THE CRURA CEREBRI. 197 



THE PONS VAROLIL 



The pons Varolii is united with the cerebrum above, the cerebellum 

 behind, and the medulla oblongata below. It consists of transverse 

 and longitudinal fibers, amidst which are irregularly scattered collec- 

 tions of gray or vesicular nervous matter. 



The transverse fibers unite the two lateral halves of the cerebellum. 



The longitudinal fibers are continuous 



1. With the anterior pyramids of the medulla oblongata, which, 

 interlacing with the deep layers of the transverse fibers, ascend to 

 the crura cerebri, forming their superficial or fasciculated portions. 



2. With fibers derived from the olivary fasciculus, some of which 

 pass to the tubercula quadrigemina, while others, uniting with 

 fibers from the lateral and posterior columns of the medulla, 

 ascend in the deep or posterior portions of the crura cerebri. 



Properties and Functions. The superficial portion is insensible 

 and inexcitable to direct irritation ; the deeper portion appears to 

 be excitable, consisting of descending motor fibers ; the posterior por- 

 tions are sensible, but inexcitdble to irritation. 



Transmits motor and sensor impulses from and to the cerebrum. 



The gray .ganglionic matter consists of centers which convert im- 

 pressions into more or less conscious sensations and originate motor 

 impulses, these taking place independent of any intellectual process ; 

 they are the seat of instinctive reflex acts, the centers which assist 

 in the coordination of the automatic movements of station and pro- 

 gression. 



THE CRURA CEREBRI. 



The crura cerebri are largely composed of the longitudinal fibers 

 of the pons (anterior pyramids, fasciculi teretes) ; after emerging 

 from the pons they increase in size, and become separated into two 

 portions by a layer of dark-gray matter, the locus niger. 



The superficial portion, the crusta, composed of the anterior pyra- 

 mids, constitutes the motor tract, which terminates, for the most 

 part, in the corpus striatum, but to some extent, also, in the cerebrum ; 

 the deep portion, made up of the fasciculi teretes and posterior 

 pyramids and accessory fibers from the cerebellum, constitutes the 

 sensor tract (the tegmentum), which terminates in the optic thalamus 

 and cerebrum. 



