THE PONS VAROLII. 409 



roots of the spinal nerves, and enter the ganglia and branches 

 of the sympathetic, by which they are conducted to the blood- 

 vessels. 



The influence which is exercised by the medulla oblongata, 

 or, at least, by its irritation, on the formation of sugar in the 

 liver, has been referred to (p. 269). 



STRUCTURE AND PHYSIOLOGY OF THE PONS VAROLII, CRURA 

 CEREBRI, CORPORA QUADRIGEMINA, CORPORA GENICU- 

 LATA, OPTIC THALAMI, AND CORPORA STRIATA. 



Pom Varolii. The mesocephalon, or pons (o, Fig. 145), is 

 composed principally of transverse fibres connecting the two 

 hemispheres of the cerebellum, and forming its principal com- 

 missure. But it includes, interlacing with these, numerous 

 longitudinal fibres which connect the medulla oblongata with 

 the cerebrum, and transverse fibres which connect it with the 

 cerebellum. Among the fasciculi of nerve-fibres by which 

 these several parts are connected, the pons also contains abun- 

 dant gray or vesicular substance, which appears irregularly 

 placed among the fibres, and fills up all the interstices. 



The anatomical distribution of the fibres, both transverse 

 and longitudinal, of which the pons is composed, is sufficient 

 evidence of its functions as a conductor of impressions from 

 one part of the cerebro-spinal axis to another. 



Concerning its functions as a nerve-centre, little or nothing 

 is certainly known. 



Crura Cerebri. The crura cerebri (I, Fig. 145), are prin- 

 cipally formed of nerve-fibres, of which the inferior or more 

 superficial are continuous with those of the anterior py- 

 ramidal tracts of the medulla oblongata, and the superior or 

 deeper fibres with the lateral and posterior pyramidal tracts, 

 and with the olivary fasciculus. Besides these fibres from the 

 medulla oblongata, are others from the cerebellum ; and some 

 of the latter as well as a part of the fibres derived from the 

 lateral tract of the medulla oblongata, decussate across the 

 middle line. 



On their upper part, the crura cerebri bear three pairs of 

 small ganglia, or masses of mingled gray and white nerve- 

 substance, namely, the corpora geniculata externa and internet, 

 and the corpora quadrigemina, or nates and testes. And in 

 their onward course to the cerebrum, the fibres of each crus 

 cerebri pass through two large ganglia, the optic thalamus and 

 corpus striatum, and in their substance come into connection 

 with variously-shaped masses and layers of gray substance. 

 Whether all the fibres of the crura cerebri end in the gray 



