Vertebrata. 303 



freely on to the surface of the head^ the latter into the mouth, 

 within the upper jaw (the premaxilla and maxilla develop in those 

 parts which have grown over the olfactory pits) . Thus the anterior 

 end of the head becomes perforated by two tubes which are usually 

 close together, and separated only by a thin septum ; in the tubes, 

 there is a limited region containing the olfactory cells. The lining 

 often projects as large folds, which may be supported within by bony 

 or cartilaginous pieces (the turbinals). For further details, see the 

 various classes.* In those Vertebrata, in which the olfactory 

 apparatus consists of two tubes leading from the upper surface 

 of the head to the mouth, they have still another function, for air 

 for the respiratory organ enters through them. 



The eye or optic bulb consists externally of the sclerotic 

 coat, a firm connective tissue sheath, varying in thickness and often 

 strengthened with cartilaginous or bony plates : it becomes trans- 

 parent anteriorly to form the cornea. Within the sclerotic there 

 lies the darkly coloured vascular coat, the choroid, and within this, 

 again, the retina, connected with the sclerotic and choroid by 

 the optic nerve, which perforates them. In the cavity of the eye- 

 ball, towards the exterior is the lens, consisting chiefly of long 

 filiform cells ; in aquatic animals it is usually almost round, 

 in terrestrial forms more flattened. Behind the lens lies the 

 vitreous humour, a gelatinous connective tissue massj the 

 cavity between the lens and the cornea is filled with lymph, the 

 aqueous humour. In front of the lens there is a circular 

 extension of the choroid, the muscular pigmented iris, whose 

 circularly arranged muscle cells respond involuntarily to the action 

 of the light, and so narrow the aperture, that less light enters when 

 the illumination is brilliant. The opening of the iris, the pupil, 

 is either round or elliptical, in the latter case either vertical 

 or horizontal. The choroid itself is provided just behind the lens 

 with numerous meridional folds, the ciliary processes, which 

 are slightly developed or wanting in Pisces. 



With regard to the development of the eye, it may be noted, 

 that at an early stage of ontogeny, the fore brain forms anteriorly 

 on either side a vesicular evagination, the primary optic 

 vesicle, which is connected with the brain by a short stalk, whilst 

 its distal portion lies close below the skin. Then follows an invagina- 

 tion of the outer part of this vesicle, so as to form a double-walled 



* In some Eeptiles and most Mammals, there is a peculiar saocvilar or tubular 

 paired organ, Jacobson's organ, in close connection with the olfactory 

 apparatus ; its epithelium contains olfactory cells, and it receives fibres from the 

 olfactory nerves. In EeptOia (Snakes and Lizards), it is a small sac lying below the 

 nose, and opening anteriorly into the mouth ; in Mammalia, it is a long tube, closed 

 behind, running below the mucous membrane of the nose, on either side of the 

 lower edge of the nasal septum ; and opening usually in a fine canal (duct of Stensen), 

 which leads into the mouth j more rarely it opens direct iijto tlie nose, 



