OH. XLII.] 



THE SPINAL CORD. 



605 



horns of each of which are called respectively the anterior and 

 posterior cornua) connected together by a narrower portion or 

 isthmus, called the posterior commissure (fig. 450). Passing 

 through the centre of this isthmus in a longitudinal direction is 

 a minute canal ; in a transverse section it appears as a hole ; this 

 central canal of the spinal cord is continued throughout its entire 

 length, and opens above into the space at the back of the 

 medulla oblongata and pons Varolii, called the fourth ventricle. 



Fig. 450. Different views of a portion of the spinal cord from the cervical region, with th 

 roots of the nerves (slightly enlarged). In A, the anterior surface of the specimen is 

 shown ; the anterior nerve-root of its right side is divided ; in it, i view of the 

 right side is given ; in o, the upper surface is shown ; in D, the nerve roots and 

 ganglion are shown from below, i, the anterior median fissure ; 2, posterior median 

 fissure ; 3, anterior lateral depression, over which the anterior nerve-roots are seen to 

 spread ; 4, posterior lateral groove, into which the posterior roots are seen to sink ; 

 5, anterior roots pawing the ganglion ; 5', in A, the anterior root divided ; 6, the 

 posterior roots, the fibres of which paw into the ganglion 6' ; 7, the united or com- 

 pound nerve ; 7', the posterior primary branch, seen in A and n to be derived in part 

 from the anterior and in part from the posterior root. (Allen Thomson.) 



It is lined by a layer of columnar ciliated epithelium, and 

 contains a fluid called cerebro-spinal fluid. 



The spinal cord consists of two symmetrical halves, separated 

 anteriorly and posteriorly by vertical fissures (the posterior 

 fissure being deeper, but less wide and distinct than the 

 anterior), and united in the middle by nervous matter which is 

 usually described as forming two commissures an anterior com- 

 missure in front of the central canal, consisting of medulluted 



