THE CEREBROSPINAL NERVOUS SYSTEM. 473 



chief portion, while the latter occupies its central or axial portion, and 

 is so arranged, that on the surface of a transverse section of the cord it 

 appears like two somewhat crescentic masses connected together by a 



FIG. 329. View of the csrebro-spinal axis of the nervous system. The right half of the cranium 

 and trunk of the body has been removed by a vertical section; the membranes of the brain and 

 spinal cord have also been removed, and the roots and first part of the fifth and ninth cranial, and 

 of all the spinal nerves of the right side, have been dissected out and laid separately on the wall of 

 the skull and on the several vertebrae opposite to the place of their natural exit from the cramo- 

 spinal cavity. (After Bourgery.) 



narrower portion, or isthmus (Fig. 330). Passing through the centre 

 of this isthmus in a longitudinal direction is a minute canal (central 



