ISTHMUS OF THE ENCEPHALON. 



523 



(ascending) and efferent (descending) fibers. The afferent fibers are the 

 axons of sensor end-nuclei located in the upper segments of the spinal cord 

 as well as of axons of sensor end-nuclei of cranial nerves. As they pass 

 upward some of the fibers, as well as collateral branches, arborize around 

 the nuclei of origin of the various motor cranial nerves of the same and 

 opposite sides. The ascending fibers thus associate anatomically sensor 

 end-nuclei of both spinal and cranial nerves with the nuclei of the motor 

 cranial nerves. The efferent fibers are the axons of nerve-cells located in 

 the corpora quadrigemina and in a special nucleus in the floor of the third 

 ventricle. From this origin the fibers soon cross the median line, pursue a 

 downward course and come into close relation with the ascending fibers. 

 In their descent fibers pass successively to the nuclei of origin of the motor 

 cranial nerves and to nuclei in the upper segments of the spinal cord. The 

 efferent fibers thus associate anatomically the nuclei from which they arise 

 with the nuclei just alluded to. 



The superior olive is a cylindric mass of gray matter situated in the pons 

 in the anterior part of the formatio reticularis. It consists of nerve-cells 

 the axons of which pass dorso-later- 

 ally, decussate in the median line, 

 and form the lateral fillet of the oppo- 

 site side. Some few axons go to the 

 lateral fillet of the same side. 



The groups of efferent nerve-cells 

 lying just beneath the floor of the 

 fourth ventricle give origin to axons 

 composing the motor portion of the 

 fifth, the sixth, the seventh cranial 

 nerves. The groups of afferent cells 



are the sensor end-nuclei of the fifth FIG. 241. SCHEME OF TRANSVERSE SEC- 

 and eighth cranial nerves from which N J DF _^ HE ^ CE J R P^7 ED A UNCL A ES * ^3 

 new axons pass as a part of the 

 mesial and lateral fillets toward the 

 cerebrum. 



The crura cerebri comprise that 



Fillet or lemniscus. RN. Red nucleus. SX. 

 Substantia nigra. III. Third nerve. Py. 

 Pyramidal tracts. Fc. Fronto-cerebellar; 

 and TOC, temporo-occipital fibers of the 



portion of the Central nerve System crusta. CC. Caudato-cerebellar^ fibers in 



upper part of crusta. {After Wernicke and 

 Gowers.} 



situated between the pons below and 

 the cerebrum above. They are com- 

 posed of strands of nerve-fibers which are divided, as shown on cross-section, 

 into a ventral and a dorsal portion by a crescentic shaped layer of gray matter, 

 the substantia nigra (Fig. 241). Of the fibers which compose the ventral 

 portion of each crus, the crusta or pes, the larger part is continuous below, 

 through the longitudinal fibers of the pons, with the pyramid of the medulla 

 and the pyramidal tract; above they assist in the formation of the internal 

 capsule. On the inner and on the outer side of each crusta there is a bundle 

 of fibers derived from the frontal, and from the temporal and occipital por- 

 tions of the cerebrum respectively. These fibers are connected directly 

 with the nuclei pontis and indirectly with the cerebellum of the same and 

 opposite sides. The fibers which compose the dorsal portion, the tegmentum, 

 are continuous with those which pass upward from the medulla and pons, 



