THE ORGAN OF SMELL. 913 



In molluscs the ears are situated on the side of the gullet, and in several 

 they are connected with the surface of the body by a fine tube (helix). In the 

 Crustacea there are otolith-saccules, partly closed and partly open. The auditory 

 bristles are supplied with nerves, and are of various lengths; they support the 

 otoliths. Other auditory bristles, supplied by the same nerve-trunk, are found 

 on the surface of the body, on the antennae, and on the tail. When a sound was 

 conducted into the water Hensen observed several bristles to be set in vibration, 

 which were attuned to various pitches. The lining membrane of the auditory 

 vesicle is lost with every shedding, and the animals voluntarily replace their 

 otoliths by grains of sand. In insects the ear is represented by a tympanic mem- 

 brane, to which a tracheal vesicle is attached, and between which there is a gan- 

 glionic nervous expansion. In the acridia (cricket) the ear lies over the base 

 of the third foot, in grasshoppers in the forefeet, in beetles at the root of the 

 hind wings, and in flies at the bases of the poisers. There are, however, also 

 in the antennae, bristles connected with ganglionic fibers, and still other formations 

 that are considered as auditory organs, as, for instance, the "auditory pencils" 

 of arthropods. In cephalopods the ear is connected with the head-cartilage, and 

 the first indications of a membranous and cartilaginous labyrinth are found. 

 The nerve passes to a plate or ledge of horn, on which ciliated epithelial cells 

 represent the end-organs. 



Historical. Empedocles (473 B. C.) referred auditory impressions to the 

 cochlea. The school of Hippocrates was familiar with the tympanic membrane; 

 Aristotle (384 B. C.) knew of the Eustachian tube. According to Cassius Felix 

 (97 A. D.) hearing is dulled during the act of yawning. Vesalius (1571) described 

 the tensor tympani muscle, Ingrassias the stapes; the latter connected the func- 

 tion of the tensor with accurate hearing. Cardanus (1560) first mentioned sound- 

 conduction through the cranial bones. More exact descriptions of the finer parts 

 of the ear were made by Fallopius (1561), who described the vestibule, the semi- 

 circular canals, the chorda tympani, the two windows, the cochlea, and the aque- 

 duct; by Eustachius (died 1570), who described the modiolus, and the bony 

 staircase of the cochlea, the Eustachian tube, and the muscles of the auricle; 

 by Plater, who described the ampullae (1583); by Casseri (1600), who de- 

 scribed the spiral lamina of the membranous cochlea. Sylvius de le Boe 

 discovered (1667) the ossicle named after him, Vesling the stapedius muscle 

 (1641). Mersenne (1618) knew of overtones. Gassendius determined the velocity 

 of sound (1658). Follius described accurately the membranous labyrinth and the 

 process of the malleus named after him (1645). Tulpius (1641) considered the 

 possibility of air passing through the ears (when the drum is perforated), a con- 

 dition that, curiously, was spoken of by Alkmaon (580 B. C.) as normal in goats. 

 Subsequently, there was much discussion as to the possible existence of a normal 

 opening in the tympanic membrane (foramen Rivini). Scarpa made a masterly 

 dissection of the ear. Perrault (1666) suggested a theory similar to that of v. 

 Helmholtz .as to the perception of pitch by the cochlea. Berzelius investigated 

 the cerumen chemically, Krimer the labyrinthine fluid. According to Authenrieth, 

 the three differently placed, semicircular canals are supposed to aid in hearing 

 sounds from the respective directions. The study of acoustics was greatly 

 advanced by Chladni (1802). A most complete work on the ear of the verte- 

 brates was written by G. Retzius (1881-84). 



THE ORGAN OF SMELL. 



STRUCTURE OF THE OLFACTORY APPARATUS. 



The entrance to the nasal cavity is formed by the vestibular region or the 

 vestibule. Its mucous membrane is covered with papillae and it is lined with 

 squamous epithelium, which reaches to the anterior extremity of the inferior 

 meatus and the inferior turbinate bone. Near the opening of the nostril there 

 are hairs (vibrissae) with greatly developed sebaceous glands. Mucous glands 

 are found toward the cartilages" The area of the terminal expansions of the 

 olfactory nerve, the olfactory region, measures about 500 sq. mm., and in man it 

 includes only the tipper part of the septum, and the islands of the superior tur- 

 binate (Fig. 330, Cs}\ detached islands or peninsulas are found in the vicinity 

 of this chief olfactory region. The remainder of the nasal cavity is desig- 

 nated the respiratory region. The differences between the olfactory and 



58 



