540 



STRUCTURE OF VERTEBRATA 



In Cyclostomata, " Ganoids," and Teleosteans, the fore-brain has no 

 nervous roof, but is covered by an epithehal pallium which resembles 

 what is called the choroid plexus of the third ventricle in higher Verte- 

 brates. This choroid plexus is a thin epithelium, with blood vessels in 

 it. But in Elasmobranchs, Dipnoi, and Amphibians the basal parts of 

 the fore-brain have grown upwards to form a nervous roof, and this 

 persists in higher Vertebrates. 



The optic thalami (thalamencephalon or tween-brain) 

 form the second region of the aduh brain. Hence arise 

 the optic outgrowths, which form the optic nerves and 



PJ 0^ 



Fig. 305. — Partial section of a Vertebrate brain (diagrammatic). 



OLF., Olfactory lobe ; C,H., cerebral hemispheres ; C, wall of cerebrum 

 cut to show ventricle, behind this the figure is that of a median 

 section; PA., parietal organ arismg from thalamencephalon; 

 PI., pineal organ ; INF., infundibulum descending from thala- 

 mencephalon ; H., hypophysis ; O.L., optic lobes ; CB., cere- 

 bellum ; C.PL., choroid plexus on roof of fourth ventricle ; M.O., 

 floor of the medulla oblongata ; C.C, central canal of spinal cord. 



some of the most essential parts of the eyes. The original 

 cavity persists as the third ventricle of the brain ; the 

 thin roof gives off the dorsal pineal outgrowth or epi- 

 physis, and, uniting with the pia mater, or vascular brain 

 membrane, forms a choroid plexus ; the lateral walls 

 become much thickened (optic thalami) ; the thin floor 

 gives off a ventral outgrowth, or infundibulum, which 

 bears the nervous and glandular pituitary body or hypo- 

 physis. The infundibulum also bears in most Teleosts 

 a peculiar posterior saccus vasculosus, which seems to be 

 a sense organ. It is not developed except in Fishes. 



The pituitary body. — This is derived partly from a downgrowth 

 from the thalamencephalon and partly from an upgrowth from the roof 

 of the mouth. The two parts unite to form a complex little organ, 

 whose morphological nature is puzzling. It forms two internal 

 secretions. 



