112 ANATOMY OF VERTEBRATES. 



spread over the mesial surface of the anterior lobes, and arc 

 homologous with those marked o in fig. 75, and in figs. 4 and 

 6, pi. vi. Lxx*. 



The corpus callosuin being removed, and the commissural 

 fibres of the hippocampi being left behind, the view of the 

 Beaver's brain now corresponds with that obtained in the dis- 

 section of the brain of the Wombat, fig. 75. The artery of 

 the plexus choroides, ib. p, in both the Beaver and Wombat, 

 enters the lateral ventricle, where the hippocampus commences 

 at the base of the hemisphere, and the plexus is continued along 

 the under surface of the ta3iiia hippocampi, and passes beneath 

 the fornix, through the usual foramen, to communicate with its 

 fellow in the third ventricle immediately behind the anterior 

 crura of the fornix, which are sent down in the Beaver, as in the 

 Wombat, from the centre of the inferior surface of the hippo- 

 campal commissure. 



If we expose the lateral ventricle by removing its outer 

 parietes in a marsupial and placental quadruped, the hippocam- 

 pus major, the tamia hippocampi, the plexus choroides, and the 

 foramen Monroianum are brought into view. If a style be 

 thrust transversely through the internal wall of the ventricle, 

 immediately above the hippocampal commissure, in the pla- 

 ceiital quadruped, it enters the opposite ventricle and perforates 

 the septum lucidum, passing below the corpus callosum. If the 

 same be done in the Marsupial brain, the style passes into the op- 

 posite ventricle, but is immediately brought into view from above 

 by divaricating the hemispheres. 



The commissure, answering to the ( lyra,' represents the begin- 

 ning of the corpus callosum ; but this determination does not 

 invalidate the fact that the great commissure which unites the 

 supraventricular masses in the Hedgehog, Beaver, Bat, and all 

 other placentally developed mammals is a definite superadditioii 

 for more effectually associating the hemispheres in whatever 

 motion or change they may undergo in the actions of the 

 brain. 



All Lissencephala show a large proportional size of the hip- 

 pocampi, fig. 79,jfand e, a small ' corpus striatum,' d, and large 

 ' bigeminal bodies, ' k, which bear the same proportion to each 

 other as in Marsupialia. The smaller the brain, the larger is the 

 share which the mesencephalon takes in its formation, as in the 

 Shrews and Moles (fig. 67). 



The rhinencephalon, fig. 79,, y, fig. 81, e, is connected by a 

 broad and complex ( crus ' with the under part of the hemispheres, 



