MYELON IN MAMMALIA. 



73 



CHAPTEE XXVIIL 



NERVOUS SYSTEM OF MAMMALIA. 



§ 203. Myelon. — The myelon in Mammals, 

 as in Birds, quits, in the course of develope- 

 ment, the hinder part of the neural canal, mov- 

 ing and concentrating forwards, and leaving 

 the concomitantly elongated roots of the nerves, 

 between their places of exit at the intervertebral 

 foramina and their places of attachment to 

 the myelon, as an indication of the primitive 

 extent of the nervous axis. 



It is remarkable that the Monotrematous 

 order, so restricted in its representative genera, 

 should present the two extremes of this deve- 

 lopemental difference in the length of the 

 myelon. The Ornithorhynchus hardly departs 

 from the condition of the lizard, the myelon 

 extending into the sacrum, and having the 

 intra vertebral nerve-roots limited to the short 

 canal of the caudal region ; whilst in the Echid- 

 na, fig. 38, the myelon moves forward to the 

 middle of the dorsal region, d } where it ends 

 in a point, aud leaves all the canal behind 

 occupied by the elongated nerve-roots and 

 shrunken emptied myelonal sheath, answering 

 to the ' cauda equina ' and ( filum terminale ' of 

 anthropotomy, but of extraordinary length. 



In the Ornithorhynchus the myelon fills 

 closely the neural canal : it is thickest at its 

 commencement and at the lower two-thirds 

 of the cervical region ; it is more slender 

 in the back, especially near the loins; it is 

 slightly enlarged in the lumbar region, and 

 gradually terminates in a point at the end of 

 the sacral canal. The short and thick myelon 

 of the Echidna presents the two usual enlarge- 



N 



Brain and spinal chord, 

 Echidna, half nat. size. 



