IN THE HUMAN FRAME. 59 



The Structure of the spine is not in general different in 

 different animals. In the serpent tribe, however, it is con- 



those of our author ; and they will also serve as additional illustrations 

 of the same great truths w^hich he has endeavoured to enforce. Ed. 



"The spinal column, as it is called, serves three purposes : it is the 

 great bond of union betwixt all the parts of the skeleton ; it forms a 

 tube for the lodgment of the spinal marrow, a part of the nervous sys- 

 tem as important to life as the brain itself; and lastly, it is a column to 

 sustain the head. 



We now see the importance of the spine, and we shall next explain 

 how tlie various offices are provided for. 



If the protection of the spinal marrow had been the only object of 

 this structure, it is natural to infer that it would have been a strong 

 and unyielding tube of bone ; but, as it must yield to the inflexion of 

 the body, it cannot be constituted in so strict an analogy with the skull. 

 It must, therefore, bend ; but it must have no abrupt or considerable 

 bending at one part ; for the spinal marrow within would in this way 

 suffer. 



By this consideration we perceive why there are twenty-four bones 

 in the spine, each bending a little ; each articulated or making a joint 

 with its fellow ; all yielding in a slight degree, and, consequently, 

 permitting in the whole spine that flexibility necessary to the motions 

 of the body. It is next to be observed that, whilst the spine by this 

 provision moves in every direction, it gains a property which it belongs 

 more to our present purpose to understand. The bones of the spine 

 are called vertebrs ; at each interstice between these bones, there is 

 a peculiar gristly substance, wdiich is squeezed out from betwixt the 

 bones, and, therefore, permits them to approach and play a little in the 

 motions of the body. This gristly substance is enclosed in an elastic 

 •binding, or membrane of great strength, which passes from the edge 

 or border of one vertebra, to the border of the one next it. When a 

 weight is upon the body, the soft gristle is pressed out, and the mem- 

 brane yields : the mom.ent the weight is removed, the membranes re- 

 coil by their elasticity, the gristle is pressed into its place, and the 

 bones resume their position. 



We can readily understand how great the influence of these twenty- 

 four joinings must be in giving elasticity to the whole column ; and how 

 much this must tend to the protection of the brain. Were it not for 

 this interposition of elastic material, every motion of the body would 

 produce ajar to the delicate texture of the brain, and we should suffer 

 almost as much in alighting on our feet, as in falling on our head. It 

 is, as we have already remarked, necessary to interpose thin plates of 

 lead or slate between the different pieces of a column to prevent the 

 edges (technically called arrises) of the cylinders from coming in con- 

 tact, as they would, in that case, chip or split off'. 



But there is another very curious provision for the protection of the 

 brain ; we mean the curved form of the spine. If a steel spring, per- 

 fectly straight, be pressed betwixt the hands from its extremities, it 

 will resist, notwithstanding its elasticity, and when it does give way, 

 it will be with a jerk. 



Such would be the effect on the spine if it stood upright, one bone 

 perpendicular to another ; for then the weight would bear equally ; 

 the spine would yield neither to one side nor to the other ; and, con- 

 sequently, there would be a resistance from the pressure on all sides 



