774 THE NERVOUS SYSTEAI 



of each varies with the locality, both being greatest in the enlargements. The grey 

 substance predominates in the conus medullaris and lower lumbar segments. The 

 white substance begins to predominate in the upper lumbar segments, not because of 

 the increased presence of cerebro- and cerebello-spinal axories, but because of the 

 increased volume of the fasciculi proprii coincident with the greater mass of grey 

 substance to be intersegmentally associated in this region. In the thoracic region the 



JT IG 573 GRAPHIC REPRESENTATION OF THE VARYING AMOUNTS OF GREY AND WHITE SUB- 



' STANCE AND OF THE VARIATIONS IN AREA OF ENTIRE SECTIONS OF THE DIFFERENT SEGMENTS 

 OF THE SPINAL CORD. (From Donaldson and Davis.) 



(Based upon measurements from several adult human spinal cords.) 



Curves showing area of cross seel ion of human spinal cord. 



W|,j fe matter. --Grey matter. Enrire secrion. 



1OO 

 80 

 6O 

 40 

 2O 



n m is y vi wm i n in IY v a YD YIII ix x xi XH i n ninni oniirw 

 CERVICAL THORACIC LUMBAR SACRAL 



predominating white substance is composed mostly of the axones of long course. The 

 greatly increased amount of white substance in the cervical region is due both to the 

 greater accumulation of cerebro- and cerebello-spinal axones in this region and to 

 the increased volume of the fasciculi proprii of the cervical enlargement. 



ORDER OF MEDULLATION OF THE FASCICULI OF THE CORD 



The axones of the spinal cord begin to acquire their myelin sheaths during the 

 fifth month of intra-uterine life. In general, axones which have the same origin and 

 the same locality of termination the same function acquire their sheaths at the 

 same time. While it has been proved that the medullary sheath does not necessarily 

 precede the functioning of an axone, it may be said that those fasciculi which first at- 

 tain complete and definite functional ability are the first to become medullated. At 

 birth all the fasciculi of the spinal cord are medullated except Helweg's fasciculus, 

 and occasionally the pyramidal tracts. As indicated by their medullation, those 

 axones by which the cord is enabled to function as an organ per se, that is, the 

 axones making possible the simpler reflex activities, complete their development 

 before those axones which involve the brain with the activities of the cord. 



According to Flechsig and van (lehuchten, and investigators succeeding them, 

 the following is the order in which the axones of the cord become medullated: 



(1) The afferent and efferent nerve-roots and commissural fibres of the grey sub- 

 stance. 



(2) The fasciculi proprii, first the ventral, then the lateral, and last the dorsal, 

 fasciculus proprius. 



(3) The fasciculus cuneatus (Burdach's column) and Lissauer's zone the area 

 of those ascending cerebro-spinal fibres which run the shorter course. 



(4) Fasciculus gracilis (Coil's column). 



(5) The cerebello-spinal fasciculus (direct cerebellar tract). 



(6) The superficial antero-lateral fasciculus ((lowers' tract). 



(7) The lateral cerebro-spinal fasciculus (crossed pyramidal) and the ventral 

 cerebro-spinal fasciculus (direct pyramidal tract). 



(8) Helweg's (Bechterew's) fasciculus. 



