LATERAL CEREBROSPINAL FASCICULUS 



783 



fewer commissural axones, it is of the same general significance as the others. 

 It is frequently divided into small bundles by the reticular formation (see fig. 616). 

 The lateral cerebro-spinal fasciculus (crossed pyramidal tract). In contrast 

 to the sensory fibres passing through the spinal cord conveying impulses destined 

 to reach the cerebral cortex, axones are given off from the pyramidal cells of the 



Fig. 618. — Diagram Illustrating the Formation of the Fasciculi Proprii (association 

 fasciculi) and the Commissural Fibres of the Spinal Cord, and the General Archi- 

 tecture OF the Cord as a Mechanism for Reflex Actfvities. 



The ventral fasciculus proprius is omitted and the lateral is shown on one side only. The lower 

 spinal ganglion neurone shown illustrates the type whose ascending branch is of much longer 

 extent than that of the upper one. 



^ Dorsal fasciculus proprius 



Commissural 

 neurone in 

 ventral fas- 

 ciculus 

 proprius 



Lateral 

 fasciculus 

 proprius 



cortex, which descend to terminate about the cells of the grey substance of the 

 spinal cord, chiefly the cells which give origin to the ventral root-fibers. 



Upon reaching the medulla oblongata in their descent, these axones are accumulated into 

 two well-defined, ventrally placed bundles, the pyramids, one from each cerebral hemisphere. 

 In passing through the brain stem the pjTamids contribute many fibres which cross the mid-line 

 to terminate in the motor nuclei of the cranial nerves of the opposite side, and thus decrease 

 appreciably in bulk. According to the estimate of Thompson, only about 160,000 of the 

 pyramidal fibres are destined to enter the spinal cord. 



_ Upon reaching the lower part of the medulla, the greater mass of the fibres of each pyramid, 

 which are destined to enter the cord, suddenly cross the mid-fine in the 'decussation of the 



