THE SPINAL CORD AS A CONDUCTING PATH 611 



ceive their medullary coverings last of all. In this assumption we are correct, 

 because the myelination of these fibers is not completed until the first month after 

 birth. Next in order follow those fibers which connect the cerebellum with the 

 spinal cord. These also belong to the long system. Following the same course 

 of reasoning, it may be assumed that the fibers composing the more primitive 

 system, which regulates the reflex life of the animal, acquire their medullary sheaths 

 long before the others. In this assumption we are also correct, because the fibers 

 connecting the centers in the spinal cord with the sensory and motor organs at the 

 periphery, are myelinated first. From here the myelination progresses to those 

 intraspinal fibers which connect the different segments of the cord. In the human 

 embryo, this process is practically completed at the time of birth. 



The third morphological method consists in tracing the course of degenerating 

 nerve fibers. ^ It has been pointed out above, that a nerve fiber, when separated 

 from its cell-body, is eventually converted into a band-fiber. This process neces- 

 sitates the conversion of the phosphorized fat of the myelin into fat which is 

 absorbed and displaced by fibrous tissue. In studying the distribution of the 

 spinal fibers, it is possible to divide the cord in places and to trace the degenerating 

 fibers by the method of staining. The sections are hardened in a bibhromate 

 solution and are then placed in a mixture of osmic acid and bichromate. Normal 

 myelin remains unstained, while its fatty derivative assumes a black color. Ob- 

 viously, the degeneration of a tract above the section implies that the trophic 

 centers (cell-bodies) of these fibers are situated below the lesion and that the de- 

 generation is ascending in its character. Quite similarly, a degeneration below the 

 cut signifies that the cell-bodies are located above the lesion and that the degenera- 

 tion is descending in its nature. This method has been employed by Waller in 

 his determination of the function of the roots of the cord. 



It should also be remembered that the localization of the cell-bodies of a given 

 tract of fibers does not always necessitate a repeated transection of the cord at 

 different levels, but may also be effected by means of staining the suspected cells. 

 It has been pointed out above that the degeneration following upon the separation 

 of a nerve fiber from its cell-body, does not remain confined to the peripheral stump 

 of the cut fiber, but also involves its central end and corresponding cell-body. 

 This central degeneration which is known as retrogressive degeneration, finds its 

 cause in a trophic disturbance of the cell-body in consequence of the inactivity 

 forced upon it by its separation from its end-organs and neighboring neurons. 

 In their final atrophic state, these cells may readily be recognized after staining 

 with methylene blue or toluidin blue. They exhibit a swollen and eccentric nu- 

 cleus as well as indistinct and diffusely stained chromophil granules. 



(b) Physiological. — The early view of VanDeen and Schiff, that the white 

 matter of the spinal cord is non-receptive to electrical stimuli, has been thoroughly 

 disproved by the work of Fick, Biedermann, and others. It must be admitted, 

 however, that the results of the direct stimulation of the different tracts of the 

 cord leave much to be desired, because the paths are not suflBciently separated 

 from one another to be able to obtain sharply differentiated effects. In spite of 

 this difficulty this method has proved distinctly helpful as an adjunct to other 

 procedures. By applying a galvanometer or capillary electrometer to the different 

 spinal paths, Eckhard, Gotch and Horsley^ have succeeded in tracing the action 

 current which is produced whenever the motor areas of the cerebrum are stimu- 

 lated. This method has been amplified by the procedure of fractional division 

 of the spinal cord. Obviously, the division of certain spinal tracts enables us to 

 determine whether these electrical variations continue even after the establishment 

 of this block between the motor area and the level of the galvanometer. This 

 procedure is also applicable to the tracing of the circuits of the common spinal 

 reflexes. 



^ Employed by Ttirck in 1851 upon sections of the diseased spinal cord of man. 

 2 Proc. Royal Society, London, 1888. 



