THE TEGMENTUM AND THE PEDUNCLE OE THE MIDBRAIN. 



303 



In man a manifest division into several portions may, in fact, be observed. 

 Quite forward, partly in the wall of the third ventricle, lies, on either side, 

 a narrow, small-celled nucleus, the nucleus anterior. It sends its few fibers 

 somewhat backward to the main trunk of the nerve. Posterior to it lies the 

 nucleus posterior, composed of large, multipolar cells and extending along 

 nearly the whole length of the aqueduct. One may recognize in this nucleus 

 an arrangement of the cells into groups. One dorsally located collection 

 of cells is clearly distinguishable. While all the other motor-oculi fibers 



Fig. 196. — Floor of the aquseductus Sylvii, looking upward. Nuclei of the 

 motor-oeuli and trochlearis nerves. Partly diagrammatic. 



emerge' from the side in which they originate, the fibers from this group, as 

 discovered by Gudden, pass toward the median line, dip ventrally, and cross 

 to the opposite side. Besides the dorsal division, a median one may be de- 

 fined. It lies exactly in the median line and sends out root-fibers both to 

 the right and left. 



Pig. 196 represents partly diagrammatically the nuclei in the floor of 

 the aqueduct and their relations to the nerve-roots. Note in the figure two 

 small nuclei joined together anteriorly (a and b). These two nuclei, first 



