GENERAL MORPHOLOGY OF THE NERVOUS SYSTEM. 27 



It is hence called the pineal or parietal eye. It will be seen later 

 (Chapter VIII) that there are in vertebrates two of these rudi- 

 mentary eyes one behind the other. In amphibia, birds and 

 mammals the parietal eye is quite rudimentary and in man is 

 known as the pineal gland or body. The anterior border of the roof 

 of the diencephalon is marked in lower vertebrates and in the 

 embryos of all vertebrates by a deep transverse fold called the 

 velum transversum (Fig. n). The velum forms the cephalic wall 

 of a larger or smaller median sac of the choroid roof of the dien- 

 cephalon, which may be called simply the dorsal sac. In mammals 

 a median sac occupying nearly the same topographical position 

 bears the name of paraphysis. It is nervous in character and its 

 significance will be treated later (compare Chapter XVIII). 



The lateral walls of the diencephalon are known as the optic 

 thalami. They are thick and in lower vertebrates are traversed 

 by the optic tracts on their way to the optic lobes, while in higher 

 forms a large part of these tracts end in the optic thalami them- 

 selves. The ventral wall of the diencephalon in lower vertebrates 

 is expanded and is divided by a median ventral furrow into lateral 

 halves, known as the inferior lobes. These lobes are relatively 

 small in cyclostome fishes and become progressively larger in 

 selachians, ganoids and bony fishes. In vertebrates above the 

 fishes the region corresponding to the inferior lobes is less expanded 

 and in mammals it forms a funnel-shaped body with apex ventrad, 

 which is called the injundibulum. (Compare Figs. 2, 7, n.) 



An evagination of the caudal wall of the inferior lobes or inf un- 

 dibulum in all vertebrates forms a pair of bodies projecting some- 

 what laterally and caudally which bear the name of mammillary 

 bodies (corpora mammillaria, Figs. 2, n). Between and ventral 

 to these the floor is thin and is produced caudally and ventrally 

 into a thin-walled sac which is supplied richly with blood spaces. 

 It is hence called the saccus vascidosus. It is present in all verte- 

 brates but is much larger in the true fishes than elsewhere. Con- 

 nected with the ventro- cephalic surface of the saccus is a glandular 

 body properly known as the hypophysis (see p. 66). The two 

 together constitute the pituitary body. The so-called optic or 

 second cranial nerve is connected with the ventral wall of the 



