GENERAL ANATOMY OF THE NERVOUS SYSTEM 09 



H -figure. So far as the cord consists of white matter, it 

 can be regarded without serious error as a huge nerve 

 conveying impulses to and from the brain. Its gray matter 

 serves to mediate simple reflexes. Yet it must also be 

 remembered that the gray matter may bear a part in the 

 transmission of impulses from one level to another. The 

 gray matter of the cord shows two expansions in the course 

 of its length, one between the shoulders and one close to 

 the lower end. It will readily be inferred that these ex- 

 pansions provide for the massing of cells related respec- 

 tively to the upper and lower extremities. 



In the medulla, as in the cord, the gray matter is 

 practically all internal. The H -figure, however, ceases 

 to be distinguishable, and instead there are more or less 

 isolated clusters of nerve-cells which in several con- 

 spicuous cases are in connection with cranial nerves. A 

 large proportion of the substance of the medulla, includ- 

 ing about all of its superficial portion, is made up of fibers. 

 For the most part these run longitudinally, but at certain 

 places, as revealed in sections, there are many which 

 sweep from left to right, and the reverse, crossing the 

 midline. 



In the restricted region which we have termed the 

 midbrain we find that the corpora quadrigemina (Fig. 

 12, 3d), the four eminences which rise upon its dorsal 

 aspect, are largely gray matter. Within, the alternation 

 of the two types is somewhat intricate, but ventrally this 

 part of the brain consists of fibers. The ventral white 

 matter of the midbrain (Fig. 12, 3?>) is seen just forward 

 of the pons for a short distance, and it is referred to under 

 the name of the crura cerebri. This is translated "the 

 legs of the cerebrum," and the fact which doubtless sug- 

 gested the odd name is that we have here a paired pedestal 

 into which the larger proportion of the fibers coming from 

 and going to the cerebrum are compressed. If a crude 

 comparison may be admitted, we may liken the cerebrum 

 to a great bouquet whose stems are gathered into the 

 crura. 



