340 



HISTOLOGY. 



appear as additional columns or horns on the dorsal part of the gray H 

 (Fig. 395) ; the inner pair are the nuclei of the gracile fasciculus, and the 

 outer ones are nuclei of the cuneate fasciculus. In them the fibers from 

 the cord terminate and others arise which cross beneath the central canal 

 to the opposite side of the medulla (Fig. 395). Then they pass forward 

 in right and left bundles known as lemnisci or fillets. The decussation of 

 the lemnisci occurs higher up in the medulla (that is, more anteriorly) than 

 that of the pyramids; and, after crossing, the fillets remain internal to the 

 pyramids. 



With the sensory and motor decussations the resemblance between 



f.c.v. 



FIG. 394. SECTION OF THE CORD AT THE 

 LEVEL OF THE FIRST CERVICAL NERVE. 



The right half of the section shows the effect 

 of Weigert's stain, the myelinated portions 

 being dark; the left half shows the gray 

 substance stippled and the white is blank. 

 f. c., Fasciculus cuneatus; t: c. 1., fascicu- 

 lus cerebro-spinalis lateralis; f. c. v., fascic- 

 ulus cerebro-spinalis ventralis; f. g., fascic- 

 ulus gracilis; d. c., dorsal column; d. p., 

 decussation of the pyramids; d. r., dorsal 

 root of first cervical nerve; v. c., ventral 

 column. 



tS.n.t 



,\ ' i 



n. ace? \ . - ,. 



FIG 395. SECTION OF THE MEDULLA. (After 

 Dejerine.) 



d. c., Dorsal column; d. 1., decussation of the 

 lemnisci; f. c., fasciculus cuneatus; n. ace., 

 nucleus of the accessory portion of the 

 vagus; n. c., cuneate nucleus; n. g. f gracile 

 nucleus; py., pyramid; t. s. n. t.,^spinal tract 

 of the trigeminal nerve; v. "c., ventral 

 column. 



the medulla and spinal cord is lost. The gray substance no longer forms 

 an H, and the dorsal fiber tracts have become ventral; the central canal 

 expands to make the fourth ventricle, as seen in Fig. 396. The lemnisci 

 form vertical bands of white substance on either side of the median ventral 

 raphe. The pyramids cause protrusions of the ventral surface. Dorsal 

 to each there is a large nucleus, the olive, which also makes an external 

 elevation (Fig. 391, B). Its gray substance forms a convoluted capsule; 

 it receives fibers from the cord and cerebellum, and gives rise to some 

 which cross through the median raphe and ascend to the cerebellum in the 

 restiform body. The restiform body, which forms the dorso-lateral portion 

 of sections of the upper part of the medulla, contains olivary fibers, those 



