DISSECTION OF THE DOG 215 



The ventral surface forms the roof of the lateral ventricles, and in the median 

 plane is connected with the septum pcllucidum, a partition separating the 

 cavities of the two ventricles. The lateral boundaries of the corpus callosum 

 are lost in the white substance of the hemispheres. 



The main part of the corpus callosum is known as the truncus corporis 

 callosi. Its aboral end is thickened and forms the splenium corporis callosi. 

 On examination of the sagittal median section of the brain, the oral end of 

 the body will be seen to make an abrupt bend. This is the genu corporis 

 callosi, from which is a recurved, tapering (in section) rostrum corporis callosi 

 connected in its turn with the lamina terminalis. 



Dissection. — It is now necessary to expose the interior of the lateral ven- 

 tricles. Make a longitudinal incision through the corpus callosum about 

 three or four mm. from the middle line. Then remove that part of the 

 body which is lateral to the incision by tearing it away with either forceps 

 or the end of the handle of the scalpel. When both ventricles have 

 been opened in this manner a narrow strip of the corpus callosum will 

 be left undisturbed. From the main part of the ventricle so exposed 

 two cornua proceed. One of these is anterior in position and can be 

 followed to its termination without difficulty. The other (inferior) 

 cornu must be followed by the piecemeal removal of portions of the 

 hemisphere. Previous to doing this, however, it will be well to note 

 that the cornu is at first directed in a lateral, backward and ventral 

 direction. Later its course is medialwards and forwards. 



A proper understanding of the lateral ventricle and its boundaries will 

 be facilitated by dissection from the medial side. To do this, remove 

 the corpus callosum and the immediately adjacent part of the hemisphere 

 from the hemi-sected brain. Though the anterior cornu may be com- 

 pletely investigated in this specimen, it is better to do no more than 

 expose the first part of the inferior cornu. 



Ventricultts lateralis.- — The lateral ventricle represents the cavity 

 of the vesicle from which the cerebral hemisphere originally developed, and 

 is in communication with the third ventricle by way of the interventricular 

 foramen of Monro (foramen interventriculare [Monroi]), a small opening imme- 

 diately beneath the fornix. Generally the cavity of the ventricle is not 

 spacious, since its roof and floor are mostly in contact. 



For descriptive purposes the lateral ventricle is divided into three parts — 

 a central part, an anterior cornu, and an inferior cornu. The central part 

 (pars centralis) has a roof formed by the corpus callosum, and a medial wall, 

 by which it is separated from the other ventricle, consisting of the septum 

 pellucidum and the fornix. There is no lateral wall, because the roof and floor 

 meet and thus form the lateral boundary. 



The floor of the central part of the ventricle is formed by several important 



