788 THE NERVOUS SYSTEM 



The ventral cerebro -spinal fasciculus (anterior or direct pyramidal tract), 

 as stated above, is the uncrossed portion of the descending cerebro-spinal system of 

 neurones. It is a small, oblong bundle, situated mesially in the anterior funiculus, 

 parallel with the anterior median fissure. Like the lateral cerebro-spinal fasci- 

 culus (crossed pyramidal tract), its axones arise from the large pyramidal cells 

 of the motor area of the cerebral cortex, and transmit their impulses to the neu- 

 rones of the ventral horns of the grey substance of the spinal cord, and almost 

 entirely to those neurones which give origin to the ventral or motor root fibres. 



It represents 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 ventral white commissure and 

 in the commissural 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 ven- 

 tral 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 anthropoid apes and certain monkeys, 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 

 anterior 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; functionally it is the ventral mesencephalo-spinal (tecto- 

 spinal) tract. 



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 quadrigeminate bodies, 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 superior quadrigeminate 

 Ijodies having to do with sight, this tract forms a second path conveying visual impulses to the 

 neurones of the spinal cord. 



The commissural bundle is situated about the floor of the anterior median i 

 fissure, and is the most dorsal tract of the anterior funiculus. It contains decus- 

 sating or commissural axones of three varieties. 



(1) It contains the decussating axones of the ventral cerebro-spinal fasciculus throughout I 

 the extent of that fasciculus; (2) it is chiefly composed of the axones of the ventral fasciculus i 

 proprius which arise in the grey substance (ventral horn) of one side, cross the mid-line as com- • 

 missural fibres, and course both upward and downward to be distributed to the neurones of I 

 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 ventral 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 f 

 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 thei 

 commissural portion of the dorsal fasciculus proprius (the cornu-commissural bundle). 



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 

 general systems. It contains axones which — (a) enter it from cell-bodies situated 

 outside its })oundaries, i. e., in the spinal ganglia and in the encephalon, and (b) 

 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 wliolly within it. Its axones comprise — (1) 

 a system for the intersegmental association of its grey substance, both ascending 

 and descending, association proper and commissural; (2) a spino-cerebral and 

 cerebro-spinal system, ascending and descending; and (3) a spino-cerebellar and 

 ccrebello-spinal system, ascending and descending. 



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

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

 of the ventral roots; those which give rise to central axones of long course, going 

 to the encephalon; and those which supply its central axones of shorter course, 

 the association and commissural systems. 



