DEVELOPMENT OF THE SENSE ORGANS 389 



is developed directly from the mesenchyme as a membrane bone. The 

 development of the acoustic nerve has been described on p. 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 rapidly 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 slit-like during the fourth month. The tympanic 

 cavity is surrounded by loose areolar connective tissue in which the 

 auditory ossicles are developed and 

 for a time are embedded. Even in 

 the adult, the ossicles, muscles, and ,if B l' !^ ch ' ]-, 



(Meeker s cartilage) 



chorda tympani nerve retain a 

 covering of mucous epithelium con- 

 tinuous with that lining the tympanic Tympanum 

 cavity. The pneumatic cells are 

 formed at the close of fetal life. (SMrtfeliavt) 



The development ot the auditory FlG 387 ._ Diagram sho wing the branchial- 



ossides has been described by arch origin of the auditory ossicles. 



Broman (1899), with- whose general 



conclusions 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 intermediate disk of tissue, which later forms an articula- 

 tion. When the malleus begins to ossify, it separates from Meckel's 

 cartilage. The incus is early connected with the anlage of the stapes, 

 and the connected portion becomes the cms longum. Between this and 

 the stapes an articulation develops. 



The stapes and Reichert's cartilage are derived from the second 

 branchial arch (Fig. 387). The mesenchymal anlage of the stapes is 

 perforated by the stapedial artery, and its cartilaginous anlage is ring- 

 shaped. This form persists until the middle of the third month, when 

 it assumes its adult structure and the stapedial artery disappears. 



The muscle of the malleus, the tensor tympani, is derived from the first branchial arch; 

 the stapedial muscle from the second arch. The further fact that these muscles are inner- 

 vated by the trigeminal and facial nerves, which are the nerves of the first and second arches 

 respectively, points toward a similar origin for the ear ossicles. These relations strengthen 

 the general belief in a branchial arch origin. 



