766 THE NERVOUS SYSTEM 



The axones of the white substance belong to three general neurone systems: 

 (1) The cerebro-spinal system, which consists of axones of long course both ascending 

 and descending, forming conduction paths between the cerebrum and the peripheral 

 organs. The ascending axones of this system collect the general bodily sensations 

 and convey them to the cerebrum, the cells of which in response contribute axones 

 which descend the cord, conveying efferent or motor impulses. (2) The ctrebdlo- 

 spinal system consists of conduction paths, both ascending and descending, which 

 are connections between cerebellar structures and the grey substance of the spinal 

 cord. (3) Axones which serve to associate the different levels of the spinal cord. 

 The axones of this system are proper to the spinal cord, i. e., they do not pass outside 

 its confines. Necessarily this system contains axones of various lengths. Some 

 merely associate different levels within a single segment of the cord; others asso- 

 ciate the different segments with each other. Axones which associate the structures 

 of the spinal cord with those of the medulla oblongata may be included in this system. 



Both the first and second systems increase in bulk as the cord is ascended. The 

 ascending axones of each system are contributed to the white substance of the cord 

 along its length, and therefore accumulate upwards; the axones descending from 

 the encephalon are distributed to the different levels of the cord along its length, and 

 therefore diminish downwards. 



The mass of the third system of axones varies according to locality. Wherever 

 there is a greater mass of neurones to be associated, as there is in the enlargements of 

 the cord, a greater number of these axones is required. Their cells of origin, being 

 in the grey substance of the cord, contribute to its bulk and thus both the cells and 

 the axones of this association system serve to make the enlargements more marked. I n 

 the lumbar and sacral regions the greater mass of the entire white substance consists 

 of axones belonging to this system. It forms a dense felt-work about the grey column 

 throughout its length. Many of these axones cross the mid-line to associate the 

 neurones of the two sides of the grey column. For purposes of distinction, such as 

 cross the mid-line are called commissural fibres, while those which course upwards 

 and downwards on the same side are association fibres. Coursing in longitudinal 

 bundles about the grey figure, they compose the fasciculi proprii or 'ground bun- 

 dle:' of the spinal cord. 



A purely anatomical examination of a normal adult cord, prepared by whatever 

 means, gives no indication of the fact that the mass of longitudinally coursing fibres 

 of the white substance is composed of more or less definite bundles or fasciculi, 

 each having a definite course, and whose axones form links (conduction paths) in a 

 definite system of neurone chains. 



Present information as to the size, position, and connections of the various fasciculi is based 

 upon evidence obtained by three different lines of investigation: 



(1) Physiological investigation. (a) Direct stimulation of definite bundles or areas in 

 section and carefully noting the resulting reactions which indicate the function and course of 

 the axones stimulated. (6) 'Wallerian degeneration' and the application of such methods as 

 that of Marchi. When an axone is severed, that portion of it which is separated from its parent 

 cell-body degenerates. Likewise a bundle of axones severed, whether by accident or design, 

 will degenerate from the point of the lesion on to the locality of their termination in whichever 

 direction this may be. By the application of a staining technique which is differential for de- 

 generated or degenerating axones and a study of serial sections containing the axones in question, 

 their course and distribution may be determined. The locality of their cells of origin, if unknown, 

 may be determined by repeated experiment till a point of lesion is found not followed by degen- 

 eration of the axones under investigation, (c) The axonic reaction or 'reaction from a distance.' 

 Cell-bodies whose axones have been severed undergo chemical change and stain differently from 

 those whose axones are intact. Thus cell-bodies giving origin to a bundle of severed axones may 

 be located in correctly stained sections of the region containing them. 



(2) Embryological evidence. In the first stages of their development axones of the cerebro- 

 spinal nervous system are non-medullated. They acquire their sheaths of myelin later. Axone 

 pathways forming different connections become medullated at different periods. Based upon 

 this fact a method of investigation originated by Flechsig is employed, by which the position 

 and course of various pathways may be determined. A staining method differential for medul- 

 lated axones alone is applied to the nervous systems of foetuses of different ages, and pathways 

 medullated at given stages may be followed from the locality of their origin to their termination. 

 In the later stages, when most of the pathways are medullated and therefore stain alike, the less 

 precocious pathways may be followed by their absence of medullation. 



(3) Direct anatomical evidence. Stains differential for axones alone are applied to a given 

 locality to determine the fact that the axones of a given bundle actually arise from the cell- 

 bodies there, or that axones traced to a given locality actually terminate about the cell-bodies 

 of that locality. For example, it may be proved anatomically that the axones of a dorsal root 

 arise from the cells of the corresponding spinal ganglion, and then these axones may be traced 



