DEVELOPMENT OF THE SENSE ORGANS 



389 



Malleus 



Br. arch. I 

 (Meckel's cartilage) 



Space. The membranous labyrinth is thus suspended in the fluid of .the peri- 

 lymph space. The bony labyrinth is produced by the conversion of the cartilage 

 capsule into bone. In the case of the cochlea, large perilymph spaces form above 

 and below the cochlear duct. The duct becomes triangular in section as its 

 lateral wall remains attached to the bony labyrinth, while its inner angle is ad- 

 herent to the modiolus. The upper perilymph space is formed first and is the 

 scala vestibuli, the lower space is the scala tympani. The thin wall separating 

 the cavity of the cochlear duct from that of the scala vestibuli is the vestibular 

 membrane (of Reissner). Beneath the basal epithelium of the cochlear duct a 

 fibrous structure, the basilar membrane, is differentiated by the mesenchyme. 

 The modiolus is not preformed as cartilage, but is developed directly from the 

 mesenchyme as a membrane bone. The development of the acoustic nerve has 

 been described on page 358 with the other cerebral nerves. 



The Middle Ear. — The middle ear cavity is differentiated from the first 

 pharyngeal pouch which appears in embryos of 3 mm. The pouch enlarges rap- 

 idly up to the seventh week, is flattened 

 horizontally, and is in contact with the 

 ectoderm (Fig. 168). During the latter 

 part of the second" month, in embryos 

 of 24 mm., the wall of the tympanic 

 cavity is constricted to form the auditory 

 (Eustachian) tube. This canal lengthens 

 and its lumen becomes sKt-like during 

 the fourth month. The tympanic cavity 

 is surrounded by loose areolar connec- 

 tive tissue in which the auditory ossicles are developed and for a time are em- 

 bedded. Even in the adult, the ossicles, muscles, and chorda tympani nerve retain 

 a covering of mucous epithelium continuous with that Hning the tympanic cavity. 

 The pneumatic cells are formed at the close of fetal life. 



The development of the auditory ossicles has been described by Broman 

 (Anat. Hefte, Bd. 11, 1899), with whose general concltlsions most recent workers 

 agree. The condensed mesenchyma of the first and second branchial arches gives 

 rise to the ear ossicles. 



The malleus and incus are differentiated from the dorsal end of the first arch 

 (Fig. 387). The cartilaginous anlage of the malleus is continuous ventrally with 

 Meckel's cartilage of the mandible. Between the malleus and incus is an inter- 

 mediate disk of tissue, which later forms an articulation. When the malleus be- 



Tym^amim^ 



X'Stapes 

 Br. arch II. 

 (Reickert's cartilage) 



Fig. 387 — Diagram showing the branchial 

 arch origin of the' auditory ossicles. 



