62 
OF MECHANICAL ARRANGEMENT 
quadrupeds the number of vertebras is from thirty to forty 
in the serpent it is nearly one hundred and fifty: where¬ 
as in men and quadrupeds the surfaces of the bones are 
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 they 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 fusee,) which aims at the same 
properties, is but a bungling piece of workmanship in com¬ 
parison with that of which we speak.| 
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 backbone, or rather to its side 
fer this model to the consideration of nautical men, as fruitful in hints 
for improving naval architecture. 
Every one who has seen a ship pitching in a heavy sea, must have 
asked himself why the masts are not upright, or rather, why the fore mast 
stands upright, whilst the main and mizzen masts stand oblique to the deck, 
or, as the phrase is, rake aft or towards the stern of the ship. 
The main and mizzen masts incline backwards, because the strain is 
greatest in the forward pitch of the vessel; for the mast having received 
an impulse forwards, it is suddenly checked as the head of the ship rises; 
but the mast being set with an inclination backwards, the motion falls 
more in a perpendicular line from the head to the heel. This advantage 
is lost in the upright position of the foremast, but it is sacrificed to a supe¬ 
rior advantage gained in working the ship; the sails upon this mast act 
more powerfully in swaying the vessel round, and the perpendicular posi¬ 
tion causes the ship to tack or stay better; but the perpendicular position, 
as we have seen, causes the strain in pitching to come at right angles to 
the mast, and is, therefore, irure apt to spring. 
These considerations give an interest to the fact, that the human spine, 
from its utmost convexity near its base, inclines backwards.”— Bell’s 
Treatise on Animal Mechanics. 
* Der. Phys. Theol. p. 396. 
t In fish, which have more elastic, but less flexible bodies, the structure 
of the spine differs. The end of each vertebra is a cup containing a viscid 
fluid, which keeps the bones from approaching nearer to each other than 
the mean state of the elasticity of the lateral ligaments ; the fluid is in¬ 
compressible, therefore forms a ball round which the bony cups move ; 
the ball having no cohesion, the centre of motion is always adapted to the 
change which the joint undergoes without producing friction .—Paxton 
