320 



ZOOLOGY 



SECT. 



of each hemisphere. On the floor of each paracoele is a thickened 

 mass of nerve-matter, the corpus striatum (c.s.), and between them 

 passes a transverse band of nerve-fibres, the anterior commissure 

 (a. c.). The diencephalon is a small rounded lobe between the 

 paracoeles and the mid-brain, containing a laterally compressed 

 cavity, the diaccele (v. 3). Its roof is extremely thin. Its lateral 

 walls are formed of two thickenings, the optic thalami, 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. Behind the optic tracts the floor is produced down- 

 wards into a tubular process, the infundibulum (inf.), ending below 

 in a rounded body, the pituitary body or hypophysis (pty^. The 

 roof is produced into a median outgrowth, the pineal apparatus 

 (Fig. 981, D, pn ; Fig. 982, Z), 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. 982, pa), lying in 

 the parietal foramen, while the other is the pineal organ or epiphysis. 

 In front of the epiphysis, in the velum transversum (v. t.), a transverse 

 fold of the thin roof of the brain marking the anterior limit of the 

 diencephalon, is another commissure, the aberrant commissure 

 (c.p.p.), which connects together the posterior and dorsal parts of 

 the parencephala ; this is not represented either in the Frog or 

 in higher Vertebrates. The mid-brain consists dorsally of two 

 oval optic lobes (corpora bigemina, Fig. 981, o. I.) and ventrally 



of a mass of longitudinal nerve- 

 fibres, the crura cerebri (c. c.), passing 

 forwards to the fore-brain. Each 

 optic lobe contains a cavity (optoccele) 

 communicating with the iter, a narrow 

 passage leading from the diaccele to 

 the metacoele. The cerebellum (cb) is, 

 like that of the Frog, of small 

 size, being a small antero-posteriorly 

 FIG. 983. Transverse section of the flattened lobe overlapping the anterior 



nasal region of the head of Lacerta rvnrfinn n f 4->, a Ynofarrolo TVio 



to show the relations of Jacobson's P ortlon ol metaCOSie. Ifte 



organs. D, nasal glands; j, j metencephalon (medulla oblongata, m. 



Jacobson's organs; N, N. nasal \ i i < i i \ , 



cavities. (From wiedersheim's com- o.), broad in front, tapers behind to 



where it passes into the anterior 



portion of the spinal cord. The metacoele is a shallow space 

 on the dorsal aspect of the medulla oblongata, overlapped 

 in front for a short distance by the cerebellum, and behind 

 covered only by the pia mater, containing a network of vessels, 

 the choroid plexus of the metacoele (Fig. 982, pch.). At the 

 point where medulla oblongata and spinal cord meet is a strong 

 ventral flexure. 



The spinal cord is continued backwards throughout the length 

 of the neural canal, becoming slightly dilated opposite the origins 



