682 



THE CENTRAL AXIS OF THE NERVOUS SYSTEM. 



INTERNAL CONFORMATION OF THE ISTHMUS. (Fig. 327.) 



The encephalic isthmus is hollowed at the thalami optici by a central 

 cavity, named the middle (or third) ventricle, which is extended backwards 

 beneath the corpora quadrigemina by a canal the aqueduct of Sylvius ; 

 this opens, below the valve of Vieussens, into the posterior (or fourth) ven- 

 tricle another cavity comprised between the cerebellum and medulla 

 oblongata. These three diverticuli will be studied in succession. 



Fig. 325 



1. Middle Ventricle, or Ventricle of the Thalami Optici. (Fig. 327, 13.) 



The middle ventricle is an irregular cavity, elongated from behind to 

 before, depressed on each side, and offering for study two walls, a floor, a 

 roof, and two extremities. 



The two walls are smooth, nearly plane, or very slightly concave from 

 above to below. 



The floor is extremely narrow, and only forms a channel whose bottom 

 corresponds to the interpeduncular fissure, which is nearer in front than 

 behind, and to the corpus albicans and tuber cinereum. The cavity of 

 the latter (Fig. 327, 20), prolonged into the pituitary stem, communicates 

 with the middle ventricle, and assists in its formation. 



The roof, as narrow as the floor, and, like it, nothing but a channel, is 

 constituted by the two thalami optici which are joined to one another above 



the ventricle, forming a thick 

 grey commissure (Fig. 327, 

 16). It is terminated at its 

 extremities by the two orifices 

 already noted as the posterior 

 and anterior common foramina. 

 The posterior common foramen 

 (Fig. 327, 15) commences be- 

 hind the grey commissure, and 

 terminates at the base of the 

 pineal gland by an irregu- 

 larly expanded cul-de-sac. It 

 is limited behind by the 

 terior white commissure, a thii 

 fasciculus of transverse fibres 

 placed in advance of the coi 

 pora quadrigemina, above tl 

 TRANSVERSE SECTION OF THE ENCEPHALON AT entrance to the aqueduct ol 



THE POSTERIOR COMMON FORAMEN. Sylvius, (or iter a tertio 



White substance of the hemisphere, or centrum quartumventriculum\fOldwlu 

 ovale of Vieussens ; 2, 2, Grey substance forming 

 the external layer of the convolutions ; 3, Section 

 of the corpus callosum ; 4, 4, Interior of the 

 lateral ventricles; 5, Section of the great vena 

 Galeni ; 6, 6, Cerebral peduncles ; 7, 7, Section of 

 the isthmus ; 8, Posterior common foramen ; 9, 

 Posterior white commissure ; 10, Entrance to the 

 aqueduct of Sylvius. 



extremities are lost in the sul 

 stance of the thalami optic 

 (Fig. 325, 9). The antt 

 common foramen, also desig- 

 nated the foramen of Me 

 (and Her ad infundibulum] 



iter 



(Fig. 327, 14), is the media 

 of communication between the middle and lateral ventricles, and afford 

 a passage to the vascular cord which unites the two choroid plexuses. It 



