SENSE OR RECEPTOR ORGANS 851 



acoustic ganglion lies to the left in the figure. The neural tube (rhombencephalic portion) 

 has not closed dorsally. (D) Left otic vesicle of 9 mm. human embryo viewed from 

 lateral aspect. The differentiating acoustic ganglion is shovi'n to the left. It is now dividing 

 into vestibular and spiral ganglia. The cochlear diverticulum is shown extending ven- 

 trally. (E) Later differentiation (11 mm. human embryo) of left otic vesicle, lateral 

 view. (F) 20 mm. human embryo, left, lateral view of differentiating otic vesicle. (G) 

 30 mm. human embryo, left, lateral view of differentiating otic vesicle. (H) 30 mm. 

 human embryo, left, median view of differentiating otic vesicle. (H-1) Semischematic 

 plan of cochlear duct and spiral ganglion of 4 month human embryo. (H-2) High- 

 powered view of basilar membrane (lamina spiralis membranacea) shown in Fig. 361 H-1, 

 portraying the spiral organ of Corti. (LI) Three-dimensional schematic drawing of the 

 human ear composed of the external ear, the middle ear, and the inner ear. The external 

 ear is composed of the pinna and external auditory meatus. The middle ear is made up of 

 the middle ear cavity or cavity of the tympanum with its auditory ossicles, the malleus, 

 incus, and stapes. The external tympanic membrane is stretched across the entrance of the 

 external auditory meatus into the middle ear cavity whereas the internal tympanic mem- 

 brane covers the fenestra rotunda (fenestra cochlea). The internal ear located within the 

 petrous bone communicates with the middle ear directly by means of the fenestra oralis 

 (fenestra vestihuli) and indirectly by means of the fenestra rotunda. The stapes is inserted 

 in the fenestra ovalis and the malleus is joined to the external tympanic membrane. By 

 means of the stapedial articulation with the incus and the latter's association with the 

 malleus, the three auditory ossicles thus extend across the middle ear cavity from the ex- 

 ternal tympanic membrane to the fenestra ovalis. 



The structural parts of the inner ear are made up of the membranous labyrinth, the 

 semicircular canals, utriculus, sacculus, and the cochlear duct (fig. 36 IF and G). Sur- 

 rounding the membranous labyrinth is the other structural part of the ear, the bony 

 labyrinth, which conforms to the general shape of the membranous labyrinth. A fluid, 

 the endolymph, is contained within the membranous labyrinth, whereas perilymph lies 

 in the space between the membranous labyrinth and the bony labyrinth. 



The development of the membranous labyrinth is shown in Fig. 361A-H, and the 

 formation of the pinna and external auditory meatus is depicted in Figs. 328A and B, 

 and 329A. It is to be observed that swellings upon the hyoid and mandibular visceral 

 arches contribute to the formation of the pinna, and that the external auditory meatus 

 develops from the invaginating hyomandibular cleft (branchial groove) between these 

 two arches. The origin of the auditory ossicles is shown in Fig. 319C-1 and C-2. Fig. 

 3611-2 shows the early relationship of these ossicles within the mesenchymal substance 

 of the developing middle ear cavity. During the formation of the middle ear cavity 

 spaces form around the developing ossicles. These spaces then coalesce to form the 

 rudiments of the middle ear cavity or cavity of the tympanum. This rudimentary 

 tympanic cavity later unites with the distal end of the first branchial pouch. The proximal 

 portion of the first branchial pouch forms the eustachian tube which connects the 

 pharyngeal area with the middle ear cavity. The extent to which the middle ear cavity 

 eventually comes to be lined with entoderm from the expanded distal end of the 

 eustachian tube (first branchial pouch) is problematical. The external tympanic mem- 

 brane is developed from the ectoderm of the external auditory meatus (hyomandibular 

 cleft invagination) and the lining of the middle ear cavity. Between these two mem- 

 branes is a layer of mesenchyme which transforms into connective tissue. The malleus 

 remains attached to the external tympanic membrane. (1-2) Schematic diagram of an 

 early stage in development of the auditory ossicles and tympanic cavity in the human 

 embryo. Observe that the first branchial pouch is expanding into the area around the form- 

 ing auditory ossicles where spaces, shown in black, are beginning to appear within the 

 mesenchyme surrounding the developing ossicles. (J) Diagram of the ear in the frog. 

 Unlike the condition in the frogs a tympanic cavity is almost entirely absent in urodeles. 

 (K) Diagram of the ear of a reptile comparable to conditions found in the snakes. It is 

 to be observed that an external tympanic membrane or external ear opening is absent. 

 Observe that the ear ossicle is composed of stapedial and extrastapedial segments. (L) 

 Diagram of the ear of the chick. The ear "ossicle" is composed of two parts, viz., a 



