THE MAMMALIAN NERVOUS SYSTEM 67 



75. The mesencephalic V root. — The fibers of this root can 

 easily be recognized in the microscopic sections; see Herrick 

 ('15), Fig. 75, and the references cited in Section 62 (8) and (9). 

 The root-fibers for the motor and chief sensory V nuclei form a 

 layer of white matter separating these nuclei. From the dorsal 

 border of this layer the fibers of the mesencephalic root can be 

 followed in the series of sections forward and dorsalward to take 

 their positions along the lateral wall of the aqueduct of Sylvius; 

 here they extend through the entire length of the midbrain. 

 These fibers are the innermost myelinated fibers in this region 

 and they are of very large size, though few in number. They 

 connect with the cells of the mesencephalic V nucleus, which can 

 be seen in favorable preparations as a row of large flask-shaped 

 nerve-cells accompanying the tract. 



This root of the V nerve has often been regarded as motor 

 and is so described in many works; but its sensory character is 

 now well established, though the function performed is unknown. 

 It is probable that it serves some form of deep sensibility of the 

 head. See Herrick ('15), Chap. IX; Johnston ('05); Morris 

 ('14), Fig. 654, p. 828; Otto May and Victor Horsley ('10). 



76. The dorsal funiculi of the cord. — In the gross specimens 

 both of the human and the sheep's brain identify the fasciculus 

 gracilis and the clava, into which its fibers run to end among the 

 cells of the underlying nucleus of the fasciculus gracilis. Iden- 

 tify also the fasciculus cuneatus and the tuberculum cuneatum, 

 within which lies the nucleus of the fasciculus cuneatus. 



Now examine the microscopic sections of the human brain in 

 the region of transition between spinal cord and medulla oblon- 

 gata and locate all of the structures mentioned in the preceding 

 paragraph. See lists of references under required drawings, Sec- 

 tion 62 (1), (2), and (3). Follow the fibers of the fasciculus gra- 

 cilis and fasciculus cuneatus downward through the four levels of 

 the spinal eord and enter them in your sketches of these levels. 

 Then, beginning at the upper end of the cord, follow these 

 fasciculi upward into their nuclei under the clava and tuber- 

 culum cuneatum respectively, where their fibers end. The 

 axones arising from the cells of these nuclei form the medial 

 lemniscus (fillet). 



77. The medial lemniscus. — These fibers immediately descend 



