NERVOUS SYSTEM OF THE VERTEBRATA. 



355 



gradually prolonged, and forms an elongated diverticulum of the third 

 ventricle, the apex of which is in contact with the pituitary body 

 (figs. 252, 254 in, and figs. 250 and 255 inf). 



Along the sides of the infundibulum run the commissural fibres 

 connecting the floor of the mid-brain wdth the cerebrum. 



In its later stages the infundibular region presents considerable varia- 

 tions in the different vertebrate types. In Fishes it generally remains very 

 large, and permanently forms a marked diverticulum of the floor of the 

 thalamenceplialon. In Elasmobranchii the distal end becomes divided into 

 three lobes — a median and two lateral. The lateral lobes apjjcar to become 

 the sacci vasculosi of the adult. 



In Teleostei peculiar bodies known as the lobi inferiores (hypoaria) 

 make their appearance at the sides of the infundibulum. They appear to 

 correspond in position with the tuber 

 ciaereum of Mammalia \ In Birds, 

 Reptiles, and Amphibia the lower part 

 of the embryonic infundibulum becomes 

 atrophied and reduced to a mere finger- 

 like process — the processus infundibuli. 



In Mammalia the posterior part of 

 the primitive infundibulum becomes the 

 corpus albicans, which is double in Man 

 and the higher Apes ; the ventral part 

 of the posterior wall forms the tuber 

 cinereum. Laterally, at the junction of 

 the optic thalami and infundibulum, 

 there are placed the fibres of the 

 crura cerebri, which are probably de- 

 rived from the walls of the infundi- 

 bulum. A special process grows out 

 from the base of the infundibulum, 

 which undergoes peculiar ch.nges, and 

 becomes intimately united with the 

 pituitary body ; in which connection it 

 will be more fully described. 



The sides of the thalamence- 

 phalon become very early thickened 

 to form the optic thalami, which con- 

 stitute the most important section of 

 the thalamencephalon. They are 

 separated, in Mammalia at all 



events, on their inner aspect from the infundibular region by a 

 somewhat S-shaped groove, known as the sulcus of Munro, which 

 ends in the foramen of Munro. They also become in Mammalia 

 secondarily united by a transverse commissure, the grey or middle 

 commissure, which passes across the cavity of the third ventricle. 



Fio. 254. Longitudinal section 



THROUGH THE BRAIN OF ScYLLIUM CANI- 

 CULA AT AN ADVANCED STAGE OF DE- 

 VELOPMENT. 



cer. cerebral hemisphere; pn. pi- 

 neal gland ; op.th. optic thalamus, con- 

 nected with its fellow by a commissure 

 (the middle commissure). In front of 

 it is seen a fold of the roof of the fore- 

 brain, which is the choroid plexus of 

 the third ventric'e; op. optic chiasma; 

 pt. pituitary body; in. infundibulum; 

 c6. cerebellum; am. v. passage leading 

 from the auditory vesicle to the ex- 

 terior; mtl. medulla oblongata; C.in. 

 internal carotid artery. 



1 For the relations of these bodies, vide L. Stieda, "Stud. iib. d. centrale Nerven- 

 system d. Knochenfische." Zeit. f. wisn. Zool. Vol. xviii. 1868. 



23—2 



