76 



ANATOMY OF VERTEBRATES. 



single delicate layer of vascular tissue in the dorsal (posterior) one, 

 ib. c. A layer of white iieurine accompanies the ventral fold, which, 

 when withdrawn, shows the fissure to be closed by such layer, 

 perforated by numerous holes for capillaries : its fibres are trans- 

 verse and form the ( white myelonal commissure.' The depth of 

 the ventral fissure is greatest at the pectoral enlargement of the 

 myelon, and gradually diminishes towards the ' cauda equina.' 

 The deeper dorsal fissure penetrates fully one-half of the dorso- 

 ventral diameter of the myelon through the greater part of its 

 course, but becomes shallower in the lumbar region : it is bounded 

 by a layer of grey neurine, connecting the same tissue in each 

 lateral moiety of the myelon,, which layer forms the ( grey mye- 

 lonal commissure.' 



In the developemeiit of the myelon, as of the encephalon, the 

 central part contains a fluid which is reduced by the endogenous 



grow 7 th of neurine, on ap- 

 proaching maturity ; it re- 

 mains in the myelon, as its 

 e canal,' which is obvious in 

 the cold-blooded Verte- 

 brates, 1 and is exposed, in 

 birds, as the ' ventricle of 

 the pelvic enlargement,' as it 

 is in the ' fourth ventricle ' 

 of all Vertebrates, where it 

 bears the name of ' calamus 

 scriptorius ' in anthropoto- 

 my. The myelonal canal is 

 more obvious in lower mam- 



ancl fourth cervical nerves. Magnified ten diameters- mals 2 than in Man, aild in 



Z 



Transverse section of the human myelon, close to the third 



XVIII". 



the foetus than in the adult ; 

 in whom, whilst unobliterated, it is surrounded, like the more 

 obvious myelonal canal in Reptiles, by the grey commissural 

 neurine. The canal is lined by ciliate cells. 3 The lateral columns 

 of this tissue, united by the commissure, are thicker but less peri- 

 pherally extended in the ventral, y, than in the dorsal, h, portions 

 of the myelon. In transverse section the grey neurine resem- 

 bles a comma, the concavity of which is directed outward, 

 the head, fig. 40, g, is surrounded by the peripheral white 

 neurine, and the tail, ib. h, i, is produced to the issue of the dorsal 

 (posterior) nerve-roots, ib. k. The proportions of the grey and 



1 vol. i. pp. 272, 296. 2 xx. vol. iii. p. 43, no. 1362. 3 xvni". 



