PATHS FROM AND TO THE CORTEX 



851 



pyramidal tract of the opposite side. A few (usually about 10 per 

 cent.) remain on the same side as the slender direct pyramidal tract. 

 The size of this tract varies much in different individuals, and it 

 is occasionally absent. Its breadth constantly diminishes as it 

 proceeds down the cord, and it disappears before the middle of the 

 thoracic region is reached, its fibres continually decussating across 

 the anterior white commissure and plunging into the opposite 

 anterior horn. They 

 either end among its 

 cells, or, passing through 

 it, reinforce the crossed 

 pyramidal tract. The 

 fibres of thiscrossedtract 

 are, in their turn, con- 

 tinually passing off into 

 the grey matter to make 

 connection (p. 848) with 

 the cells of the anterior 

 horn, whose axis-cylin- 

 der processes enter the 

 anterior roots of the 

 spinal nerves. The losses 

 which it suffers as it 

 descends the cord may 

 be in some slight degree 

 compensated by the bi- 

 furcation of some of its 

 fibres (geminal fibres), 

 but ultimately the whole 

 tract forms synapses 

 with cells in the grey 

 matter, and dwindles 

 awav as the lumbar re- Fi S- 346. Horizontal Section tnrough the Right 

 . Hemisphere to show the Constituents of the 



glOnisreached(Flg.333). i nte rnal Capsule (von Monakow). A, knee of 



A certain number of the 

 pyramidal fibres do not 

 decussate either in the 

 bulb or in the cord. 

 These are called homo- 

 lateral fibres. They run 

 down in the lateral py- 

 ramidal tract, and are 

 represented by the fibres 



Cdlc. 



corpus callosum ; B, anterior, B', posterior, horn of 

 lateral ventricle; C, knee of internal capsule; 

 S, sensory fibres; V, visual tract; AH, Ammon's 

 horn ; Cole, calcarine fissure ; T, first, T', second, 

 temporal convolution; OR, optic radiation; Aud, 

 auditory tract ; D, retrolenticular region of internal 

 capsule; lo, lenticulo-optic division of internal 

 capsule; Cl, claustrum; op., operculum; I, island 

 of Reil; E, external capsule; Is, lenticulo-striate 

 division of internal capsule ; F, fibres from frontal 

 lobe; F', inferior part of third frontal convolution; 

 Th, optic thalamus; Put, putamen. 



that degenerate in that 



tract after a lesion in the ' motor ' area of the same side (p. 847). 



This would explain the escape in hemiplegia (paralysis of one side 



