FIBER TRACTS OP THE SPINAL CORD 99 



of which the sensations of touch, heat, cold, and pain are elaborated. An 

 important problem which in great measure awaits solution is this: How are the 

 fibers of the different functional varieties distributed in the spinal cord and 

 along what paths are these various types of afferent impulses carried toward 

 the brain? 



The proprioceptive fibers, which terminate at the periphery in neuromus- 

 cular and neuro tendinous spindles and in Pacinian corpuscles, are known to 

 be myelinated. They must, therefore, pass through the well myelinated medial 

 division of the dorsal root into the posterior funiculus. As shown by Brown- 

 Sequard in 1847 by a study of patients with unilateral lesions of the spinal 

 cord, sensations from the muscles, joints, and tendons reach the brain without 

 undergoing a crossing in the spinal cord. This and other evidence points un- 

 mistakably to the long ascending branches of the dorsal root fibers, which are 

 continued uncrossed in the posterior funiculus to the medulla oblongata, as the 

 conductors of this type of sensation. When these fibers are destroyed by a 

 tumor or other lesion confined to the posterior funiculus, muscular sensibility 

 and the recognition of posture are abolished, while touch, pain, and tempera- 

 ture sensations remain intact (Dejerine, 1914). 



No better exposition of the proprioceptive functions could be furnished than 

 by describing the sensory deficiencies found in cases of tabes dorsalis or loco- 

 motor ataxia, a disease in which there is degeneration of the posterior funiculi. 

 Lying in bed, with eyes closed, a tabetic may not be able to say in what posi- 

 tion his foot has been placed by an attendant because afferent impulses from 

 the muscles, joints, and tendons fail to reach the cerebral cortex and arouse 

 sensations of posture. Not only are the sensations of this variety lacking, but 

 the unconscious reflex motor adjustments initiated by proprioceptive afferent 

 impulses are also impaired. Standing with feet together and eyes closed, the 

 patient loses his balance and sways from side to side. In walking his gait is 

 uncertain and the movements of his limbs poorly coordinated. All of this 

 motor incoordination is explained by a loss of the controlling afferent impulses 

 from the muscles, joints, and tendons. 



The long ascending fibers of the posterior funiculus, which reach the brain 

 and end in the nucleus gracilis and cuneatus, are for the most part proprio- 

 ceptive in function (Fig. 235). The connections which they make there can 

 best be considered in another chapter. Collaterals and many terminal branches 

 end in the gray matter of the cord, entering into synaptic relations with the neu- 

 rons of the spinocerebettar paths and with neurons belonging to spinal reflex arcs. 



