178 HUMAN PHYSIOLOGY. 



12. A center for certain ocular functions, governing the closure of the eyelids 

 and dilatation of the pupil. 



13. A sweat center is also localized in the medulla. 



THE PONS VAROLII. 



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

 hind, and the medulla oblongata below. It consists of transverse and longi- 

 tudinal fibers, amidst which are irregularly scattered collections 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 ventral 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 lat- 

 eral 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, con- 

 sisting of descending motor fibers; the dorsal portions are sensible, but in- 

 excitable to irritation. 



Transmits motor and sensor impulses from and to the cerebrum. 



The gray ganglionic matter consists of centers which convert impressions 

 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 progression. 



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 pyramids, 

 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 



