34 ANATOMY AND PHYSIOLOGY 



contact, there will be an oval or spherical space 

 between them, as the case may be, depending on 

 the kind of motion required at any particular 

 place. In this cavity is placed a strong sac, con- 

 taining a gelatinous fluid ; the bones move round 

 it, like a surveyor's compass with the ball and 

 socket joint. The sac is more or less convex, 

 according to the necessary mobility of the section 

 where it may be found. Near the tail, the organ 

 of locomotion in propelling the body forward, 

 they are nearly globular. 



Towards the chest and head, where less motion 

 is requisite, the sacks assume the appearance of 

 slightly convex lenses. Being incompressible, 

 though yielding, and remarkably, withal, confined 

 to their places by strong elastic ligaments, both 

 symmetry and power are beautifully and effectual- 

 ly combined.* 



* Between the joints of the spinal column of quadrupeds, 

 as well as man, there are pieces of what the books term in- 

 tervertebral substance, resembling in shape a common sun- 

 glass, but thicker in the centre. If it were not for the inter- 

 vention of this elastic stuff between every two bones of the 

 spine, which is built up of twenty-four bones, every time we 

 take a step, the meeting of the foot with the ground would 

 produce such a jar in the body, as to destroy the action of 

 many vessels, and perhaps break down the brain. Construct- 

 ed as it is, there is no jerk felt in the system — the weight of 

 the body is transmitted so easily, from bone to bone, through 

 these twenty-four cushions, that no sensation whatever is 



