274 INNERVATION. [CHAP. x. 



sists of a series of curved fibres, which pass from one cms cerebelli 

 to the other. When the brain lies with its base uppermost, these 

 fibres appear to cross over the upward continuations of the anterior 

 pyramids, as a bridge over a stream. Hence the term pans was 

 applied to them by Varolius.^ The fibres form a series of curves, 

 convex forwards, concave towards the medulla oblongata, the pos- 

 terior being much less curved than the anterior. At either side 

 they become more closely packed, taper, and form the inferior 

 layer of each crus cerebelli. Along the middle line a groove 

 traverses the surface of the pons from its posterior to its anterior 

 margin, in which the basilar artery usually lies. 



The fibres of the pons are always developed in the direct ratio 

 of the hemispheres of the cerebellum. In animals which have only 

 the median lobe, there is no pons ; and when the hemispheres are 

 small, the pons is small likewise. Hence these fibres must be 

 regarded as especially belonging to the cerebellum, and as serving, 

 whatever other office they may perform, to connect the hemi- 

 spheres of opposite sides. They constitute, therefore, the great 

 transverse commissure of the cerebellum, and are to the hemispheres 

 of that organ what the corpus callosum is to those of the brain. 



These transverse fibres do not form merely a superficial plane, 

 which covers the pyramids in their upward passage : on the con- 

 trary, they extend to more than one half of the depth of the meso- 

 cephale, as is apparent on a transverse section of it. The more 

 superficial fibres simply cross from one side to the other; the 

 deeper-seated ones interlace with those of the pyramids. The 

 fibres are irregularly disposed in planes, and vesicular matter is 

 interposed between the more deeply-seated ones. From this gray 

 matter it is not improbable that some of the fibres of the pyramids 

 may take origin. 



On the superior surface of the mesocephale are the quadrigemi- 

 nal bodies (nates and testes\ and beneath these the olivary columns. 

 A slight longitudinal groove separates the quadrigeminal bodies 

 into a right and left pair, and a transverse groove indicates their 

 division into an anterior and posterior pair. They are gangliform 

 bodies, of a grayish white colour, containing fibrous and vesicular 

 matter. The anterior (nates) are somewhat elliptical in shape; 

 they are the larger in man. The posterior (testes) are hemi- 

 spherical, and somewhat lighter in colour. These bodies are much 

 more developed in the lower animals than in man. In mammalia 



* The terms annular protuberance, isthmus encepfiali, nodus encephali, are 

 also frequently used. 





