386 THE PERIPHERAL NERVOUS SYSTEM 



cochlea and proceeds toward the apex. The internal spiral sulcus is formed by 

 the degeneration and metamorphosis of the cells of the inner epithelial thicken- 

 ing which He between the labium vestibulare and the spiral organ (Fig. 367 B, C). 

 These cells become cuboidal, or flat, and line the spiral sulcus, while the membrana 

 tectoria loses its attachment to them. The membrana tectoria becomes thickest 

 over the spiral organ and in full term fetuses is still attached to its outer cells 

 (Fig. 367 C). 



According to Hardesty (Amer. Jour. Anat., vol. 8) the membrana tectoria is not devel- 

 oped from the cells of the spiral organ and therefore is not attached to it at any time. From what 

 is known of the development of the spiral organ in human embryos, it follows the same lines 

 of development as described for the pig. It must develop relatively late, however, for in the 

 cochlear duct of a new-born child figured by Krause the spiral sulcus and the spiral tunnel are not 

 yet present. 



The mesenchyme surrounding the labyrinth is differentiated into a fibrous 

 membrane directly surrounding the epithelium, and into the perichondrium of 

 the cartilage which develops about the whole internal ear. Between these two 

 is a more open mucous tissue which largely disappears, leaving the perilymph 

 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 356 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. During the latter part of the second month, in embryos of 24 mm., 

 the wall of the tympanic cavity is constricted to form the tubo-tympanic (Eus- 

 tachian) canal. This tube lengthens and its lumen becomes slit-like during 

 the fourth month. The tympanic cavity is surrounded by loose areolar connec- 



