ANATOMY, PHYSIOLOGY, OF NERVOUS SYSTEM. 415 



in the form of a capital "H." The commissure of this H 

 is the gray commissure just mentioned, which contains the 

 central canal. Each limb of the H shows, however, a 

 shorter and thicker anterior branch and a somewhat more 

 slender posterior branch. These are called respectively the 

 anterior and posterior horns or cornua. The remaining 

 portion of the cord outside of this central H is composed of 

 white matter which examination with the microscope re- 

 veals to be cross-sections of nerve fibers which are here 

 passing along the cord. The central gray-shaped H owes 

 its grayish color to the fact that it contains aggregations of 

 nerve cells, which are always gray, and of nerve fibers 

 which do not possess the white medullary coat, and so are 

 also gray. On the other hand, the white appearance of the 

 surrounding portions is due to the presence of the white 

 medullary coats of the nerve fibers. Just anterior to the 

 gray commissure is the white commissure already men- 

 tioned, formed by fibers connecting the white matter of one 

 side with the white of the other. By means of the horns 

 of the gray matter the area of the cord is divided into 

 several easily distinguishable regions. The white matter 

 included between the anterior horns is spoken of as the 

 anterior white column, that included between the posterior 

 horns is called the posterior white column, while that por- 

 tion on each side lying in the hollow of each of the cres- 

 cent-shaped limbs is called the lateral column. These 

 columns are of the deepest interest in the discussion of the 

 course of fibers through the brain and cord, a point to be 

 treated further on 



The Spinal Nerves. 



If a cross-section of the cord had been made between the 

 origins of the spinal nerves, the section as just described 

 would have included all the points visible. If, however, the 

 section should have passed through that region of the cord 

 from which a pair of spinal roots takes its issue several ad- 

 ditional points would appear. Running out from the an- 

 terior and posterior horns, fibers might be traced leaving 



