490 THE XEPiVOUS SYSTEM. 



ganglia 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 cylindrifbrm column of nerve-sub- 

 stance, 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 grey or vesicular substance, thefilum terminate 

 which lies in the midst of the roots of many nerves form- 

 ing the cauda 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 portion, 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 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 laj'er 

 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 dis- 

 tinctly 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 



