THE NERVOUS SYSTEM ^xji? 



(vertebra prominens) is opposite the roots of the first thoracic nerve. From 

 the third to the tenth thoracic vertebrae the spinous processes correspond to 

 the second root below. The eleventh thoracic spine corresponds to the 

 first and second lumbar nerves. The twelfth thoracic spine corresponds to 

 the third, fourth, and fifth lumbar nerves. The first lumbar spine correspcmds 

 to the first, second, and third sacral nerves. (Gowers, from an original 

 investigation). 



Mode of distinguishing the Anterior and Posterior Surfaces of 

 the Spinal Cord. — ^These surfaces may be recognised by attending 

 to the following points : 



Anterior Surface. Posterior Surface. 



1. Linea splendens in median line. i. Ganglion on each posterior nerve- 



root. 



2. Anterior spinal artery in 2. Fasciculi of posterior nerve-roots 



median line. lie in a straight line, and enter 



through postero-lateral sulcus. 



3. FascicuU of anterior nerve- 3. Arterial anastomotic chain be- 



roots spread over a wide hind and in front of the pos- 



area. tenor nerve-roots. 



Internal Structure of the Spinal Cord. — The spinal cord, as 

 seen in transverse section, consists of a central portion com- 

 posed of grey matter, and an external portion composed of white 

 matter. 



Grey Matter. — ^This is arranged in the form of two irregular 

 crescents, the concavities of which are directed outwards, and the 

 convexities inwards, the latter being connected across the middle 

 line by the grey commissure. The arrangement has been likened 

 to the letter M . The grey commissure lies at the bottom of the 

 posterior median fissure, and presents about its centre the minute 

 opening of the central canal of the cord. The part of the commis- 

 sure in front of this canal is known as the anterior grey commissure, 

 and the part behind as the posterior grey commissure. In front of 

 the anterior grey commissure there is a transverse band of white 

 matter, called the anterior white commissure, which lies at the 

 bottom of the anterior median fissure. Each crescent of grey 

 matter consists of two comua, anterior and posterior, the former 

 bemg in front of, and the latter behind, the grey commissure. 

 The anterior cornu is broad and blvmt, and it stops short of the sur- 

 face of the cord, being separated from the surface by white matter 

 which is traversed by the fasciculi of the anterior ner\'e-roots. 

 The blunt extremity of the anterior cornu is called the caput cornu, 

 and the portion adjoining the grey commissure, which is slightly 

 constricted, is called the cervix cornu. The posterior cornu is, for 

 the most part, long, narrow, and tapering, and its pointed ex- 

 tremity almost reaches the surface of the cord at the bottom of the 

 postero-lateral sulcus. This pointed extremity is called the apex 

 ccrnu, and it contains a translucent substance, known as the 

 substantia gelatinosa of Rolando, which forms a cap for the caput 



