CH. XLIV.] MAIN TRACTS 639 



tudinal bundle, with which its fibres ultimately mix in the antero- 

 lateral descending tract of the spinal cord. 



Seventh section. This is through the crus. It is made up of 

 crusta (which contains the motor fibres), tegmentum (which contains 

 the sensory fibres, especially the bundle called the mesial fillet), and 

 the substantia nigra, the grey matter which separates them. 



The destination of one of the spinal cord tracts we have not yet 

 mentioned ; this is the tract of Gowers. This 

 is continued up through the ventral part of 

 the pons lateral to the pyramidal bundles ; 

 when it reaches the superior cerebellar pe- 

 duncles the main part of the tract takes a 

 sharp backward turn and enters the middle 

 lobe or vermis of the cerebellum by the superior 

 peduncle and superior medullary velum. Some 

 crusta; s.N., substantia O f the fibres of the tract are continued, how- 



nigra ; T, tegmentum. . . , . 



ever, into the corpora quadngemma. 



The Tracts of the Bulb, Pons, and Mid-Brain. 



In the preceding description we have had occasion to mention 

 the main tracts which are seen in transverse section. It will now 

 be convenient to summarise matters by enumerating them again as 

 well as certain others which are of less importance, or concerning 

 which we know less. The tracts may be divided into two main 

 groups, those which are descending and those which are ascending. 



Descending tracts. The principal descending tract is (a) the 

 pyramidal tract. This has already been sufficiently described, so also 

 have (b) the posterior and (c) the ventral longitudinal bundles. The 

 remaining tracts are : 



(d) Monakow's bundle. These fibres start from the cells of the red nucleus, 

 cross the raphe in Forel's fountain decussation ; they eventually pass into the lateral 

 column of the cord as the prepyramidal tract. 



(e) The ponto-spinal lateral tract starts from the large cells of the formatio 

 reticularis, and runs down the lateral portion of this formation through the pons and 

 bulb. In the spinal cord the fibres, mixed with many others of different origin, lie in 

 the lateral column between the grey matter and the tracts of Gowers and Monakow. 

 They pass like the fibres of the posterior and ventral longitudinal bundles into the 

 grey matter of the anterior cornu. 



(/) The vestibulo-sp'mal tract fibres are similar in origin to those of the posterior 

 longitudinal bundle ; its fibres lie mixed with those of the two last-mentioned tracts, 

 and their destination is the grey matter of the anterior horn. 



(ff) The central tract of the tegmentum; this is a distinct bundle which lies in the 

 middle of the reticular formation, but its origin and destination are both unknown. 



(h) Other longitudinal fibres of the tegmentum are (1) the fasciculus retroflexus, 

 which passes obliquely from the ganglion of the trabecula (a collection of cells 

 near the middle of the optic thalamus) to the interpeduncular ganglion of the opposite 

 side (a collection of cells just where the peduncles diverge from the transverse fibres 

 of the pons) ; (2) Von Guddens bundle, which runs from the corpora mammillaria 

 to end in the tegmentum ; these fibres decussate, and their intercrossing together 

 with that of Monakow's bundle constitutes the fountain decussation of Fore]. 



