Mammalia. 119 



NEEVOtJS SYSTEM. Geneeal, 



The Spinal Cord, or Myelon (of Owen), exists in varioits 

 proportions relativel}^ to the neural canal in different Mam- 

 mals : but the mass of the Myelon, with but few exceptions, 

 bears a direct ratio to that of the body throughout the series, 

 has essentially the same structure, and is much less than 

 that of the brain. It extends considerably further posteriorly 

 in other Mammalia than in Man, as a rule reaching as far as 

 the Sacrum*. As in the rest of Yertebrata, a central canal 

 is often found in the Mammalia, especially in the foetus ; it 

 is more obvious in lower Mammals than in Man. Two en- 

 largements are generally found in the Cord ; one in the region 

 of the attachment of the anterior limbs (brachial); the other 

 where the lumbar and sacral nerves arise, which form the 

 plexus for the hind limbs (pelvic) : the pelvic enlargement is 

 wanting when the hind limbs are rudimentary ; it never ex- 

 hibits a rhomboidal sinus {cf. Aves) : the brachial enlargement 

 is distant from the medulla oblongata, according to the length 

 of the *neck.' The Spinal Cord generally terminates pos- 

 teriorly by breaking up into threads, forming the so-called 

 ' Cauda equina,' which is scarcely ever met with in the cord 

 of the other Yertebrata. 







The Oruithorhj-ncns hardly departs from the condition of the Lizard, the 

 Myelon extending into the sacrum, and having the intravertehral nerve roots 

 limited to the shori canal of the caudal region. 



