THE TEGMENTUM OF THE CRUS CEREBRI AND ITS GANGLIA. 445 



prehended segmentation forms a scheme for the course of all 

 the projection masses which course through the anterior tract 

 of the crus cerebri, that is, its crusta, to the entire series of 

 motor nerve roots. 



The size of the fasciculi, and consequently the number of 

 fibres these afferent fasciculi of the crus cerebri contain, as 

 well as the calibre of the fibres themselves, is far smaller 

 than that of the emerging roots. These facts, therefore, ob- 

 viously support the frequently verified statement of Deiters, 

 to the effect that with the interruption of the fibres that 

 takes place in a central nucleus, a change in their nature also 

 occurs. 



3. The roots of the fourth nerve arise from the same nucleus. 

 The place of decussation of this pair of nerves lies, as Stilling 

 demonstrated, not to the central side of the nuclei of origin, 

 but results from the interweaving of the root fasciculi in the 

 velum medullare (Valve of Vieussens), immediately before their 

 emergence below the corpora quadrigemina (fig. 251, 4). 



The representation of the trochlearis and the oculomotorius 

 by decussating fibrse rectse in the crus cerebri cannot therefore 

 be common to both nerves, unless we admit a recrossing which 

 is certainly not well established from a physiological point of 

 view. The trochlearis roots, which are spread in a brush-like 

 manner, arise chiefly from that somewhat deeper-lying compact 

 region of the common nucleus which lies in an excavation 

 of the posterior longitudinal fasciculi. In transverse sections 

 made through the upper half of the testes, they form marginal 

 fasciculi running round the nucleus posteriorly. In transverse 

 sections through the lower half they appear as compact 

 masses cut across at the ends of the transverse diameters of 

 fibres surrounding the wall of the aquseductus (fig. 250, 4). 



The entire tract of the trochlearis, the nucleus of origin of 

 which lies in front of the aqueduct and in the vicinity of the 

 nates, and whose point of emergence is behind the aqueduct 

 and below the testes, must necessarily encircle the aquse- 

 ductus Sylvii in a plane sloping from above downwards and 

 also backwards (Stilling). On the other hand, the admission 

 of a second root of origin for the fourth nerve, which Stilling 

 and Deiters have regarded as the so-called inferior tract, rests 



