HUMAN PHYSIOLOGY. 



the contact of mustard, pepper, &c. with the 

 tongue are not those of taste, but rather exag- 

 gerated forms of touch. 



Smell. The organ of the sense of smell is the 

 mucous membrane lining a part of the nasal 

 cavities supplied with nerves from the olfactory 

 bulbs or first pair of cranial nerves. Attached to 

 the side-walls of each nasal cavity are two delicate 

 scroll-like bones, called turbinated bones, which 

 to a great extent divide each cavity into three 

 spaces, lying one above the other. The two 

 uppermost of these constitute the true olfactory 

 chambers, while the lowest passage is merely used 

 for respiratory purposes. The whole of this bony 

 framework is covered by moist mucous membrane, 

 having imbedded in it flat elongated plates at- 

 tached to the ramifications of the olfactory nerves. 

 By the contact of certain substances with these, a 

 sensation of smell is produced. According to 

 Graham, 'all odorous substances are in general 

 such as can be readily acted on by oxygen.' 

 Animal effluvia keep near the soil, hence the 

 bloodhound runs with the nose to the ground. 

 The sense of smell is extremely delicate in most 

 individuals. It is soon blunted, and consequently 

 many who live among disagreeable odours do not 

 perceive them. A distinction must be drawn 

 between a smell proper, like that of a violet, and 

 the irritation produced by the fumes of ammonia. 

 The close stuffy sensation experienced on enter- 

 ing an ill-ventilated crowded apartment, is due 

 chiefly to interference with the free play of respir- 

 ation. 



Sight or Vision. The sensation of light results 

 from the influence produced on the sensitive expan- 

 sion of the filaments of the optic nerve by vibra- 

 tions of a delicate and subtle substance known as 



Fig. 8. A longitudinal section of the Coats of the 

 Eye: 



i, the sclerotic, thicker behind than in front ; 2, the comea ; 

 3, the choroid ; 6, the iris ; 7, the pupil ; 8, the retina ; 10, the 

 anterior chamber of the eye ; n, the posterior chamber ; 12, the 

 crystalline lens, inclosed in its capsule ; 13, the vitreous 

 humour, inclosed in the hyaloid membrane, and in cells formed 

 in its interior by that membrane; 15, the sheath; and 16, the 

 interior of the optic nerve, in the centre of which is a email 

 artery. 



' ether.' But the falling of light upon the optic 

 nerve itself will produce no sensation. An inter- 

 mediary apparatus is necessary the retina, which 

 is an expansion of nervous matter placed behind 

 the optic nerve. 



The globe of the eye is placed in the anterior 

 part of the orbit, in which it is held in position by 



its connection with the optic nerve posteriorly, 



and with the muscles which surround it, and by 



the eyelids in front: It is further supported 



behind and on the sides by a quantity of fat. 



The eyeball is composed of several investing 

 ' membranes, and of certain transparent structures, 

 | which are inclosed within them. These trans- 

 ! parent structures act as refractive media of 



different densities, sd that rays of light entering 

 j the eye are so bent as to come to a focus on the 

 I retina. Thus a distinct image is formed. These 



refractive structures are from before backwards 



ist, the cornea, like transparent horn ; 2d, the 



aqueous humour, like water; 3d, the lens, like 



glass ; and lastly, the vitreous humour, like clear 



jelly. 



The outermost coat of the eye is the sclerotic 



(from skleros, hard). It is a strong, dense, white, 



fibrous structure. Posteriorly, it is perforated by 



the optic nerve. This coat, by its great strength 



and comparatively unyielding structure, maintains 



the inclosed parts in their proper form, and serves 



to protect them from external injuries. 



The choroid coat is a dark-coloured vascular 



membrane, containing pigment cells. In front, it 



ends by means of the ciliary processes, which 



consist of about sixty or 



seventy radiating folds. MM^BHH^MB 



These fit into depressions 



in the suspensory ligament 



of the lens, and assist in 



keeping it in its proper 



position. 



The iris may be regarded 



as a process of the choroid, 



with which it is continu- 



ous. It is a thin flat cur- 



tain, hanging vertically in 



the aqueous humour in 



front of the lens, and per- 



forated by the pupil for 



the transmission of light. 



It is composed of un- 



striped muscular fibres, 



one set of which being 



arranged circularly round 



the pupil, and, when neces- 



sary, effecting its contrac- 



tion ; while another set lie 



in a radiating direction 



from within outwards, and 



by their action dilate the 



pupil. Thus more or less 



light may be admitted into 



the eye, and its function is 



like that of the diaphragm 



in many optical instru- Fig. 9- A Vertical Sec- 



men t s . 



The varieties of colour 



in the eyes of different 



individuals and Of diner- 



ent kinds of animals, 

 mainly depend upon the 

 colour of the pigment, 

 which is deposited in cells 

 in the substance of the 



tion of 

 Retina: 



the Human 



external granular layer ; 3, 

 the intervening layer be- 

 tween 2 and 4, the inter- 

 nal granular layer; 5, finer 

 granular layer; 6, layer of 

 nerve-cells ; 7, fibres of the 

 optic nerve; 8, limitary n> em ' 

 brane. 



iris. 



Within the choroid is the retina. With the 

 naked eye it is seen to be a delicate semi-trans- 

 parent sheet of nervous matter, lying immedi- 

 ately behind the vitreous humour, and extending 





