PHYLUM CHORDATA 



329 



thickened mass of nerve-matter 

 and between them 

 passes a transverse 

 band of nerve-fibres, 

 the anterim- comviis- 

 sure (ffl.c). The dien- 

 cephalon is a small 

 rounded lobe between 

 the paracceles and the 

 mid-brain, and con- 

 taining a laterally 

 compressed cavity, the 

 diacoele (v. 3). Its roof 

 is extremely thin. Its 

 lateral walls are 

 formed of two thicken- 

 ingSj the optic ihalami, 

 behind which passes 

 a transverse band, the 

 posterior commissure 

 (p c). Behind and 

 below the thalami 

 are the optic tracts 

 (o. t.) continued into 

 the optic nerves. Be- 

 hind the optic tracts 

 the floor is produced 

 downwards into a 

 tubular process, the 

 infundihulum, {inf.), 

 ending below in a 

 rounded body, the 

 pituitary hody or hypo^ 

 physis(pty.). The roof 

 is produced into a 

 median process, the 

 epiphysis (Fig. 970, i3, 

 iJTi; Fig. 971,^), which 

 is divided into two 

 parts, one of which 

 has connected with its 

 distal extremity an 

 eye-like structure, the 

 parietal organ or 

 pineal eye (Fig. 971, 

 pa), lying in the parietal foramen 

 in the velum transversum (v. t.), a 



m.o 



Fig. 970.— Brain of Iiacerta viridis. A, from above, with 

 the left parencephalou (c. h.) and optic lobe (o. I.) opened. 

 B, from beneath. C, from the left side. D, in longitudinal 

 vertical section, a. c. anterior cummissure ; aq. s. aque- 

 duct of Sylvius : ch, cerebellum ; c. c. crura cerebri ; c. k. 

 cerebral hemispheres; ch.p. choroid plexus ; c. s. corpus 

 striatum ; /. m. foramen of Monro ; inf. ii.fundibulum ; 

 m. 0. medulla oblongata ; o. c. optic chiasma ; o. L optic 

 lobes ; olf. olfactory bulbs with their peduncles or tracts ; 

 o.t. optic tracts; o.v. apeiture between aqueduct of 

 Sylvius and optic ventricle ; p, c. posterior commissure ; 

 p-a. pineal body ; jjfy. pituitary body ; v3, diacoele ; r4, 

 metacoeie ; I — XTI, cranial nerves. (From Parker's 

 Zootomy.) 



In front 

 transverse 



of the epiphysis, 

 fold of the thin 



