218 ANIMAL BIOLOGY. [Part I. 



In the pigeon the thickened floor is not well differentiated from 

 that of the hind-brain. The optic lobes are pressed outwards 

 to the sides by the large cerebral hemispheres and cerebellum, 

 the band of fibres uniting them being known as the optic com- 

 missure. In the rabbit the floor is differentiated into two strong 

 diverging bands of white fibres, the crura cerebri (c. ce.), pro- 

 ceeding outwards and forwards. The optic lobes are differen- 

 tiated into two pairs nates (na.) anteriorly and testes (te.) pos- 

 teriorly forming the corpora quadrigemina. In this region the 

 central canal becomes narrowed to form the iter connecting the 

 fourth and third ventricles. The cavities contained within the 

 optic lobes of the frog and pigeon communicate with the iter. 

 In the optic lobes the grey matter is external. 



(0.) Fore-Brain 4. Thalamencephalon, or primary fore-brain. 

 In the frog the hinder part of the floor in this region gives rise 

 to a funnel-shaped depression, the infundibulum (inf.), leading 

 down to a small mass of tissue (pinched off in development from 

 the stomodseum), known as the pUuita/n/ body (pit.). In front 

 of this the floor gives rise to the optic cfiiasma (op. ch.), formed 

 of optic tracts (op. tr.) posteriorly, and the optic nerves (op. n.) 

 anteriorly. The sides give rise to thickened masses of mixed 

 grey and white matter, the optic thalami (68, A., th.). The roof 

 becomes very thin anteriorlyP^orming the velum interposition. 

 Posteriorly it is thickened to form a transverse band of fibres, 

 the posterior commissure (p. c.), the anterior portion of which rises 

 on either side to fotm the peduncles of the pineal gland (pn. gl.). 

 In the pigeon the infundibulum is small, the optic chiasma well 

 marked, and the optic tracts thick. The optic thalami are 

 small, covered by the velum interpositum, and connected by 

 the posterior commissure, above which on either side are the 

 peduncles of the pineal gland. 1 In the rabbit the infundibulum 

 is small, and behind it is a rounded mass, the mammillary body. 

 (c. m.). The optic chiasma is well marked. The optic thalami 



1 The function of the pineal body has long been problematical. It has recently 

 been discovered that in the reptile (Hatteria) the pineal gland is the seat of an 

 eye constructed on the invertebrate type. The saccular brow-spot between the 

 eyes of the frog is probably a vestige of the same organ. 



