ENCEPHAION. 



77 



tebral substance is to prevent the jar, that 

 would necessarily be communicated to the 

 delicate parts within the cavities of the 

 spine and cranium, were the spine composed 

 entirely of one bone. In falls from a height 

 upon the feet or breech, these elastic cushions 

 are forcibly compressed ; but they immedi- 10- 



ately return to their former condition, and 

 deaden the force of the shock. In this they 

 are aided by the curvatures of the spine, 

 which give it the shape of the Italic/, and 

 enable it to resist in the same manner as 

 a steel spring any force acting upon it in 

 a longitudinal direction. So well is the 

 medulla spinalis protected by the strong 

 bony processes jutting out in various direc- 

 tions from the spine, that it is extremely 

 rare to meet with lesions of the marrow; 

 and it is comparatively of late years that 

 any ex professo treatises have appeared on 

 the subject. 



Besides the protection afforded by the 

 bony structure to the delicate medulla, M. 

 Magendie 1 has pointed out another, which 

 he was the first to detect. The canal, 

 formed by the dura mater around the spinal 

 cord, is much larger than is necessary to , IQ/ 

 contain that organ ; but, during life, the 

 whole of the intermediate space is filled 

 with a serous fluid, which strongly distends 

 the membrane, so that it will frequently 

 spirt out to a distance of several inches, 

 when a puncture is made in the membrane. 

 To this fluid he has given the epithet ce- 

 phalo- spinal ; and he conceives, that it may 

 act as one of the tutamina of the marrow 

 which is, as it were, suspended in the fluid 

 and exert upon it the pressure necessary 

 for the healthy performance of its functions. 



Beneath the dura mater is a very delicate 

 membrane, the arachnoid, belonging to the 

 class of serous membranes. It surrounds 

 the encephalon in every part ; but is best 

 seen at the base of the brain. 



ItS Chief USe is to Secrete a thin fluid, to Lateral View of the Spinal Column. 



1 Precis, &c., edit. cit. i. 181. For an elaborate descrip- 

 tion of the fluid, see Magendie, Recherches Physiolo- 

 giques, &c., sur le Liquide cephalo-rachidien, Paris, 1842; 

 and Dr. Todd, Cyclop, of Anat. and Physiol., part xxv. 

 p. 639, Lond., 1844. 



1. Atlas. 2. Dentata. 3. Seventh 

 cervical vertebra. 4. Twelfth dorsal 

 vertebra. 5. Fifth lumbar vertebra. 

 6. First piece of sacrum. 7. Last 

 piece of sacrum. 8. Coccyx. 9. A 

 spinous process. 10, 10. Interverte- 

 bral foramina. 



