SCYLLIUM CANICULA 253 



behind the point of origin of the pineal stalk is a second transverse 

 band of fibres, the posterior commissure, marking the point of 

 juncture of thalamencephalon and mesencephalon. 



The anterior end of the third ventricle is limited by the lamina 

 terminalis, a strip of nervous tissue on each side of which lies a well- 

 marked opening, the foramen of Munro, through which the third 

 ventricle is continuous with the lateral ventricle. The lamina 

 terminalis runs backward, also forming the floor of the anterior part 

 of the ventricle. It terminates at a thick transverse thickening, 

 the optic chiasma, which sticks up into the ventricular cavity pro- 

 ducing a pre-optic recess in front of it, and this marks the ventral 

 boundary between telencephalon and thalamencephalon. The 

 floor of the latter commences with the chiasma, in which the fibres 

 of the optic nerve cross one another, those from the right eye crossing 

 over to the left side of the brain and vice versa. From the sides of 

 the chiasma the two stout optic nerves pass sharply outwards. 

 Just behind this point the floor of the thalamencephalon projects 

 downwards to form a backwardly running lobe, the infundibulum, 

 containing a prolongation of the third ventricle. The anterior part 

 of the infundibulum swells out laterally to form two sac-like diver- 

 ticula, the lobi inferiores, whose cavities communicate with that of 

 the infundibulum by oval apertures, while the postero-dorsal portion 

 enlarges to form a wide extremely vascular bag, the saccus vasculosus. 

 Closely adherent to the ventral wall of the indundibulum is the 

 pituitary body or hypophysis cerebri. This consists of an anterior 

 lobe, narrow and stalk-like, attached to the infundibulum between 

 the lobi inferiores, and a much broader posterior lobe which is 

 highly glandular. It is slightly more dorsal than the former, and 

 extends back beyond the saccus. 



The mesencephalon or mid-brain, although small, is easily 

 distinguished. Dorsally it comprises the two optic lobes or corpora 

 bigemina ; these are small oval swellings separated in the middle 

 line by a furrow and overhung to a large extent by the anterior end 

 of the cerebellum. They are hollow, their cavities, the optic 

 ventricles, being offshoots from the brain cavity, and their walls 

 termed the tectum opticum, are plentifully supplied with ganglion 

 cells related to the terminations of the fibres from the optic nerves. 

 The sides and floor of the mesencephalon are constituted by nervous 

 masses, the cruri cerebri, continuous with the optic thalami in front 

 and the medulla behind. The oculo-motor nerves arise from the 

 ventral surface of the crura. The cavity of the mid-brain is con- 

 stricted laterally, forming the Iter or Aqueduct of Sylvius. 



The hind-brain in Scyllium is large, occupying more than half 

 the entire length of the whole brain. The roof of the front part, or 



