THE SPINAL CORD. 513 



former may be mentioned the sensations of pain, touch, pressure, tem- 

 perature, passive position and movements of parts due to the activity of 

 skeletal muscles; of the latter may be mentioned hunger, thirst, fatigue, 

 well-being, etc. 



Though all the impulses that give rise to these varied sensations are 

 contained within the fibers of the afferent peripheral nerves, they are on 

 reaching the cord distributed by the intraspinal mechanisms to different 

 tracts of nerve fibers, each of which' transmits to localized areas of the cerebral 

 cortex, the somesthetic areas, a special group of impulses which give rise to 

 sensations of various kinds, but especially to sensations of pain, temperature 

 (heat and cold), touch, passive position, and movement of parts, due to the 

 action of skeletal muscles. 



The pathways through the spinal cord that conduct these afferent im- 

 pulses to the brain are ill-defined and imperfectly known, and only for a few 

 sensations can it be said that their pathways have been determined. The 

 reason for this obscurity lies partly in the difficulties of experimentation, 

 partly in the difficulties of interpretation. Clinical observations are for 

 special reasons more or less untrustworthy. 



As the outcome of many investigations it may be said that a transverse 

 section of one lateral half of the cord in the monkey, or a lesion involv- 

 ing the one lateral half in man, as a rule abolishes many if not all forms of 

 cutaneous sensibility on the opposite side below the injury. This would seem 

 to prove that the nerve impulses cross the median line of the cord immedi- 

 ately or very shortly after entering and then ascend the corresponding half 

 of the cord on their way to the thalamus. At the same time, muscle sen- 

 sibility is abolished on the same below the injury. This would seem to 

 prove that the fibers of the posterior roots that enter and cross the 

 column of Burdach and ascend in the column of Goll to terminate around the 

 cells of the gracile and cuneate nuclei are derived mainly from the muscles. 

 It is, however, believed by some investigators that those fibers which sub- 

 serve the sense of touch do not decussate at once, but ascend in the column 

 of Goll as far as the medulla oblongata, where they, in common with the 

 fibers coming from the muscles, arborize around the nerve-cells in the gracile 

 and cuneate nuclei. The afferent path originating in the spinal cord in- 

 creases in size at successive levels as it passes upward, to and through the 

 medulla and successive structures, to the thalamus. The fibers that com- 

 pose the afferent path originating in the cells of the gracile and cuneate 

 nuclei, cross over the median plane and after decussating with the fibers 

 coming from the opposite side, join the afferent path from the spinal cord. 

 These fibers are known as the internal arcuate fibers and assist in the forma- 

 tion of the lemniscus or fillet. (Fig. 236.) The sensor pathway decussates 

 in part at different levels of the spinal cord and in part at the level of the gra- 

 cile and cuneate nuclei. The former is often termed the lower, the latter 

 the upper sensor decussation. 



The afferent pathway on passing toward the thalamus receives addi- 

 tional fibers at the level of the medulla and pons from the cells with which 

 the terminations of the afferent cranial nerves, the trigeminal, glosso- 

 pharyngeal, and vagus, are associated. 



The pathways for the Impulses that give rise to the different sensation 



33 



