FISSURES OF THE CORD. 



381 



vical to the first dorsal vertebra ; the other in the Back is smaller, and is 

 on a level with the last dorsal vertebra. In the upper enlargement the 

 greatest thickness is from side to side ; but in the lower swelling the 

 measurement from before back rather exceeds the other. 



Whilst the pia mater remains on the cord, the anterior surface is dis- 

 tinguished from the posterior by a central fibrous band and the anterior 

 spinal artery ; and by the irregular line of the anterior nerve-roots, which 

 approach the middle towards the lower end. 



Dissection. For the examination of the structure the student should 

 possess a piece of the medulla which has been hardened in spirit, for the 

 cord which is obtained from the spinal canal at this period, is not fitted for 

 the purpose of dissection. Supposing the pia mater removed from the 

 surface, with the roots of the nerves left on one side, the student will be 

 able to observe the following divisions of the medulla. 



FISSURES OF THE CORD (fig. 125). On the anterior and posterior 

 aspects of the cord is a median longitudinal cleft the anterior and poste- 

 rior median fissures, which mark its division into halves ; and along the 

 line of the posterior roots of the nerves, in each half, is another slit the 

 lateral fissure. 



Fig. 125. 



A SECTION OP THE SPINAL CORD, 

 TO SHOW ITS COMPOSITION, AND 

 ITS DIVISIONS. FISSURES IN THB 

 MIDDLE LINE ARE THE ANTERIOR 

 AND POSTERIOR MEDIAN THB 

 ANTERIOR BEING THE WIDER. 



d. The lateral fissure. 



e. The slight groove between the lateral and the posterior 



median fissure, which marks the limit of the posterior 

 median column. 



Columns : 

 a. Antero-lateral. 

 6. Posterior. 

 c. Posterior median. 



Composition : 



g. Gray crescent, surrounded by white fibres. 

 h. Gray transverse commissure, and i, canal of the cord 



in it. 

 j. Posterior, and k, anterior root of a nerve entering the 



gray crescent. 



The anterior median fissure (fig. 125) is wider than the posterior, and 

 penetrates about one-third of the thickness of the medulla: it is lined by 

 a piece of the pia mater, and is deepest towards the lower end of the cord. 

 White medullary substance lines the fissure; and in the bottom of it the 

 white fibres are transverse, and are separated by apertures for blood- 

 vessels. 



The posterior median fissure (fig. 125) is not so wide, or so well marked 

 as the anterior; but it is best seen at the upper part of the neck, and in 

 the lower or dorsal enlargement. Vessels of the posterior surface of the 

 cord enter it. 



The lateral fissure is situate along the line of attachment of the fibrils 

 of the posterior roots (fig. 125, rf). 



Between the lateral and posterior median fissures is a slight groove on 

 the surface (fig. 125, e~). 



Sometimes a lateral fissure is described along the line of origin of the 

 anterior roots (k), but there is not any cleft in that situation. 



