558 THE MEDULLA SPINALIS. 



CHAPTER XVII. 



OF THE SPECIAL ANATOMY OF THE SPINAL MARROW AND 

 BRAIN, AS DESCRIED FROM BELOW UPWARDS. 



HAVING investigated the general anatomy of the nervous 

 system, the student may enter with advantage upon the study 

 of its particular parts; and for the convenience of demonstra- 

 tion, it will be best to retain the artificial divisions, commonly 

 made, viz. : 1st. The brain and spinal marrow, which forms 

 the cerebro-spinal axis of Meckel, and the forty-two pairs of 

 encephalic and spinal nerves which they give off; and 2d. The 

 ganglia and nerves of the great sympathetic or ganglionic 

 system. 



Of the Medulla Spinalis of Man. 



The medulla spinalis, is a cylindrical cord, slightly flattened 

 in its antero-posterior diameter. In the adult it is on an 

 average about sixteen inches in length, and extends from the 

 first or second lumbar vertebra,* to the basilar foramen of the 

 occipital bone, where it is directly continuous with the medulla 

 oblongata, and through the latter with the cerebrum and cere- 

 bellum. 



The transverse or larger diameter of the cord, is, at the mid- 

 dle of the back, five lines ; but at the lower part of the neck, 

 and the lower part of the back, where the great plexuses of 

 nerves are given off to the upper and lower extremities, it is a 

 line or two more. It is surrounded by three membranes, like 

 the brain ; viz. the dura mater, tunica arachnoidea, and the pia 

 mater, the latter of which closely embraces the medulla. It is 

 enclosed its whole length in the spinal canal, the diameter of 

 which is much greater than that of the medulla ; it doos not 



* Keuffel has seen it commencing opposite the eleventh dorsal vertebra in 

 one case, and the third lumbar in another. 



