388 THE XERVOUS SYSTEM. 



on those nerves. It will be convenient to speak first of the 

 spinal cord and its nerves. 



Spinal Cord and its Nerves. 



The spinal cord is a cylindriform column of nerve-substance, 

 connected above with the brain through the medium of the 

 medulla oblongata, terminating below, about the lower border 

 of the first lumbar vertebra, in a slender filament of gray or 

 vesicular substance, thefilum terminate, which lies in the midst 

 of the roots of many nerves forming the eauda equina. The 

 cord is composed of fibrous and vesicular nervous substance, 

 of which the former is situated externally, and constitutes its 

 chief portion, while the latter occupies its central or axial por- 

 tion, and is so arranged, that on the surface of a transverse 

 section of the cord it appears like two somewhat crescentic 

 masses connected together by a narrower portion or isthmus 

 (Fig. 141). 



Passing through the centre of this isthmus in a longitudinal 

 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 an- 

 teriorly and posteriorly by a* vertical fissure ; the posterior fis- 

 sure being deeper, but less wide and distinct 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 be- 

 tween the lateral and posterior column arise the posterior 

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

 to each vertebra (Fig. 141). 



The fibrous part of the cord contains continuations of the 

 innumerable fibres of the spinal nerves issuing from it, or en- 

 tering it ; but it is, probably, not formed of them exclusively ; 

 nor is it a mere trunk, like a great nerve, through which they 

 may pass to the brain. It is, indeed, among the most difficult 



right side, have been dissected out and laid separately on the wall of the skull and 

 on the several vertebrae opposite to the place of their natural exit from the cranio- 

 spinal cavity. 



