118 HUMAN PHYSIOLOGY. 



some of which pass to the tubercula quadrigemina, while others, uniting 

 with fibres 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 portions appear to be excitable, 

 consisting of descending motor fibres ; the posterior portions are sensible 

 but inexcitable to irritation. 



Transmits motor impulses and sensory impressions from and to the 

 cerebrum. 



The. gray ganglionic matter consists of centres which convert impres- 

 sions into conscious sensations, and originate motor impulses, these taking 

 place independent of any intellectual process ; they are the seat of instinct- 

 ive reflex acts ; the centres which assist in the coordination of the auto- 

 matic movements of station and progression. 



CRURA CEREBRI. 



The Crura Cerebri are largely composed of the longitudinal fibres 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 por- 

 tion, made up of the fasciculi teretes and posterior pyramids and accessory 

 fibres from the cerebellum, constitute the sensory tract (the tegmentum], 

 which terminates in the optic thalamus and cerebrum. 



Function. The crura are conductors of motor impulses and sensory 

 impressions ; the gray matter, the locus niger, assists in the coordination of 

 the complicated movements of the eyeball and iris, through the motor oculi 

 communis nerve. They also assist in the harmonization of general muscu- 

 lar movements ; section of one cms giving rise to peculiar movements of 

 rotation and somersaults forward and backward. 



CORPORA QUADRIGEMINA. 



The Corpora Quadrigemina are four small, rounded eminences, two 

 on each side of the median line, situated immediately behind the third 

 ventricle, and beneath the posterior border of the corpus callosum. 



The anterior tubercles are oblong from before backward, and larger than 



