772 THE NERVOUS SYSTEM 



entirely to those neurones which give origin to the ventral or motor roots. It repre- 

 sents merely a delayed decussation of the pyramidal fibres, for instead of crossing 

 to the opposite side in the lower portion of the medulla oblongata, as do the fibres 

 of the lateral fasciculus, its fibres decussate all along its course, crossing in the ven- 

 tral white commissure and in the cornmissural bundle of the cord to terminate about 

 the ventral horn cells of the opposite side. Hoche, employing Marchi's method, 

 found that a few of its fibres terminate in the ventral horn of the same side. This 

 conforms to the pathological and experimental evidence that there are homolateral 

 or uncrossed fibres in the crossed pyramidal tracts also. Like the crossed tract, the 

 ventral pyramidal tract diminishes rapidly in volume as it descends the cord. Its 

 loss is greatest in the cervical enlargement, and it is entirely exhausted in the 

 thoracic cord. With the exception of the monkey, none of the mammalia below 

 man, which have been investigated, possess this* ventral pyramidal tract. 



Lying between the ventral cerebro-spinal fasciculus and the pia mater of the ante- 

 rior median fissure is a thin tract of descending axones continuous ventrally with the 

 anterior marginal fasciculus. From its position it is known as the sulco-marginal 

 fasciculus. The extent of its course in the spinal cord is uncertain. It arises from 

 the cells of the grey substance of the superior pair of the corpora quadrigemina, and 

 there, in largest part at least, it crosses the mid-line, and in the so-called 'optic 

 acoustic reflex path' descends through the medulla oblongata into the spinal cord of 

 the opposite side. 



The commissural bundle is situated about the floor of the anterior median fis- 

 sure, and is the most dorsal tract of the anterior funiculus. It contains decussating 

 or commissural axones of three varieties: (1) It contains the decussating axones 

 of the ventral cerebro-spinal fasciculus throughout the extent of that fascicu- 

 lus; (2) it is chiefly composed of the axones of the ventral fasciculus pro- 

 prius which arise in the grey substance (ventral horn) of one side, cross the 

 mid-line as commissural fibres, and course both upwards and downwards to be dis- 

 tributed to the neurones of different levels of the grey substance of the opposite side ; 

 (3) it contains decussating axones which arise from cell-bodies in the grey substance 

 of one side and cross the mid-line to terminate about cell-bodies in practically the 

 same level of the opposite side. The latter are merely axones belonging to the ven- 

 tral white commissure which course without the confines of the grey figure. The 

 commissural bundle is present throughout the length of the spinal cord, and is 

 largest in the enlargements, i. e., where the association and commissural neurones 

 occur in greater number generally. In its two last-mentioned varieties of axones 

 it corresponds to the commissural portion of the dorsal fasciculus proprius. 



The ventral fasciculus proprius is but a continuation of the lateral fasciculus 

 proprius, and is composed of ascending and descending association fibres of the 

 same general significance. 



SUMMARY OF THE SPINAL CORD 



The spinal cord contains two general classes of axones arranged into three gen- 

 eral systems. It contains axones which (1) enter it from cell-bodies situated out- 

 side its boundaries, i.e., in the spinal ganglia and in the encephalon, and (2) axones 

 which arise from cell-bodies situated within its own grey substance, some of which 

 axones pass outside its boundaries both to the periphery and into the encephalon ; 

 some of which remain wholly within it. Its axones comprise (1) a cerebro-spinal 

 system, ascending and descending; (2) a cerebello-spinal system, ascending and 

 descending, and (3) a system for the intersegmental association of its grey sub- 

 stance, both ascending and descending, association proper and commissural. 



For these relations the grey substance of the cord contains three general classes 

 of nerve-cells: (1) those which give rise to the peripheral efferent or motor axones 

 of the ventral roots; (2) those which give rise to central axones of long course, those 

 going to the encephalon, and (3) those which supply its central axones of shorter 

 course, the association and commissural systems. 



In transverse sections of the spinal cord the relative area of white substance as 

 compared with that of the grey increases as the cord is ascended. The absolute area 



