iio6 



HUMAN ANATOMY. 



not suggested by its size. Its upper limit corresponds with an oblique plane passing 

 through the base of the pineal body and the posterior border of the corpora mam- 

 millaria ; its lower one is indicated on the ventral surface by the upper border of the 

 pons and on the dorsal aspect by the upper margin of the superior medullary velum. 

 As seen in sagittal sections (Fig. 938,) the mid-brain is about li mm. in length, 

 although when measured on the ventral surface it is slightly shorter (9 mm.) and 

 on the dorsal aspect a little longer (13 mm.). Its greatest breadth is approximately 

 23 mm. The mid-brain is traversed longitudinally by a canal, the Sylvian aqueduct, 

 which, however, lies much nearer the dorsal than the ventral surface of the brain-stem. 

 When the several parts of the brain are undisturbed, only a portion of the ventral 

 aspect of the mid-brain can be seen. Its dorsal and lateral surfaces are hidden by 

 the overhanging cerebral hemispheres, the splenium of the corpus callosum and the 

 pulvinar of the thalamus being in close relation with these surfaces respectively. 

 Notwithstanding its ventral position and apparent removal from the exterior of the 

 brain behind, the dorsal surface of the mid-brain is, in fact, directly continuous with 



Fig. 957. 



Thalamus 

 Trigonum habeiiulse 



Pulvinar 

 Colliculus superior 



Cerebral peduncle 



Fourth nerve 



Pons 



Superior cerebellar peduncle 



Taenia thalami 



Commissura habenulae 

 Pineal body 



Median geniculate body 

 Brachium inferior 

 Colliculus inferior 



Frenulum veli 



Lingula 



Cerebellum, cut surface 



Mid-brain viewed from behind ; upper part of cerebellum has been removed to expose superior medullary 



velum with lingula. 



and a part of the free posterior surface of the brain. It is, therefore, covered with 

 the pia mater, as may be demonstrated by drawing aside the overhanging cerebral 

 hemispheres. In situ the mid-brain occupies the opening bounded by the tento- 

 rium and thus connects the divisions of the brain which lie within the posterior cra- 

 nial fossa (cerebellum, pons and medulla) with those (cerebral hemispheres) that lie 

 above. Its cavity, the Sylvian aqueduct, establishes direct communication between 

 the third and fourth ventricles. The mid-brain includes two main subdivisions, a 

 smaller dorsal part, the quadrigcminal plate, which roofs in the Sylvian aqueduct and 

 bears the corpora quadrigemina, and a much larger ventral part, made up by the 

 cerebral peduncles. 



The quadrigeminal plate lies behind the plane of the roof of the Sylvian 

 aqueduct and extends from the base of the pineal body above to the upper margin 

 of the anterior medullary velum below. Its dorsal surface is subdivided into four white 

 rounded elevations, the corpora quadrigemina, by two grooves, one of which is 

 a median longitudinal furrow and the other a transverse furrow that crosses the first 

 one at right angles and slightly below its middle point. The upper part of the longi- 

 tudinal groove, between the upper pair of elevations, broadens into a shallow trian- 

 gular depression, the pineal fossa (trimonum suhpinealc) in which rests the pineal 

 body. Below, the mid-furrow ends at the base of the frenum of the superior medul- 

 lary velum. 



