IX.] 



THE NERVOUS SYSTEM. 



The anterior bony nares may be continued on by a single 

 passage having a single opening on the surface (as in the 

 Porpoise), the channel being also connected with a complex 

 appendage of sacs. 



Each nostril may still have two apertures, and yet both of 

 these may open on the external surface, as in Fishes. Finally, 

 the nasal organ may be absolutely single and median, as in 

 the Marsipobranchs, ending below in a blind termination, as 

 in the Lamprey, or opening into the pharynx, as in the 

 Myxine. 



13. The envelope of the THIRD VENTRICLE of the brain 

 is a most constant and morphologically important structure, 

 small and subordinate as it appears in adult man. We have 

 seen, however, that the fore-brain is relatively large at first, 

 even in him, and it so remains in all the members of the 

 lowest class of Vertebrates. 



In Fishes indeed this part attains a great relative size, and 

 by some anatomists it is believed to combine with part of 



FIG. 334. BRAIN OF T IE PERCH 

 (Ptrt*JltanatiJts)SE&n FROM ABOVE. 



i, one of the olfactory lobes ; 2, one 

 of the cerebral hemispheres ; 3, the 

 pineal gland ; 4, one of the hollow 

 vesicles commonly called " optic 

 lobes " ; 5, the posterior median 

 prominence commonly termed the 

 cerebellum, but by some deemed to 

 be a special development of the 

 "mid-brain." 



FIG. 335. VERTICAL LONGITUDINAL 

 SECTION OF THE BRAIN OF THE 

 PERCH. 



i, an olfactory lobe ; 2, one of the cere 

 bral hemispheres ; 3, pineal gland 



4, the so-called "optic lobes," show 

 ing the large cavity they contain 



5, the median structure commonly 

 termed the cerebellum ; 6, the pitui 

 tary body ; 7, one of the lobi in 



Jeriores. 



the mid-brain to form a large pair of hollow vesicles in 

 proximity to the cerebral hemispheres, and to have various 

 complexities of structure within its cavity. This segment 

 of the brain is well marked out by the pineal gland abo've 

 and the pituitary body below. 



