DISSECTION OF THE BRAIN 257 



h. The olfactory lobes have much the same appearance 

 as from the dorsal surface. 



c. The optic chiasma is formed by the crossing of the 



optic nerves on the under surface of the thalam- 

 encephalon. Beyond the chiasma the nerves 

 diverge outwards towards the foramina in the 

 skull-wall which lead to the orbits. 



d. The infundibulum consists of a pair of prominent 



oval swellings, sometimes called lobi inferiores, 

 on the under surface of the thalamencephalon, 

 immediately behind the optic chiasma. 



e. The pituitary body is a thin-walled sac immedi- 



ately behind the infundibulum, and attached to 

 it. It is produced laterally into two thin-walled 

 pouches, the sacci vasculosi; and it has along 

 its ventral surface a median tubular body of 

 unknown function, the anterior end of which is 

 connected with the infundibulum, and the pos- 

 terior end, in the natural condition of the parts, 

 attached to the floor of the skuU. 



f. The roots of the third nerves arise from the ventral 



surface of the brain close to the middle line, and 

 opposite the junction of the infundibulum with 

 the pituitary body. 

 Lift up the pituita/ry body and infundibulum so as to 

 expose the third nerves fully. 



g. The medulla is narrow in front : it widens consider- 



ably opposite the roots of the fifth and seventh 

 nerves; and then narrows again as it passes 

 back into the spinal cord. 

 h. The roots of the sixth nerves arise from the ventral 

 surface of the medulla, near the middle line, and 

 a short distance behind the level of the roots of 

 the fifth and seventh nerves. 

 3. The cavities of the brain. 

 Bisect the brain by a median vertical incision along its 

 whole length. Slice away the inner surface of one half of the 

 prosencephalon until the cavity within it is fully exposed. 



