80 SPINE. 



in the neighborhood of the neck, at the shoulder 

 blade ; and no other kind is ever employed in the 

 body, for a revolving joint, except in the single in- 

 stance of the neck. They are all liable, however, to 

 slip in some degree, in the socket ; which, while of no 

 importance in any other situation, would have been 

 dangerous to the spinal marrow. A smart blow might 

 easily dislocate such a joint. But nothing can slip a 

 pin joint, short of breaking the pin. 



B. I do not see how any atheist could ever have 

 been acquainted with his own structure. 



T. Do you think of any striking resemblance to 

 the works of art, in the mechanism of the neck ? 



A. When we bend the head downward, we make 

 use of the hinge joint, which is situated between the 

 head and the first vertebra of the neck. In moving the 

 head horizontally, that is, in turning it round on either 

 side, we make use of the pin joint, which lies immedi- 

 ately under the former, and plays between the first 

 bone of the • neck and the second. The same two 

 kinds of joints, similarly .'situated, and exactly resem- 

 bling those of the human head, are employed in the 

 frame or mounting of a telescope. It is occasionally 

 requisite to move the telescope up and down, as when 

 we want to point it to a star which lies higher or lower 

 than another. It is also required, that it should be 

 able tctake a circular motion, as when we want to re- 

 move it from one star and point it to another star, by 

 the side of it. For the first motion, there is a hinge 

 upon which the telescope plays up and down ; for the 

 next, there is an axis or pivot on which the hinge and 

 the telescope upon it turn round together. This is 



