06 EQUINE ANATOMY. 



volutions, and showing the mastoid lobe, the fissure of 

 Sylvius and the olfactory or ethmoidal lobes ; an ante 

 rior extremity lodged in the ethmoidal fossa, a posterior 

 resting on the cerebellum. 



(h.) Internal surface. 1 — Corpus callosum, an arch 

 thrown over the lateral ventricles uniting the two 

 hemispheres, and offering on its superior face the 

 tractus longitudinalis ; 2 — lateral or cerebral ventricles. 

 Division : An anterior and a posterior or reflexed por- 

 tion ; they are separated by the septum lucidum, a thin 

 median lamella, standing vertically between them, and 

 by the fornix or trigonum cerebralis, which serves to 

 support the septum. On the floor of the lateral ven- 

 tricles are seen from forward backwards ; 1 — the cor- 

 pora striata, showing inwardly and posteriorly the 

 taenia semi-circularis in which floats the choroid plexus ; 

 2 — the hippocampi or cornu Ammonis, occupying the 

 floor of the anterior region of the lateral ventricle, and 

 is prolonged in its reflected portion and having their 

 external extremities resting on the mastoid lobes ; 3 — 

 the velum interpositum or choroid plexus, a vascular 

 expansion of the pia mater penetrating in the ventri- 

 cles by the transverse fissure. A fine membrane, the 

 ventricular arachnoid, lines the cavity of the ventricles ; 

 it secretes a liquid analogous to the cerebro-spinal fluid. 

 Structure : the grey substance forms the cortical layer 



