112 THE NERVOUS SYSTEM 



in the medulla oblongata and conveys impulses concerned in the maintenance 

 of equilibrium. Some of its fibers can be traced as far as the lower lumbar 

 segments. They end in the gray matter of the anterior column. 



Hemisection of the spinal cord in man produces a characteristic symptom 

 complex known as the Brown-Sequard's syndrome which the student is now in 

 position to understand. Below the level of the lesion and on the same side 

 there is found a paralysis of the muscles with a loss of sensation from the mus- 

 cles, joints, and tendons; while on the opposite side of the body, beginning two 

 or three segments below the level of the lesion, there is loss of sensations of 

 pain and temperature. Tactile sensibility is everywhere retained (Dejerine, 

 1914). 



Order of Myelination. The fiber tracts of the spinal cord do not all become 

 myelinated at the same time. By a study of the fetal spinal cord at various 

 developmental stages Flechsig was able to identify and trace many of these 

 tracts because of the difference in the tune of myelination. His results agree 

 in general with those derived frorh a study of spinal cords showing ascending 

 and descending degeneration (Figs. 79, 80). Myelination begins during the fifth 

 month of intra-uterine life. The order in which the fibers of the spinal cord 

 acquire their myelin sheaths is as follows: (1) afferent and efferent root fibers, 

 (2) those of the fasciculi proprii, (3) the fasciculus cuneatus, (4) the fasciculus 

 gracilis, (5) the dorsal spinocerebellar tract, (6) the ventral spinocerebellar fas- 

 ciculus, (7) the corticospinal tracts. 



