774 THE CENTRAL AXIS OF THE NERVOUS SYSTEM. 



h. The infimdihulum is only a short conical prolongation, with its base attached 

 to the tuber cinereum, and its apex to the superior face of the pituitary gland. 

 The cavity of the tuber cinereum is continued into the infundibulum, and termi- 

 nates in a cul-de-sac towards its summit. This prolongation, also formed of grey 

 substance, is distinguished by its great fragility ; so that it requires some care to 

 preserve it intact when opening the cranium at its base. 



c. The pituitary gland is lodged in the sella Turcica, where it is enveloped by 

 the supra-sphenoidal duplicature of dura mater. It is a small, nearly circular 

 body, flattened above and below, and more or less thick, according to the subjects. 



Its inferior f me rests on the sphenoid bone through the medium of the dura 

 mater, to which it is strongly adherent ; the nqjerior covers the corpus albicans, 

 with a portion of the cerebral peduncles, and in front receives the insertion of 

 the pituitary stem. Its circumference is related to the supra-sphenoidal duplicature, 

 the interior of which forms the cavernous sinus. 



There is no cavity in the pituitary gland. 



The matter composing it is yellow in the anterior half of the organ, and 

 brown in its posterior portion. Its structure resembles that of the supra-renal 

 capsules, and there is a tendency to regard it as a lymphatic organ (though it 

 has been surmised that it may have a function in connection with the blood- 

 supply to the brain). 



Internal Conformation of the Isthmus (Figs. 427, 428). 



The isthmus is hollowed at the thalami optici by a central cavity, named the 

 middle (or thvrcV) ventrich, 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) ventricle — another cavity comprised 

 between the cerebellum and medulla oblongata. These three diverticuli will be 

 studied in succession. 



1. Third or Middle Ventricle, or Ventricle of the Thalami Optici 

 (Fig. 429, 13). 



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

 depressed on each side, and offering for study two ivalls, 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, the bottom of which 

 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. 

 429, 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 con- 

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

 ventricle, forming a thick grey commissure (Fig. 429, 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. 429, 15) commences 

 behind the grey commissure, and terminates at the base of the pineal gland by 

 an irregularly expanded cid-de-sac. It is Hmited behind by the posterior tvhite 



