33° 



ZOOLOGY FOR MEDICAL STUDENTS 



degenerate and membranouSj with a choroid plexus of blood-vessels 

 closely apposed to it. Its cavity, the " third ventricle," is a narrow 

 vertical slit, on each side of which is a thick lateral wall known as 

 the optic thalamus and composed for the most part of sensory nerve- 

 fibres on their waj- from the eye towards the optic lobe. 



From the posterior portion of 

 the roof of the thalamencephalon 

 there projects a slender tubular 

 outgrowth which stretches for- 

 wards and ends blindly close to the 

 skin of the dorsal surface a little in 

 front of the hemisphere region. 



X.V. 



B 



Fig. 138. 



Brain of Acanthias with the cranial nerves (after Purser), c, Cerebellum ; h, hemisphere region ; 

 hm, hyomandibular branch of VII ; i.l, inferior lobe ; m.o, medulla oblongata ; o.l, olfactory bulb ; 

 o.p, deep ophthalmic branch of V ; op,l, optic lobe ; pit, pituitary body ; th, thalamencephalon. 

 II, Optic ' nerve ; III, oculomotor ; IV, pathetic ; V, trigeminal, V.o, its superficial ophthalmic 

 branch ; VI, abducent ; VII, facial, VIIo, its superficial ophthalmic branch ; VIII, auditory ; 

 IX, glosso - pharyngeal ; X, vagus, 67, 62, etc., its branchial branches, X.iaf, its lateral line 

 branch, X.u, its visceral branch. 



This is the pineal organ — of great interest from the fact that in most 

 vertebrates it functions as a ductless gland while in a few members of 

 the group it takes the form of a third eye. It will be referred to in 

 this connexion later on. 



The floor of the thalamencephalon dips downwards into a large 

 backwardly projecting pocket — the infundibulum. The side-walls of this 

 are thickened to form the inferior lobes (Fig. 138, B, iX) while its terminal 



