52 OF MECHANICAL ARRANGEMENT 



CHAPTER VIII. 



OF MECHANICAL ARRANGEMENT IN THE HUMAN FRAME. 



We proceed, therefore, to propose certain examples ta- 

 :.en out of this class; making choice of such, as amongst 

 those which have come to our knowledge, appear to be the 

 most striking, and the best understood ; but obliged, per- 

 haps to postpone both these recommendations to a third, 

 that of the example being capable of explanation without 

 plates or figures, or technical language. 



OF THE BONES. 



I. I challenge any man to produce, in the joints and 

 pivots of the most complicated, or the most flexible, ma- 

 chine, that was ever contrived, a construction more artifi- 

 cial, or more evidently artificial, than that which is seen 

 in the vertebrae of the human neck. [PI. VII. fig. 1.] Two 

 things were to be done. The head v.as to have the power 

 of bending forward and backward, as in the act of nodding, 

 stooping, looking upward or downward ; and, at the same 

 time, of turning itself round upon the body to a certain 

 extent, the quadrant, we will say, or rather, perhaps, a hun- 

 dred and twenty degrees of a circle. For these two pur- 

 poses, two distinct contrivances are em.ployed. [PI. VII. 

 fig. 2, 3, 4.] First, the head rests immediately upon the 

 !ippermost of the vertebras, and is united to it by a hinge, 

 joint; upon which joint the head plays freely forward and 

 backward, as far either way as is necessary, or as the liga- 

 ments allow ; which was the first thing required. But 

 then the rotatory motion is unprovided for. Therefore 

 secondly, to make the head capable of this, a further me- 

 chanism is introduced , not between the head and the up- 

 permost bone of the neck, where the hinge is, but between 

 that bone, and the bone next underneath it. It is a me- 

 chanism resembling a tenon and mortice. This second, 

 or uppermost bone but one, has what anatomists call a pro- 

 cess, viz. a projection, somewhat similar, in size and shape, 

 to a tooth ; which tooth, entering a corresponding hole or 

 socket in the bone above it, forms a pivot or axle, upon 

 which that upper bone, together with the head which it 

 supports, turns freely in a circle ; and as far in the circle, 

 as the attached muscles permit the head to turn. Thus 

 are both motions perfect, without interfering with each 



