Chap. XIX] 



THE NERVOUS SYSTEM 



379 



verse section of the spinal cord, the gray matter is seen to be ar- 

 ranged in the form of a butterfly with extended wings. The tips 

 of each wing are called its horns or cornua, the anterior horns being 

 thicker and larger than the posterior. The transverse bar of gray 

 matter found in the isthmus is called the gr^y commissure, and it 

 connects the two lateral masses of gray matter. The white matter 

 is arranged around and between the gray matter, tne proportion 

 of gray and white varying in different regions of the cord. The 



VCNTRO-MCDIAN 

 FISSURE 



VENTRAL ROOTS 



VENTRAL HORN 



LATERAL HORN 



RETICULAR 



OORSAL HORN 



OORSO-LATERAL 

 FISSURE 



OORSAL ROOTS 



OORSO-MEOIAN 

 FISSURE 



Fig. 180. — Transverse Section of the Spinal Cord at the Middle of 

 THE Thoracic Region. The neuroglia septum has been removed from between 

 the dorsal columns. (Gerrish.) 



white matter is composed of medullated nerves, and the gray 

 matter consists of cell-bodies, dendrites, axis cylinder processes, 

 and collaterals, all held together and supported by neuroglia. The 

 medullated nerve-fibres are grouped in bundles known as tracts 

 or columns, and the majority run in a longitudinal direction. 

 These tracts are classified under two main headings: (l)sensory, 

 or those which carry impulses upward to the brain ; they begin 

 in the gray matter of the cord, ascend, and terminate in the gray 

 matter of the brain ; (2) motor, or those which carry impulses 



