



THE MEDULLA SPINALIS OR SPINAL CORD 761 



neurons the fibers of the cerebrospinal fasciculi correspond not to individual muscles, 

 but to associated groups of muscles. 



The anterior and lateral cerebrospinal fasciculi constitute the motor fasciculi 

 of the medulla spinalis and have their origins in the motor cells of the cerebral 

 cortex. They descend through the internal capsule of the cerebrum, traverse the 

 cerebral peduncles and pons and enter the pyramid of the medulla oblongata. 

 In the lower part of the latter about two-thirds of them cross the middle line and 

 run downward in the lateral funiculus as the lateral cerebrospinal fasciculus, while 

 the remaining fibers do not cross the middle line, but are continued into the same 

 side of the medulla spinalis, where they form the anterior cerebrospinal fasciculus. 

 The fibers of the latter, however, cross the middle line in the anterior white com- 

 missure, and thus all the motor fibers from one side of the brain ultimately reach 

 the opposite side of the medulla spinalis. The proportion of fibers which cross 

 in the medulla oblongata is not a constant one, and thus the anterior and lateral 

 cerebrospinal fasciculi vary inversely in size. Sometimes the former is absent, 

 and in such cases it may be presumed that the decussation of the motor fibers in 

 the medulla oblongata has been complete. The fibers of these two fasciculi do 

 not acquire their medullary sheaths until after birth. In some animals the motor 

 fibers are situated in the posterior funiculus. 



(6) The rubrospinal fasciculus (Monakow) (prepyramidal tract), lies on the ventral 

 aspect of the lateral cerebrospinal fasciculus and on transverse section appears 

 as a some\vhat triangular area. Its fibers descend from the mid-brain, where they 

 have their origin in the red nucleus of the tegmentum of the opposite side. Its 

 terminals and collaterals end either directly or indirectly in relation with the motor 

 cells of the anterior column. The rubrospinal fasciculus is supposed to be concerned 

 with cerebellar reflexes since fibers which pass from the cerebellum through the 

 superior peduncle send many collaterals and terminals to the red nucleus. 



(c) The olivospinal fasciculus (Helweg) arises in the vicinity of the inferior 

 olivary nucleus in the medulla oblongata, and is seen only in the cervical region 

 of the medulla spinalis, where it forms a small triangular area at the periphery, 

 close to the most lateral of the anterior nerve roots. Its exact origin and its mode 

 of ending have not yet been definitely made out. 



2. Ascending Fasciculi. (a) The dorsal spinocerebellar fasciculus (fasciculus 

 cerebeUospinalis; direct cerebellar tract of Fkchsig) is situated at the periphery of the 

 posterior part of the lateral funiculus, and on transverse section appears as a 

 flattened band reaching as far forward as a line drawn transversely through the 

 central canal. Medially, it is in contact with the lateral cerebrospinal fasciculus, 

 behind, with the fasciculus of Lissauer. It begins about the level of the second or 

 third lumbar nerve, and increasing in size as it ascends, passes to the vermis of the 

 cerebellum through the inferior peduncle. Its fibers are generally regarded as 

 being formed by the axons of the cells of the dorsal nucleus (Clarke's column) ; they 

 receive their medullary sheaths about the sixth or seventh month of fetal life. Its 

 fibers are supposed to conduct impulses of unconscious muscle sense. 



The superficial antero-lateral fasciculus (tract of Gowers) consists of four fasciculi, 

 the ventral spinocerebellar, the lateral spinothalamic, the spinotectal and the 

 ventral spinothalamic. 



(&) The ventral spinocerebellar fasciculus (Gowers) skirts the periphery of the 

 lateral funiculus in front of the dorsal spinocerebellar fasciculus. In transverse 

 section it is shaped somew r hat like a comma, the expanded end of which lies in front 

 of the dorsal spinocerebellar fasciculus while the tail reaches forward into the 

 anterior funiculus. Its fibers come from the same but mostly from the opposite 

 side of the medulla spinalis and cross both in the anterior white commissure and 

 in the gray commissure; they are probably derived from the cells of the dorsal 

 nucleus and from other cells of the posterior column and the intermediate portion 



