IX.] 



THE NERVOUS SYSTEM. 



385 



solid hemispheres, often with lobi inferiores (Fig. 335, 7), an 

 enlarged roof to the third ventricle, and a pair of large hollow 

 optic lobes, often with a fold of brain-substance running for- 

 ward into their cavity, while what is commonly reckoned as the 

 cerebellum is more or less developed, but without an arbor vita?. 



In the lowest forms we find instead of, as in man, a series 

 of segments enclosed by an enormous expansion of what 

 were primitively terminal bodies, merely a number of seg- 

 ments plainly arranged in a linear series and undisguised 

 by the excessive development of any one constituent part. 



1 8. The SPINAL MARROW in man has been described in the 

 " Elementary Physiology " (Lesson XI. 4) how the cylindri- 

 cal nervous mass is divided by two vertical median fissures 



FIG. 339. THE SPINAL CORD. 



A, Front View of a portion of the Cord. On the right side, the anterior roots 

 of spinal nerves, A R, are entire ; on the left side they are cut, to show the 

 posterior roots, PR. 



B, A Transverse Section of the Cord : A, the anterior fissure ; P, the posterior 

 fissure ; G, the central canal ; C, the grey matter ; W, the white matter ; 

 AR, the anterior root ; PR, the posterior root ; Gn, the ganglion ; and T, the 

 trunk, of a spinal nerve. 



(one before and the other behind) into two lateral halves, each 

 of which is again subdivided by the two vertical series of 

 spinal nerve-roots (one series being made up of the anterior 

 roots, the other of the posterior ones) into three subordinate 

 and less-defined vertical segments. 



The spinal marrow extends (Fig. 35 i)from the foramen mag- 

 num nearly to the second lumbar vertebra, being continuous 

 above with the medulla oblongata, and so occupies about 

 the upper two-thirds of the neural canal. Below this region 

 the canal is occupied by a bundle formed of the elongated 

 roots of those spinal nerves which pass out at the lower lum- 

 bar and sacral foramina : this bundle is called the cauda 

 equina. In the earlier stages of existence there is no such 

 bundle, the spinal marrow then occupying the whole length 

 of the neural canal. A relic of this early condition is seen 

 C C 



