GUIDE TO THE HEAD AND NECK 1593 



upper surface of the optic thalamus, resting upon which there are (a) the 

 sharp lateral border of the fornix, and {h) the choroid plexus ot the lateral 

 ventricle ; and (4) the bulb of the posterior comu, produced by the fibres 

 of the forceps major, and beneath this the calcar avis, or hippocampus minor, 

 produced by the anterior calcarine fissure on the mesial surface of the cerebral 

 hemisphere. 



The middle or descending comu of the lateral ventricle, situated within 

 the temporal lobe, is to be laid freely open by removing a large portion of 

 that lobe. The remarkable course of this comu is to be noted, and 

 the following structures, which occupy its floor, are to be studied : (i) the 

 hippocampus major, or comu Ammonis ; (2) the fimbria, or taenia hippo- 

 campi ; (3) the eminentia coUateralis, or pes accessorius ; and (4) the choroid 

 plexus of the descending comu. The swelling in which the hippocampus 

 major terminates, called the pes hippocampi, is to be noted ; and the choroidal 

 fissure which extends from the foramen of Monro to the lower extremity of 

 the descending cornu is to receive attention. 



The corpus callosum is now to be divided transversely about its centre. 

 On raising the anterior part the septum lucidum will be exposed, as it extends 

 vertically bet^veen the under surface of the corpus callosum and the upper 

 surface of the body of the fornix. The relation of the septum lucidum to 

 the lateral ventricles having been studied, the septum is to be carefully 

 divided with a pair of scissors, and the interval beti^'een its two layers, called 

 the fifth ventricle, or ventricle of the septum, is to be examined. It will be 

 evident to the dissectors that this ventricle is a closed space. 



The posterior part of the corpus callosum may now be turned backwards. 



The fornix falls to be studied at this stage. Thereafter it is to be divided 

 transversely about its centre, and the two parts are to be carefully raised 

 from the subjacent velum interpositum, and turned forwards and back- 

 wards. The two posterior pillars of the fornix will be seen to be at first 

 connected with the under surface of the corpus callosum, but subsequently 

 each enters the descending comu of the lateral ventricle under the name of 

 the fimbria, or taenia hippocampi. On the under surface of the corpus cal- 

 losum, between the diverging posterior pillars of the fornix, the dissector is 

 to note the lyra, or psalterium. On turning forwards the anterior part of 

 the fornix, its two anterior pillars will be seen descending towards the base 

 of the brain, and in the interval between them a partial view will be obtained 

 of the anterior commissure. After reflection of the fornix the velum inter- 

 positum is exposed, in connection with which the two veins of Galen are to 

 be studied, these subsequently joining to form the vena magna Galeni. The 

 transverse fissure oi the brain, by which the pia mater enters, to form the 

 velum interpositum, may be studied at this stage, its lateral portions being 

 the choroidal fissures. 



Third Ventricle. — To expose this ventricle at the present stage the velum 

 interpositum is to be removed with the greatest care. It is to be laid hold 

 of with the forceps anteriorly, and turned bactwards. In doing so the two 

 choroid plexuses of the third ventricle on the under surface of the velum 

 interpositum, as well as the ependymal roof of the third ventricle, are neces- 

 sarily removed. In reflecting the back part of the velum interpositum the 

 dissector must be careful not to disturb the pineal body, which rests upon 

 the mesencephalon at the back part of the ventricle. This dissection will 

 expose fully (i) the upper surface of each optic thalamus ; (2) the chink -like 

 cavity of the third ventricle, lying in the mesial line between the two optic 

 thalami ; and {3) the middle, or grey, commissure, which extends between the 

 inner surfaces of the optic thalami. This commissure, however, being very 

 soft, often gives way. At this period the dissectors should make themselves 

 familiar (by revision) with the various structiires which form the roof of 

 the third ventricle. This ventricle itself is to be fully studied, and the 

 relation of its sloping floor to the interpeduncular region at the base of the 

 brain is to be carefully noted. The optic thalamus is to be studied, and in 

 connection with it the following parts are to be noted : (i) the anterior 

 tubercle ; (2) the posterior tubercle, or pulvinar ; (3) the corpus geniculatum 

 externum ; emd (4) the stria pinealis, or mednUaris, as well as the sulcus of 



