256 INNERVATION. [CHAP. x. 



and not extending beyond the space corresponding to two vertebra?. 

 These enlargements correspond to the situations at which the large 

 nerves to the extremities emerge, in conformity with a law that the 

 physical development of any portion of the cord is in the direct 

 ratio of the sensitive and motor power of the parts which it supplies 

 with nerves. 



The spinal cord is divided along the median plane by an anterior 

 and posterior fissure into two equal and symmetrical portions, of 

 which one may be called the right, the other the left spinal cord. 

 A transverse bilaminate partition, extending throughout the entire 

 length of the cord, separates these fissures, and serves to unite its 

 lateral portions. This partition is composed of a vesicular or gray 

 and a white or fibrous lamina or commissure, the gray being situate 

 posteriorly. When examined in a transverse section, the anterior 

 fissure appears evidently wider but of less depth than the posterior; 

 it is penetrated by a distinct fold of pia mater ; its floor is formed 

 by the white commissure, which has a cribriform appearance, from 

 being perforated by numerous blood-vessels. The posterior fissure 

 is much more delicate than the anterior, and about the middle of 

 the cord its existence may be doubted; its depth, in the upper 

 part of its course, is equal to fully one half of the thickness of the 

 cord. A single, very delicate layer of pia mater enters it and pene- 

 trates to its floor, which is formed by the gray commissure. 



On further examination of a transverse section of the cord, we 

 observe that the interior of each half of it is occupied by vesicular 

 matter, disposed somewhat in a crescentic form. The concavity of 

 this crescent is directed outwards: its anterior extremity, or horn, is 

 thick, but its margin has a dentated or stellate appearance, which 

 is very distinct in some situations. The gray matter is prolonged 

 backwards in the form of a narrow horn, which reaches quite 

 the surface of the cord, near which it experiences a slight enlarge- 

 ment. This enlargement appears to consist of a gray matter, paler 

 and softer than that of the remainder of the crescent, which has 

 been distinguished by Rolando as substantia cinerea gdatinosa, sur- 

 rounded by a layer of reddish-brown substance (see fig. 66, D, where 

 the central part of the posterior horn is pale). An exact symmetry 

 exists between the gray crescents of opposite sides, so that the 

 description of one is applicable to the other. 



The prolongation of the posterior horn of each gray crescent 

 to the surface divides each half of the cord into two portions. 

 All that is anterior to the posterior horn is called the antero- 

 lateral column: and this comprehends the white matter forming 



