346 THE ANIMAL KINGDOM. 



Touch cells, i. e. ganglion cells, terminating a nerve filament, are found 

 in the cutis of Anuran Amphibia and higher forms, often united within 

 a common sheath forming touch corpuscles. Finally, in Sauropsida and 

 Mammalia Pacinian corpuscles or terminal nerve filaments surrounded 

 by greatly developed sheaths are found in the skin, among the muscles, 

 &c. The olfactory mucous membrane in some Pisces, e.g. Flying Fish, 

 Pike, and Urodele Amphibia, e.g. Newt, appears to consist of end-buds 

 intermingled with non-sensory epithelium, but in most Vertebrata it consists 

 of sensory epithelium mingled with supporting cells. The former are 

 terminated by sensory hairs, except in Pisces and Mammalia. The nose 

 is originally a depression, but becomes in the Dipnoi and higher forms 

 a sac, with an external opening in the face and an internal opening into 

 the mouth. A portion of the nasal cavity becomes separated off from 

 the nose proper. It is known as Jacobson's organ, and is supplied by 

 the fifth nerve as well as by the olfactory. It exists in many Lacertilia, 

 Ophidia, and Mammalia, and is represented in Amphibia. The retina of 

 the eye, the retinal epithelium and optic nerve are derived from a vesicle 

 of the brain, originating from the thalamencephalon, i. e. secondarily from 

 the epiblast. The retina is composed, proceeding from within outwards, 

 of an inner layer of nerve-fibres derived from the optic nerve ; a layer 

 of ganglion cells ; an inner granular, or cell layer ; and an outer granular, 

 or cell layer the cells of which give origin to the visual rods, or rods 

 and cones. The outer free ends of the latter are immersed in a layer 

 of pigment cells, also derived from the optic vesicle. The retinal elements 

 are supported by connective tissue. Capillary blood-vessels enter the 

 retina in Mammalia, Chelonia, and the Eel. There is a spot lying in the 

 axis of vision known as fovea centralis, or in Primates among Mammalia 

 from its colour as macula lutea, in which the cones with their cells are 

 alone present, the other layers thinning away. Between the retina and 

 the lens intervenes a jelly-like vitreous humour, into which projects in 

 many Pisces, Reptilia, and in Aves a pigmented vascular process the 

 pecten, marsupium or processus falciformis. The lens is cellular, and is 

 formed by an involution of the epiblast There is an adjustable fibro- 

 muscular diaphragm, or iris, placed in front of it. The eye is enveloped 

 in two protective coats, an inner vascular pigmented choroid, and an outer 

 fibroid or cartilaginous sclerotic, continuous in front with a transparent 

 cornea, which is in part an epiblastic, in part a mesoblastic, structure. The 

 eye-ball is moved by four recti and two oblique muscles. It is usually 

 protected by an upper and lower eyelid, and, in very many instances, by a 

 third lid or nictitating membrane, which has frequently special muscles of 

 its own. Two glands, a lacrymal in connection with the upper lid, a 

 Harderian with the third lid, are commonly present ; and their secretion 

 is carried to the nasal cavity by a special lacrymal duct which arises at 



