EYE 273 



turbinal-like ridge, supported by cartilage continuous with that 

 enveloping the organ and covered with ciliated epithelium, and 

 numerous glands are present in the mucous membrane. In other 

 Mammals (H, i) it becomes more or less reduced, though often 

 well-marked, even in the adult, consisting of two tubes lying at 

 the base of the septum nasi, usually enclosed by separate para- 

 septal cartilages, which, as in Lizards and Monotremes, are 

 differentiations of the nasal septum (e.g. Marsupials, Edentates, 

 Insectivores, Rodents, Carnivores, Ungulates). A branch of the 

 olfactory nerve enters the tube posteriorly, and anteriorly the 

 cavity of the organ communicates with the mouth through the 

 incisive or naso-palatine canals. Vestiges of the organ exist even 

 in Man (Fig. 199> 



The function of Jacobson's organ may be concerned with bring- 

 ing the food taken into the mouth under the direct control of the 

 olfactory nerve. 



EYE. 



As already mentioned, the first rudiment of the eye aiises as 

 a paired outgrowth from the primary fore-brain, known as the 

 primary optic vesicle (Fig. 201, A). It, therefore, like the olfactory 

 lobe, represents a part of the brain, and in this respect differs 

 from the Invertebrate eye, which arises by a differentiation of the 

 cells of the superficial ectoderm. 



At the point where the vesicle touches the ectoderm, the latter 

 becomes thickened, and the outer wall of the vesicle is asymmetri- 

 cally invaginated to form a double-walled cup, the secondary optic 

 reside (Fig. 201, B), at first open below at the slit-like choroid 

 fissure. The inner and outer walls of the cup then become 

 fused, the former giving rise in its deeper part to the sensory 

 epithelium of the retina, and the latter to the pigment epi- 

 thelium, and also to the muscles of the iris, which are thus 

 of ectodermal origin (p. 275). As the optic vesicle grows out- 

 wards towards the outer skin of the embryo, the portion which 

 connects it with the brain becomes constricted and by degrees 

 loses its cavity, giving rise to a solid cord, the optic stalk. The 

 fibres of the optic nerve are first differentiated in the retinal 

 portion, and grow centripetally along the optic stalk towards the 

 brain ; centrifugal fibres also arise later. 



In the adult brain, the optic nerve is seen to arise from the 

 diencephalon, its fibres extending upwards and backwards to the 

 optic lobes, and three more or less sharply-differentiated portions 

 of it may in most cases be distinguished ; these are spoken of, 

 from the proximal to the distal end respectively, as the optic tract, 

 chiasma, and nerve. 



