SPINAL CORD. 8 



white substance, and, as in man, its contour resembles that of the capi- 

 tal letter H. In ophidians and tailed batrachians, this resemblance is 

 less close on account of the fusion of the superior horns in the former, 

 and the interposition of the substantia reticularis, between the two halves 

 of gray matter, in the latter. 



It is remarkable that Reissner, who first described the substantia 

 reticularis in the brachial region, failed to find it in other parts of the 

 spinal cord. Stieda* speaks of it as existing in the brachial, dorsal and 

 crural regions, and as being broadest in the crural enlargement. Plates 

 XXXIV. & XXXV. show that, while in the latter region of the cord it 

 surrounds the central canal, its area is no larger than in the brachial re- 

 gion. Possibly this is a point of difference between the European and 

 American species. 



The central canal, in cross sections, appears in the form of a dis- 

 tinctly limited opening, surrounded by conical, ciliated epithelium. The 

 form of these epithelial cells is best seen on the linings of the cavities of 

 the brain, including that of the fourth ventricle. See Plates LXXX. 

 LXXXI. XCIIL & XCIV. In the frog's spinal cord, (Plate XXXII.) 

 these cells send out processes, which are seen to be continuous with the 

 net-work of the substantia reticularis. 



The structure of this net-work, as well as its outline and position in 

 regard to the central canal, at different planes, can be studied in Plates 

 XXVI. XXXV. It is an arrangement of fibres peculiar to the spinal 

 cord of batrachians, and affords probably the best example of what is 

 almost universally regarded as connective tissue. 



In regard to the comparative amounts of white and gray matter in 

 various regions of the spinal cord, it may be safely affirmed that no ex- 

 planation has yet been given, which accounts for all the variations to be 



* ''Studien iiber das Centrale Nervensystems der Wirbelthiere," 1870, p. 6. 



