682 



TEE 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. 



Fia;. 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 loalls, 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 iu 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 bilt 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 pos- 

 terior white commissure, a thin 

 fasciculus of transverse fibres 

 placed in advance of the cor- 

 pora quadrigemina, above the 

 entrance to the aqueduct of 

 Sylvius, (or iter a tertio ad 

 quartmn ventriculum), and whose 

 extremities are lost in the sub- 

 stance of the thalami optici 

 (Fig. 325, 9). The anterior 

 common foramen, also desig- 

 nated the foramen of Monro 

 (and 



TEANSVEESB SEOTIOIf OF THE ENOEPHALON AT 

 THE POSTERIOR COMMON FORAMEN. 



1, White substance of the hemisphere, or centrum 

 ovale of Vieussens ; 2, 2, Grey substance forming 

 the external layer of the conrolutions ; 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. 



iter ad infundihulum) 

 (Fig. 327, 14), is the medium 

 ot communication between the middle and lateral ventricles, and affords' 

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



