492 THE NERVOUS SYSTEM. 



stance, connected above with the brain through the medium 

 of the medulla oblongata, terminating below, about the 

 first or second lumbar vertebra, in a slender filament of 

 grey or vesicular substance, the filum terminate, which lies 

 in the midst of the roots of many nerves forming the cauda 

 equina. The cord is composed of fibrous and vesicular 

 nervous substance, of which the former is situated exter- 

 nally, and constitutes its chief portion, while the latter 

 occupies its central or axial portion, and is so arranged, 

 that on the surface of a transverse section of the cord it 

 appears like two somewhat creseentic masses connected 

 together by a narrower portion or isthmus (fig. 134). 



Passing through the centre of this isthmus in a longitu- 

 dinal direction is a minute canal, which is continued through 

 the whole length of the cord, and opens above into the 

 space at the back of the medulla oblongata and pons 

 Varolii, called the fourth ventricle. It is lined by a layer 

 of cylindrical ciliated epithelium. 



The spinal cord consists of two exactly symmetrical 

 halves united in the middle line by a commissure, but 

 separated anteriorly and posteriorly by a vertical fissure ; 

 the posterior fissure being deeper, but less wide and dis- 

 tinct than the anterior. Each half of the spinal cord is 

 marked on the sides (obscurely at the lower part, but 

 distinctly above), by two longitudinal furrows, which divide 

 it into three portions, columns, or tracts, an anterior, middle 

 or lateral, and posterior. From the groove between the 

 anterior and lateral columns spring the anterior roots of the 

 spinal nerves ; and just in front of the groove between 

 the lateral and posterior column arise the posterior roots 

 of the same : a pair of roots on each side corresponding 

 to each vertebra (fig. 134). 



The fibrous part of the cord contains continuations of the 

 innumerable fibres of the spinal nerves issuing from it, or 

 entering it ; but it is, probably, not formed of them exclu- 

 sively ; nor is it a mere trunk, like a great nerve, through 

 which they may pass to the brain. It is, indeed, among 



