392 SPECIAL PHYSIOLOGY. 



transported, enters these nasal culs-cle-sac, and there comes in contact with 

 the delicate membrane supplied by branches from the large olfactory lobes. 

 This membrane often has its surface increased by being thrown into variously 

 folded or plaited laminae ; sometimes forming longitudinal, and sometimes 

 singularly complicated, radiated plicae. 



In the Cyclostomata, the nasal apparatus is single, and is sometimes closed 

 at the bottom ; but in the Myxinoids, for example, the cavity is prolonged 

 backwards, by a special trachea-like canal, which perforates the palate, and 

 is there provided with a membranous valve, which can be opened or shut. 

 The lepidosiren is another example of the communication of the nose with the 

 mouth, in fishes. In the minute and simple amphioxus, the nasal cul-de-sac 

 is single, median, very superficial, and ciliated in its interior. 



The Mollusca, being chiefly aquatic, must receive odors through the water. 

 In the Cephalopods, the organs of smell are supposed to be two cavities, placed 

 near the back of the eye, each containing a papilla ; the nerves which pass to 

 them arise from the side of the optic nerve or ganglion, and perforate the car- 

 tilaginous capsule of the eye, before entering the papillae. The cuttle-fish is 

 said to exhibit a strong aversion to certain odorous bodies. It has been sug- 

 gested that the smaller buccal tentacles of the nautilus are possibly connected 

 with the sense of smell. In the other Mollusca, the sense of smell is also sup- 

 posed to reside in the sensitive tentacles, often found at the entrance of the 

 mouth and respiratory apparatus, beyond which no special organ for this 

 sense has yet been discovered in those animals. 



In some of the Annulosa, as in the Crustacea, the habits of the animal (as 

 of the lobster, for example, which enters the lobster-pot in deep water, prob- 

 ably attracted by the smell of the bait), justify the inference, that they possess 

 an olfactory sense ; but, by what part or organ, unless by the smaller an- 

 tennae, this is exercised, is unknown. The open cavity in the base of these 

 antennae, which admits the water to its interior, may be, as Rosenthal thought, 

 the organ of smell. In the Insects, as in the carrion-flies and others, there is 

 also reason to infer the existence of a very perfect sense of smell ; for they are 

 attracted by putrid meat, some of them even depositing their ova in plants 

 possessing that odor. Bees are possibly attracted to very distant clover- 

 fields, or other feeding-grounds, by means of an olfactory sense. In the In- 

 secta, it is also conjectured that the antennas are the smelling organs. Dug; s 

 found that, after the removal of the antennae, insects did not manifest their 

 usual cognizance of the vicinity of smells. Possibly the palpi may also be 

 concerned in the exercise of this sense. In the necrophorous beetles a curious 

 double, cushion-like structure exists in a cavity in the broad upper lip, which 

 is well situated for an organ of smell. Many insects suffer irritation from 

 fumes or vapors entering their respiratory tubes or trachea ; but there is no 

 reason to consider such sensations as allied to true smell. In certain of the 

 Molluscoida and Annuloida ciliated recesses, or disc-like spots, situated on 

 the head, may serve as olfactory organs ; but in many of them, as well as in 

 the Coalenterata and Protozoa, the existence of smell is doubtful, and certainly 

 no special organ of that sense is known. 



THE SENSE OF HEARING. 



The Organs of Hearing. 



The auditory apparatus is usually described as consisting of three 

 parts: the external ear, Fig. 74, b, m; the middle ear, or tympanum, 

 t; and the internal ear, or labyrinth, s, c. 



The external ear consists of the pinna or auricle, and the external 

 auditory meatus, or canal. The pinna, the part usually called the 

 ear, Fig. 74, presents an outer border or rim, called the helix; a 

 curved ridge, internal to this, is the antihelix ; within the antihelix is 



