C. EMBRYOLOGY AND COMPARATIVE ANATOMY.

The auricle is formed (at the end of the first month) from six auricular protuberances (three glenoid, three hyoid), which are formed in the neighborhood of the first pharyngeal cleft in the upper lower jaw and lower hyoid arch, and from a fold appearing behind it (free ear fold); the embryonic auricle shows the satyr point at the top, the Darwinian tip above and behind, the angular curve of the helix below and behind. The external auditory canal arises at the interior partition of the auricular pro

FIG. 16.-Schematic development of the ear: 1, 2, 3, Glenoid auricular protuberance; 4, 5, 6, hyoid auricular protuberance; 7, a, b, c, free fold of the ear; 1, tragus; 2, crus helicis; 3, helix ascendens; 4, crus anthelicis inferiosus; 5, cymba conchæ ; 6, antitragus; 7a, crus superiosus anthelicis and antihelix; 76, helix; 7c, lobe; 8, Darwinian protuberance; 9, incisura intertragica; 10, triangular fossa; 11, scapha; 12, cavum concha.

tuberance, and from the gelatinous closing lamina of the pharyngeal cleft, the drumhead, which becomes attenuated later on. The labyrinth is formed (at the end of the first month) above the first pharyngeal cleft, beside the metencephalon, by segmentation of a hollow vesicle

from the ectoderm. The semicircular canals and auricular sacs are formed from the upper portion of the ectodermal otic vesicle, the cochlea from the lower. The otic vesicle is surrounded by mesoderm, which changes (sixth month) inside to perilymph and outside to an ossifying cartilaginous capsule (bony labyrinth). The auditory nerve develops from the brain and extends to the otic vesicle. Between the otic vesicle and the drumhead the narrow, tubotympanal sinus, projecting from the pharynx, becomes patent in the first pharyngeal cleft, and from this sinus the tubal and tympanic cavities are formed. The walls of this sinus are thick and gelatinous, its lumen is narrow and covered with cuboid epithelium. On its walls-that is, on the exterior of the tympanic cavity-are formed: from the maxillary arch, first, the hammer, which has a long continuation of the cartilage leading to the lower jaw (Meckel), and, second, the incus ; from the hyoid arch, the crura of the stapes. The basis of the stapes grows out of the labyrinth cartilage itself (Hertwig). With the absorption of the gelatinous tissue in the walls of the tubotympanal sinus the tympanic cavity is enlarged; its mucous membrane forms a thin covering for the bone, and invaginates the auditory ossicles, so that the latter become patent in the tympanic cavity, between the drumhead and the labyrinth (Hertwig).

Among the invertebrate hydromedusa an otic vesicle, resembling the human rudiment, serves as a most primitive auditory organ, and to the otic vesicle the auditory nerve is attached; in the vesicle lies an otolith, the movements of which are excited by the cilia of the epithelial tissue of the vesicle. Water animals need no external ear, as they hear by bone-conduction. The external ear is developed in land animals only, these hearing by airconduction (Kuhn). The higher one ascends in the vertebrate kingdom the more perfectly the auditory organ is developed.

II. PHYSIOLOGY OF THE ORGAN OF HEARING.

THE organ of hearing subserves the double purpose of hearing and equilibration. It reacts to the stimulus of sound and movement. Sound is produced by periodic (tones) or irregular (noises) condensation and rarefaction of air-waves, usually produced by the vibrations of elastic bodies. Sound is composed of tones, a deep ground tone and the higher-pitched (partial) overtones. The simplest sound phenomenon, therefore, is a tone or musical notesuch, for instance, as is produced by the simple pendulum-like vibrations of a tuning-fork. Our speech consists of tones (vowels), noises, and other sounds (consonants) formed by the vibrations of the voice-producing organs.

The pinna collects and intensifies the sound-waves and directs them into the auditory meatus. Binaural hearing is necessary to enable us to recognize the direction of the sound (Bloch). In man the muscles of the ear have practically lost their power of moving the ear in the direction whence the sound proceeds. By means of the external auditory meatus the sound-waves are guided toward the drumhead, which, owing to its conical shape, is extremely resonant, and the drumhead and chain of ossicles as a whole are thrown into transverse vibrations (Politzer).

The vibrating drumhead is prevented from sounding its own note (e) by its rigidity and the resistance offered by the chain of ossicles. The latter also guard against aftervibration of the drumhead and concussion of the labyrinth. It transmits the vibrations of the drumhead, especially in low-pitched tones, to the labyrinthine fluid. When the drumhead is wanting, the foot-plate of the

stapes may be directly thrown into vibration by the sound.

Ordinarily the sound vibrations are conducted to the labyrinth by the air through the pinna; more rarely, through the Eustachian tube (air-conduction: aërotympanal). Stimuli may also reach the labyrinth by way of

FIG. 17.-Air- and bone-conduction (schematic): 1, Cranium; 2, cerebrum; 3, auditory nerve going to the temporal lobe; 4, labyrinth; 5, tympanum and ossicles; 6, auditory meatus; 7, pinnæ; a, tuning-fork placed on the vertex; ab, osteal bone-conduction, ac, craniotympanal bone-conduction; d, tuning-fork held in front of the ear; de, air-conduction.

the bone. If a vibrating body is brought in contact with the bones of the skull, as when a tuning-fork is placed on the vertex, the sound-waves emanating from the tuning-fork excite the auditory nerve, both directly through the cranial bones and indirectly through the vibrations of the ear ossicles (osteo- (cranio-) tympanal) (Fig. 17).

For the proper conduction of sound a certain tension of the drumhead and chain of ossicles is desirable. This tension is regulated by the intrinsic muscles of the ear. The tensor tympani muscle during the act of hearing brings the drumhead into the best position for conducting sound; in other words, it accommodates. By contracting and forcing the foot-plate of the stapes inward it produces a momentary rise in the intralabyrinthine pressure, which is at once neutralized by the aqueducts. The act of yawning is accompanied by contraction of the tensor tympani muscle. The stapedius muscle by its contraction lifts the foot-plate of the stapes out of the oval window and thus prevents concussion of the labyrinth. Closure of the eyelids is accompanied by contraction of the stapedius muscle (Luca).

A moderate degree of pressure in the tympanum facilitates vibration of the drumhead; the pressure is regulated by the opening of the Eustachian tube, which accompanies the act of swallowing and speaking. The pressure in the auditory meatus is counterbalanced by the pressure of the air in the postnasal space, which reaches the middle ear through the tube. Whenever the interchange of air between the mouth and the tympanum is interrupted by closure of the tube, the air contained in the middle ear undergoes absorption and the pressure in the auditory meatus becomes greater than that in the tympanic cavity. As a result the drumhead is retracted, a temporary rise in the intralabyrinthine pressure takes place, and symptoms of stasis and transudation result in the tympanum (hydrops ex vacuo). The ciliary movement of the epithelium lining the Eustachian tube is directed toward the pharynx and thus assists in the drainage of the middle ear, which is the principal function of the Eustachian tube. The intratympanic pressure can be raised at will by forcible expiration while the mouth and nose are held shut (Valsalva's experiment); this forces the drumhead outward and gives rise to temporary deafness and a feeling of fulness in the ears. Conversely, the intratympanic