fixed, 

 the consequence of the obliquity, or the inclination down- 

 wards, is, that when they come to move, whatever pulls 

 the ribs upwards, necessarily, at the same time, draws 

 them out ; and that, whilst the ribs are brought to a right 

 angle with the spine behind, the sternum, or part of the 

 chest to which they are attached in front, is thrust forward. 

 The simple action, therefore, of the elevating muscles, does 

 the business ; whereas, if the ribs had been articulated with 

 the bodies of the vertebrae at right angles, the cavity of the 

 thorax could never have been further enlarged by a change 

 of their position. If each rib had been a ridged bone, ar- 

 ticulated at both ends to fixed bases, the whole chest had 

 been immovable. Keill has observed, that the breast-bone 

 in an easy inspiration, is thrust out one tenth of an inch ; 

 and he calculates that this, added to what is gained to the 



*^Der. Phys. Theol. p. 396. 

 i In fish, which have more elastic, but less flexible bodies, the 

 etructure of the spine differs. The end of each vertebra is a cup con- 

 taining a viscid fluid, which keeps the bones from approaching nearer 

 to each other than the mean state of the elasticity of the lateral llga. 

 ments ; the fluid js incompressible, therefore forms a ball round which 

 the bony cups move ; the ball having no cohesion, the centre of mo- 

 tion is always adapted to the change which the joint undergoes with- 

 .out producing friction, " Paxton. 



