THE SPINAL COKD. 137 



2. Internal structure. As in other vertebrates, the spinal cord 

 consists of white and grey matter, the latter being surrounded by 

 the former ; the relative amount of the one to the other varies in 

 different parts of the cord. 



A transverse section of the cord presents the same general 

 characteristics as a similar section from a bird or mammal. The 

 grey matter lies in the middle, surrounding the central canal, and is 

 prolonged into each half of the section by ventral and dorsal horns 

 or cornua. The grey matter is surrounded by the white, but is 

 not so sharply marked off from this in amphibia as in birds and 

 mammals. 



A section through the anterior enlargement is almost quad- 

 rangular in outline, and somewhat broader below than above ; the 

 ventral longitudinal fissure is well marked and deep, while the 

 dorsal is indistinct. The dorsal horns are narrow and short, the 

 ventral longer and much broader. The space between the dorsal 

 horns is narrow and deep, that between the ventral shallow; the 

 outer boundary of the grey matter is almost straight, and the central 

 canal is placed above the centre of the section. 



In the constricted portion of the cord [pars media) the relative pro^ 

 portion of the white matter is increased at the expense of the grey; 

 the horns are less distinct, the dorsal being directed outwards and 

 the ventral somewhat flattened. The central canal is in the centre 

 of the section. 



The posterior enlargement shows the ventral horns projecting out- 

 wards, very much enlarged, and approaching the periphery; the space 

 between them is semilunar. The dorsal horns are well developed, 

 especially at their upper parts ; the space between them is narrow 

 and deep. The outer margin of the grey matter is again nearly a 

 straight line. The central canal is below the centre of the section. 



The arrangement of the parts in the C07iu8 medullaris approaches 

 that in the pars media : the horns are much diminished in size, and 

 posteriorly they entirely disappear, the ventral horns persisting longer 

 than the dorsal. The central canal approaches the lower surface. 



No cornua can be recognised in ih.e Jihim terminale behind the 

 origin of the last pair of spinal nerves. The grey matter has here 

 a circular outline, with the exception of a very slight indentation 

 below ; the white matter is almost absent : the central canal is on 

 the lower border of the section in the anterior part, while posteriorly 

 it occupies almost the entire space below thejy/a mater. 



The Central Canal lies in the median line, and always presents a 



