. The comparative anatomy of the domesticated animals. Veterinary anatomy. 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 corjjora quadrigemina by a canal—the aqueduct of Sylvius; this opens, below the valve of Vienssens, into the posterior (or fourth) ven- tricle—another cavity comprised between the cerebellum and medulla oblongata. These three diverticuli will be studied in succession. 1. Mi
. The comparative anatomy of the domesticated animals. Veterinary anatomy. 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 corjjora quadrigemina by a canal—the aqueduct of Sylvius; this opens, below the valve of Vienssens, into the posterior (or fourth) ven- tricle—another cavity comprised between the cerebellum and medulla oblongata. These three diverticuli will be studied in succession. 1. Middle Ventricle, or Ventricle of the Thalami OpHci. (Fig. 327, 13.) The middle ventricle is an irregular cavity, elongated from behind to before, depressed on each side, and oifering for study two loalls, a floor, a roof, and two extremities. The two icalls 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 pos- terior ivhite 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 te
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