420 THE BRAIN OF MAMMALS, BY TH. MEYNERT. 



trochlearis nerves, after an antecedent decussation analogous to 

 the decussation of the pyramids of the spinal cord. We may 

 therefore suppose that the loop or collar of the crus for the most 

 part embraces the lenticular ganglion of origin for the motor 

 cranial nerves. 



In frontal transverse sections carried, as in fig. 248, through 

 the posterior extremity of the third ventricle, the area of the 

 transverse section of the crusta has become, not only complete 

 in its fascicular mass proceeding from the caudate and the 

 lenticular nuclei, but it has also associated with itself the fasci- 

 cular mass coming directly from the cortex of the occipital and 

 temporal lobes, which as the most external fasciculi of the crusta 

 become invested by the tractus opticus, the pulvinar, and 

 the corpora geniculata, and therefore enter the transverse sec- 

 tion of the crus cerebri (fig. 243) immediately in front of 

 ;these masses. During its descent through the region of 

 the mesocephalon, however, a fresh addition to the crusta is 

 made from the third of its ganglia of origin, the grey matter of 

 Soemmering (figs. 248, 249, 8). This superficially expanded 

 ganglion forms the boundary between the crusta and the 

 tegmentum of the crus cerebri, and is continuous centrally 

 with a thin fan-like corona radiata, which terminates in it. 

 Peripherically fasciculi originate therefrom (fig. 248), which tra- 

 verse the internal and middle regions of the transverse section 

 >of the crus cerebri, forming a clear plexus dividing it into small 

 areas. This includes in part individuals of the large form of 

 pigmented cell, to the presence of which the ganglion owes its 

 name of substantia nigra, and in part cells of very small size. 

 The external part of the crus, springing directly from the cor- 

 tex, obtains no accession of fibres from the substantia nigra. 



Thus, upon the whole, there are four territories of origin for 

 the crusta of the cerebral peduncle : 1. The cortex of the occipital 

 and temporal lobes of the cerebrum ; 2. The nucleus caudatus 

 (corpus striatum internum) ; 3. The nucleus lenticularis (corpus 

 striatum externum) ; and 4. The substantia nigra. The parts 

 taking origin from these centres are so distributed over the 

 transverse section of the crusta that the most external area 

 proceeds from the cortex of the cerebrum, and the innermost 

 from the lenticular nucleus, whilst the intermediately placed 



