XIII 



PHYLUM CHORDATA 



207 



(arg.), which owes its colour to minute crystals in the cells of which 

 it is composed. There are no choroid processes. In the posterior 

 part of the eye, between the choroid and the argentea, is a thickened 



A 



B 



LoT, 



L.ol. 



Fia. 877. Salmo fario. Dorsal (A ), ventral (B), and lateral (C) views of brain. BG., Bas. 

 G. corpora striata ; ch, crossing of optic nerves ; G. p, pineal body ; HH. cerebellum : 

 Hyp, pituitary body ; Inf. infundibulum ; L. ol. olfactory bulbs ; Med, spinal cord ; MR. 

 optic lobes ; NH. medulla oblongata ; Pall, non-nervous roof of prosencephalon ; Sc. 

 saccus vasculosus ; Tr. Opt. optic tracts ; UL. lobi inferiores ; VH, prosencephalon ; 

 I -X, cerebral nerves ; XII, 1, first spinal (hypoglossal) nerve ; 2, second spinal nerve! 

 (From Wiedersheim's I'ertebrata.) 



ring-shaped structure (ch. gld.) surrounding the optic nerve, and 

 called the choroid gland : it is not glandular, but is a complex network 

 of blood-vessels, or rete mirabile. It is supplied with blood by the 



