HUMAN PHYSIOLOGY. 



and indistinctly seen objects a concave lens, with 

 a curvature sufficient to throw the images of 

 external objects at the ordinary distance of distinct 

 vision backwards upon the retina. In long-sighted- 

 ness, on the other hand, there is an abnormal 

 diminution of the refractive power, so that the 

 focus is behind the retina. This defect is corrected 

 by a convex lens, which increases the convergence 

 of the rays of light. 



Position of Objects on the Retina. In conse- 

 quence of the bending of rays of light by the re- 

 fractive media, the image of an external object is 

 inverted on the retina, and yet we see objects 

 erect. The probable explanation is, that the mind 

 may perceive as correctly from an inverted as 

 from an erect image. When we glance at a 

 column from top to base, we move the eyeball 

 downwards so as to bring successive parts on the 

 yellow spot, and it is the feeling of movement 

 which informs us which is top and which is base, 

 not the inverted position on the retina, of which 

 we are really unconscious. 



Single Vision with two Eyes. This pheno- 

 menon is explained by the fact that there are 

 corresponding points on the retina, so that when, 

 by the regular action of the muscles of the eyeball, 

 an image is formed on a corresponding point in 

 each eye, the mind is conscious of one image. If 

 we alter the direction of the axis of one eye by 

 pressing gently on the ball, an image is formed on 

 a point of the retina of that eye which does not 

 correspond, and consequently we squint, or see 

 two images. 



Hearing. The organ of hearing is composed of 

 three portions, the external, middle, and internal 

 ear. The external ear consists of the auricle, which 



'ibrations of the air. The tympanum communi- 

 :ates with the back of the throat by the Eusta- 

 :hian tube, the function of which is to equalise 

 atmospheric pressure on both sides of the vibrat- 

 ng membrane. When this tube becomes stopped 

 mechanically by enlargement of the tonsils, partial 

 deafness is the result, and when cleared so as 

 again to allow air to pass into the tympanum, 

 icaring at once returns to its normal state. Across 

 he tympanum, we find a chain of small bones, 

 one of which, the malleus, or hammer, is attached 

 >y a long handle to the drum ; this unites by a 

 oint with another, the incus, or anvil ; which in 

 urn bears the stapes, or stirrup, the base of this 

 jeing fixed to a small oval membrane closing an 

 aperture, called the fenestra ovalis, which commu- 

 nicates with the internal ear. The function of this 



Fig. 10. General view of the External, Middle, and In- 

 ternal Ear, shewing the interior of the auditory canal, 

 tympanic cavity, and Eustachian tube : 



a, the auditory canal ; 6, the tympanum ; c, the Eustachian tube, 

 leading to the pharynx ; d, the cochlea ; and e, the semicircular 

 canals and vestibule, seen on their exterior by the removal of the 

 surrounding bony tissue. 



presents elevations and depressions, the functions 

 of which are to receive and reflect the vibrations 

 of the air which constitute sound, and to transmit 

 these by a tube, partly cartilaginous, partly bony, 

 called the auditory canal, to the middle ear. The 

 middle ear is named the tympanum or drum. It 

 is a cavity in the petrous or hard portion of the 

 temporal bone. It is shut off from the auditory 

 canal by the membrane of the drum, a thin struc- 

 ture capable of vibrating when acted on by the 



Fig. 1 1. Ossicles of the Left Ear, as seen from the out- 

 side and below : 



m, head of the malleus ; g, the slender process, or processes gracila ; 

 A, the manubrium or handle ; sc, the short crus, and ic, the lone 

 cms of the incus ; a, the position of the lenticular process, through 

 the medium of which it articulates with the head of the stapes ; 

 *, the base of the stapes. Magnified three diameters. 



chain of bones is to convey vibrations from the 

 membrane to the internal ear. The internal ear, 

 or labyrinth, so called on account of its complexity 

 of structure, is the essential part of the organ of 

 hearing, because here we find the filaments of the 

 auditory nerve which are ultimately to receive 

 impulses originally produced by vibrations of the 

 air, and which are conveyed by the intermediate 

 structures already described. It is made of three 

 parts the vestibule, or central part ; the semicir- 

 cular canals, three in number, which communicate 

 posteriorly by five openings with the vestibule ; and 

 the cochlea, so called from its resemblance to a 

 snail-shell. Each of these parts is excavated from 

 the substance of the bone, and forms the bony or 

 osseous labyrinth ; but within this we have a fibrous 

 structure, exactly corresponding in shape, the 

 membranous labyrinth. The osseous is separated 

 from the membranous labyrinth by a fluid called 

 the perilymph, and within the membranous por- 

 tion there is another fluid, called the endolymph. 

 The terminations of the auditor)' nerve are dis- 

 tributed on the walls of the membranous portion, 

 and by the presence of the two fluids just men- 

 tioned, the most delicate vibrations of the air 

 communicated directly to the drum and chain of 

 bones, or indirectly through the bones of the head, 

 are conveyed to the nerves. The structure of ti 

 cochlea is very remarkable. It consists of a central 

 pillar, round which a tube makes two and a half 

 coils. This tube is divided into two compartments 



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