GENERAL SURVEY OF THE STRUCTURE OF THE BRAIN. 375 



projection system displays a multiplicity of termini, the second 

 segment of the projection system, the cms cerebri at its exit 

 from the ganglionic masses already becomes reduced to the 

 merely double tract, anterior and posterior, of the peduncle of 

 the brain, i.e., the crusta (figs. 230 and 232, P 2 ) and the tegmen- 

 tum of the crus cerebri (figs. 230 and 232, Tg), which are 

 continued into the anterior (figs. 231, Pzd) and posterior regions 

 (figs. 230, 231, and 232, P a r) of the pons and medulla oblongata, 

 and ultimately unite into a morphologically single tract con- 

 stituting the medullary investment of the medulla spinalis. 



The segmentation of the projection system, its interruption 

 by the grey masses, certainly does something more than effect 

 the simple transference of the condition of excitation from one 

 series of ganglion cells to another superjacent to it, like the 

 handing of buckets from one person to another on a ladder. 

 The morphological significance of the interruption consists in 

 most cases in the circumstance that the intercalated cells, 

 besides continuing the segmented but still direct centripetal or 

 centrifugal path, constitute circuits to centres situated apart 

 from the direct continuation of the projection system. 



An exemplification of this is afforded by the centrifugal tract 

 shown by innumerable anatomico-pathological observations 

 to issue from the cortex of the cerebrum, and to reach the 

 anterior nerve-roots of the spinal cord by passing through 

 the corpus striatum and lenticular nucleus, the crusta of 

 the crus cerebri, the pons and the anterior pyramids of the 

 medulla oblongata (figs. 230 and 232, P lt Cs, P Z ; fig. 231, Pd). 

 The size of the crusta (Hirnschenkelfuss) of the crus cerebri 

 above its entrance into the pons, is reduced to the small dimen- 

 sions of its continuation in the projection system forming the 

 pyramids, in consequence of a thick fascicular portion bending 

 back into the cerebellum through the lateral portion of the pons, 

 and thus quitting the tract of the projection system. 



The tract of uniform size which passes from the cortex of the 

 cerebrum into, for instance, the lenticular nucleus, passes like- 

 wise in this ganglion into cells from which, although there is 

 at the same time a reduction of fibres, there still arise two sub- 

 sequently diverging tracts, one running into the spinal cord, 

 the other into the cerebellum. 



