870 NEUROLOGY 



backward in the groove between the caudate nucleus and the thalamus to the 

 amygdaloid nucleus. Other fibers are said to pass in the opposite direction from 

 the amygdaloid nucleus to the thalamus. 



PATHWAYS FROM THE BRAIN TO THE SPINAL CORD. 



The descending fasciculi which convey impulses from the higher centers to the 

 spinal cord and located in the lateral and ventral funiculi. 



The Motor Tract (Fig. 764), conveying voluntary impulses, arises from the 

 pyramid cells situated in the motor area of the cortex, the anterior central and the 

 posterior portions of the frontal gyri and the paracentral lobule. The fibers are 

 at first somewhat widely diffused, but as they descend through the corona radiata 

 they gradually approach each other, and pass between the lentiform nucleus and 

 thalamus, in the genu and anterior two-thirds of the occipital part of the inter- 

 nal capsule; those in the genu are named the geniculate fibers, while the remainder 

 constitute the cerebrospinal fibers; proceeding downward they enter the middle 

 three-fifths of the base of the cerebral peduncle. The geniculate fibers cross the 

 middle line, and end by arborizing around the cells of the motor nuclei of the cra- 

 nial nerves. The cerebrospinal fibers are continued downward into the pyramids 

 of the medulla oblongata, and the transit of the fibers from the medulla oblongata 

 is effected by two paths. The fibers nearest to the anterior median fissure cross 

 the middle line, forming the decussation of the pyramids, and descend in the 

 opposite side of the medulla spinalis, as the lateral cerebrospinal fasciculus (crossed 

 pyramidal tract} . Throughout the length of the medulla spinalis fibers from this 

 column pass into the gray substance, to terminate either directly or indirectly 

 around the motor cells of the anterior column. The more laterally placed portion of 

 the tract does not decussate in the medulla oblongata, but descends as the anterior 

 cerebrospinal fasciculus (direct pyramidal tract) ; these fibers, however, end in the ante- 

 rior gray column of the opposite side of the medulla spinalis by passing across in the 

 anterior white commissure. There is considerable variation in the extent to which 

 decussation takes place in the medulla oblongata ; about two-thirds or three-fourths 

 of the fibers usually decussate in the medulla oblongata and the remainder in the 

 medulla spinalis. 



The axons of the motor cells in the anterior column pass out as the fibers of the 

 anterior roots of the spinal nerves, along which the impulses are conducted to the 

 muscles of the trunk and limbs. 



From this it will be seen that all the fibers of the motor tract pass to the nuclei 

 of the motor nerves on the opposite side of the brain or medulla spinalis, a fact 

 which explains why a lesion involving the motor area of one side causes paralysis 

 of the muscles of the opposite side of the body. Further, it will be seen that there 

 is a break in the continuity of the motor chain; in the case of the cranial nerves 

 this break occurs in the nuclei of these nerves; and in the case of the spinal nerves, 

 in the anterior gray column of the medulla spinalis. For clinical purposes it is 

 convenient to emphasize this break and divide the motor tract into two portions : 

 (1) a series of upper motor neurons which comprises the motor cells in the cortex 

 and their descending fibers down to the nuclei of the motor nerves; (2) a series 

 of lower motor neurons which includes the cells of the nuclei of the motor cerebral 

 nerves or the cells of the anterior columns of the medulla spinalis and their axis- 

 cylinder processes to the periphery. 



The rubrospinal fasciculus arises from the large cells of the red nucleus. The fibers 

 cross the raphe of the mid-brain in the decussation of Forel and descend in the 

 formatio reticularis of the pons and medulla dorsal to the medial lemniscus and as 

 they pass into the spinal cord come to lie in a position ventral to the crossed pyram- 

 idal tracts in the lateral funiculus. The rubrospinal fibers end either directly or 



