SPINAL MARROW. 387 



show, step by step, the relative dependency of the several 

 parts, and the inseparable union of the anatomy and phys- 

 iology of the whole, in the great business of continuing 

 and preserving life. 



The brain and spinal marrow are not only the sources 

 of power to the different parts of the head, but they are 

 further the great centres of action to the whole nervous 

 system and all its dependencies. 



We shall consider the cerebro-spinal axis in the natural 

 order of its development, beginning with the spinal marrow. 



SECTION I. \ 



SPINAL MARROW. (Fig. 14.) 



The situation of this organ within the spinal canal, 

 together with its divisions, structure, and functions, has 

 already been partly considered under the head of nervous 

 tissue. A few more additional remarks will be all that is 

 necessary in this place. 



The spinal marrow, (medulla spinalis,) in common with 

 the brain, has three membranes surrounding it, the dura 

 mater, tunica arachnoidea, and pia mater the whole en- 

 closed in a strong bony case, called the vertebral or spinal 

 canal. 



Dissection. After removing the soft parts covering the 

 spine, divide, with the saw, the crura of the spinous pro- 

 cesses, close to the roots of the transverse, the whole length 

 of the canal; then raise this portion with an elevator, 

 when will be first seen a considerable amount of soft, red- 

 dish cellular and adipose tissue, between the bones and 

 the membranes of the spinal cord, which at this point have 

 a very loose connection while in front the dura mater is 

 closely attached to the bodies of the spine, by means of the 

 posterior ligaments. It will also be noticed that the spinal 

 marrow does not fill, by a considerable space, the whole 

 diameter of the bony canal. 



The Dura Mater ^ (mater, mother,) so called from being 

 formerly supposed, by the older anatomists, to be, along 



