THE OBLONGATA AND THE NUCLEI OE THE CRANIAL NEHVES. 



83 



cortico-spinalis. This tract is present only in mammals, and will, therefore, 

 be specially treated later under the description of the mammalian brain. It 

 may be stated here, however, that its fibers begin in the medulla, for the 

 most part decussate, and then, as two large columns, — the Pyramids, — lie 

 ventral to the anterior columns and the fillet, making there a simple addition 

 to the picture which has already been presented. Frequently a child is 

 born without a cerebrum, and it lacks, therefore, the pyramidal columns, 

 and its medulla is similar to that of the other classes of vertebrates. 



One meets these fiber-systems in all sections of the medulla farther 

 toward the brain, until the cerebellar tract turns toward the cerebellum 

 and the spinal portions of the cranial nerves pass oiit of their respective 

 roots. Furthermore, bundle after bundle from the processus reticularis 



Fig. 43. — From the caudal end of the Oblongata of Alligator Indus. Note 

 the origin of the Hypoglossus (N. XII) from the anterior horn (col. ventr.) and 

 the terminal nucleus of the Trigeminus [nucl. term. N.T. spin.), dorso-lateral 

 to which lies the Trigeminus root (Rad. spin. 7). Note, also, the nucleus of 

 the posterior columns {nucl. f. dors.), in which the fasciculus from the posterior 

 root ends {Fun. dors.). Further study of the figure will reveal many features 

 already discussed. 



(Associations-feld) of -the medulla ends, and bundle after bundle arises to 

 end farther forward. However, the section, as a whole, varies little as to 

 the parts already described. 



But another factor causes very marked changes in the structure of 

 the medulla as one studies sections near the brain; namely, the nuclei of 

 the cranial nerves and the arcuate fibers, which pass into these from the 

 fillet and from the cerebellum. 



The study of the nuclei of the cranial nerves in the medulla speaks 

 conclusively for the position already taken that there is in the animal king- 



