808 THE NERVOUS SYSTEM. 



ization means the grouping of cell bodies of neurones for specific function. A fter the 

 known localizations are eliminated, there still remains unaccounted for the greater part 

 of the cerebral cortex, and our present belief is that these neurones are neurones of 

 association by which the various centres are brought, into physiological relationship. 

 Some understanding of this neurone grouping, and especially of the arrangement of 

 their ueuraxones as fibre tracts of the cord is essential to an appreciation of those 

 degenerations which affect definite systems of neurones. 



PERIPHERAL MOTOR NEURONES. These neurones have their cell bodies in the gray 

 matter of the anterior horn and in the motor nuclei of the cranial nerves. Their neur- 

 axones pass out as the motor fibres of the cranial and of the spinal nerves. The entire 

 neurone lies upon the same side of the body ; that is, the peripheral motor neurone is 

 " direct. " 



THE UPPER OR CORTICO-SPJNAL MOTOR NEURONES have their cell bodies situated 

 mainly in the cerebral cortex near the fissure of Rolando. Their ueuraxoues converg 

 ing, pass through the internal capsule, pons, and medulla, sending off fibres to the 

 motor nuclei of the cranial nerves. In the medulla the tract comes to the surface as the 

 anterior pyramids. At the junction of medulla and cord occurs the pyramidal decussa- 

 tion in which most of the fibres of the tract cross to the opposite lateral region of the 

 cord, to be known as the crossed pyramidal tract, while a minority remain on the same 

 side, to pass down the cord as the direct pyramidal tract. As the tracts descend, fibres 

 continuously leave them to terminate, those of the crossed tract in the gray matter of 

 the same side, those of the direct tract, after passing through the anterior commissure, 

 in the gray matter of the opposite side. The cortico-spinal motor tract is a crossed 

 tract. 1 These tracts present variations in size and length. They are often asymmetrical. 

 The crossed tract extends to the lower sacral cord. The direct tract usually ends about 

 the mid -dorsal region, though it has been followed as low as the second lumbar segment. 



THE SENSORY (AFFERENT) TRACT, like the motor, consists of two segments, an 

 upper and a lower. 



The lower or peripheral sensory neurone tract lias its cell bodies in the ganglia of the 

 spinal and of the cranial nerves. Their peripheral arms, which Van Gehuchten con- 

 siders protoplasmic processes, are axis cyliuders'of cranial or of spinal nerves. Their 

 central processes or axones pass into the cord as the fibres of the posterior roots and 

 enter the posterior columns. Here they divide into ascending and descending arms. 

 The descending arm is short, and with its collateral branches soon terminates in the 

 gray matter of the same side of the cord. The ascending arm may also be short, and 

 with its collaterals terminate as does the descending. It may pass a considerable dis- 

 tance up the cord and then end in the gray matter. It may, as one of the long fiin-es 

 of the posterior columns, continue upward to the medulla, where it terminates in one of 

 che posterior column nuclei. These long fibres pass inward as they pass upward, so 

 that the lower the origin of the fibre the more mesial is its position in the upper part of 

 the cord. The entire neurone lies upon the same side of the cord. The lower sensory, 

 like the lower motor neurone, is direct. 



Upper Sensory Neurone*. The arrangement of these neurones is extremely complex, 

 and only those whose axones enter into the formation of distinct tracts of the cord will 

 be here mentioned. Of the above-described central arms of the peripheral sensory neu- 

 rones, some of the shorter enter the gray matter of the cord and with their collaterals 

 terminate around motor cells (reflexes). Others terminate around cells whose axoues 

 cross to the opposite side of the cord and pass upward as the antero-lateral ascending 

 tract, or tract of Gowers. Other fibres terminate around cells of the column of Clarke, 

 the axones of which pass upward as the direct cerebellar tract. 



In addition to these main fibre tracts, there are in all regions of the cord fibres 

 which are commissural in character. These fibres are the axones of cells situated in the 

 gray matter of the cord, and after passing a short distance up or down and sending col- 

 laterals into the gray matter, themselves re-enter the gray matter and terminate there. 



1 Certain recent observations tend to show that there may be a small number of 

 "direct" fibres in the so-called crossed pyramidal tract. 



