566 LIGAMENTUM DENTICUL ATUM, 



ing much among themselves in regard to structure and function., 

 and which have been concisely but very clearly described in 

 their general outlines, at page 371. 



It has there been shown that the brain and spinal marrow, 

 are similarly inclosed within three continuous protecting 

 membranes ; the dura mater, tunica arachnoidea and pia 

 mater, which terminate in ctesed sacs over the top of the brain, 

 and in an acute extremity at the lower portion of the spinal 

 marrow. 



The dura mater and tunica arachnoidea are analogous in 

 structure^ throughout the whole of their extent; but the pia 

 mater is considerably modified in the spinal canal. 

 The line of division established between the medulla spi- 

 nalis, and the medulla oblongata, is within the ring formed by 

 the atlas vertebra ; this division is altogether arbitrary, and 

 Cruvielhier has shown, by a number of experiments, that in flex- 

 ion of the head, the spinal marrow is drawn up above that level. 

 In the whole extent of the movable chain of bones forming the 

 spinal column, flexion takes place in various directions, and 

 it is to admit of this movement without risk of compressing 

 the medulla, that the cavity of the bony canal, and that of the 

 spinal dura mater and arachnoid membrane, are so much 

 larger in diameter than the medulla itself. The immediate 

 protecting organ of the medulla in flexion and extension of 

 the spine is the pia mater, which in the spine tightly embraces 

 the medulla, and is there so changed, that instead of the deli- 

 cate cellular and vascular appearance, which it presents before 

 it descends from the brain, it has become a firm, resisting, 

 fibrous membrane. A close inspection of this membrane as 

 it exists in the spine of the elephant and bullock, as well as 

 in man, has convinced me of a fact, which has generally 

 escaped the observation of anatomists ; viz. that it forms like- 

 wise, by a duplicature of itself upon the sides of the medulla, 

 the ligamentum denticulatum, described in page 395, which 

 ligament is fastened at its outer margin by processes of insertion 

 in the dura mater. 

 Fig. 223, is a drawing I have made from the cervical portion 



