The sight would lead us into many er rors concerning the distance and figure of objects, were it not corrected and assisted by the touch. The person born blind, mentioned by Cheselden, thought when he had gained his sight, that all ob jects touched his eyes. A square tower, at a great distance, appears round ; and lofty trees, in a distant perspective, seem no larger than small bushes that are nearer to us.
Hearing. The undulations of the at mosphere, excited by the vibrations of sonorous bodies, are collected in the ex ternal ear and auditory passage, as in a hearing trumpet, and are conveyed to the membrana tympani, which they cause to vibrate. The effect is transmitted through the small bones to the watery fluid that fills the internal ear, in which the delicate filaments of the auditory nerve float; and by this nerve the sensa tion is conveyed to the brain. Muscles attached to the small bones of the tym panum have the power of stretching or relaxing the membrane; and probably thereby adapt the organ to various quan tities of sound, by diminishing acute, and augmenting the force of grave sounds, as the changes in the pupil of the eye ac commodate that organ to a greater or less number of rays, according to the effect they produce.
An entire state of the membrana tym pani is not essential to hearing; for the sense remains, where an opening has taken place in that part ; yet it. is neces sary that the tympanum should communi cate with the fiances, for an obstruction of the eustachian tube causes deafness.
Vibrations may be transmitted to the auditory nerves through the bones of the head; thus a watch placed between the teeth is heard very distinctly, although the ears are stopped, &c.
Smelling. The odorous effluvia of bo dies are disseminated in the atmosphere. The latter fluid passes through the nose in respiration, and thereby brings the odorous particles into contact with the olfactory nerves, which convey the im pressions of odours to the brain. It is in the first pair of nerves only that the sense of smelling is supposed to reside, while the numerous twigs of the fifth pair that are distributed in the nose, are merely for the purpose of general sensibility. Hence we see two very distinct modes of sensibility in this part, one of which may be entirely obliterated, while the other is augmented; in violent coryza the ordi nary feeling is very acute, for the pitui tary membrane is painful ; but the per son at the same time is not conscious of the strongest odours.
As air is the vehicle of odours, its pas sage through the nose, in ordinary respi ration, is sufficient for the purpose of smelling ; but when any odour is particu larly agreeable, we make short and re peated inspirations, and at the same time shut the mouth, that the air, which enters the lungs, may pass entirely through the nose. On the contrary, we breathe by the mouth, or entirely suppress respiration, when odours are unpleasant to us.
The small distance between the origin of the olfactory nerves in the brain, and their termination in the nose renders the transmission of impressions very sudden and easy. This induces us to apply to the nose stimuli that are proper to revive sensibility when life is suspended, as in cases of fainting, suffocation, &c.
Tasting. No body can affect the organ of taste, that is not soluble at the ordi nary temperature of the saliva. Hence the chemical maxim, " corpora non agunt nisi soluta," may be very justly applied to this sense. If the tongue be completely dry, and a body applied to it be also dry, no sense of taste ensues, as any one may convince himself by wiping his tongue dry, and applying sugar to it. The of the tongue's surface, which is well known to depend much on the condition of the stomach, also impairs our sense of taste ; hence in some disorders every thing tastes bitter.
No sense approaches more nearly to feeling than this does ; and the organ bears a considerable analogy to that of the sense of touch. The superior papil Ions surface of the tongue is the organ of taste, but we cannot deny the power of discerning savours to other parts of the mouth ; bitter substances are particularly tasted about the throat; and in some in stances, where large portions, or the whole tongue, have been cut away or destroyed, a perception of tastes has still remained. The lingual branch of the fifth pair is considered as the true gusta tory nerve, while those sent to the tongue by the eighth and ninth are regarded as merely nerves of motion. Although the tongue appears to be a single organ, it consists of two symmetrical halves ; and should be considered as a distinct eight' and left organ closely applied to each other. This is shewn in hemiplegia, where one-half only is paralysed.