Hardesty, spinal Cord of tJie Eiini. 85 



of the mammals and especially man. In the human, with 

 the exception of the first two or three cervical nerves, all 

 the spinal nerves of the adult pass through their intervertebral 

 foramena caudad t<> the levels of their attachment to the cord 

 and all the fila of their dorsal and ventral roots are distributed 

 cephalad. In the iiuman loetus the cord occupies the entire 

 vertebral canal and the foramen of each nerve is then opposite 

 the segment or level of the cord at which the nerve is attached, 

 but, owing to the fact that the vertebral column grows more 

 rapidly than the spinal cord and continues to grow after the 

 cord has attained its adult length, the foramena become displac- 

 ed caudad. The cord being attached to the encephalon. the 

 effects of the increase in the length of the vertebrae are super- 

 imposed so that the nerves become pulled downward and the 

 nerve roots are necessarily drawn caudad into progressively in- 

 creasing lengths till the final effect is that the conus meduUaris 

 seldom reaches lower than the upper end of the first lumbar 

 vertebra and the whole lumbar and sacral portion of the verte- 

 bral canal contains only the resulting Cauda equina. Like the 

 ostrich, the emu may be said to have two brains, the enceph- 

 alon and the " lumbar brain ", or the remarkably large in- 

 tumesccntia hnnbalis. The latter, necessary for the control of 

 the chief mass of ihe body musculature, lies in an especially en- 

 larged portion of the vertebral canal. Thus the two ends of the 

 central nervous system must become mechanically fixed long 

 before the growth of the animal is complete, and as the verte- 

 brae increase in length the intervening portion of the spinal 

 cord must also increase in length. This results in remarkablv 

 long segments in the cervical and thoracic regions of the spinal 

 cord of the emu, and as compensation, explains the fact that the 

 ner\es and nerve roots are not drawn caudad instead, as they are 

 in man. Further, between foetal life and maturity the lower thor- 

 acic and the lumbar vertebras of man undergo a proportionately 

 greater gain in length than do the cervical vertebrae,^ thus accen- 



' Measurements of the length of the first five cervical vertebrae and of the 

 lumbar vertebrae of several human skeletons at birth and several adults show 



