THE OR'GANS OF SPECIAL SENSE. 



599 



(trista ampullaris) and in the saccule and utricle (macula 'acustica) (see 

 pC 596). The nerve itself becomes divided into certain branches, as indicated 

 in the following table (Streeter). The peripheral terminations of the various 

 branches are indicated in parentheses. Compare with Fig. 512, I, m, n, and 



Fig- $13, a > b > c - 



N. vestibularis' 



f ramus ampul, sup. (crista ampul.) 

 pars superior ' j ramus ampiil. ext. (crista ampul.) 

 [ ramus recess, utric. (macula acust.) 



rf ramus saccule (macula acust.) 

 pars inferior \ ramus ampul. . (crista ampul.) 



The vestibular ganglion cells; instead of remaining in a compact mass, come 

 to form two fairly distinct masses in the course of the nerve (Fig. 513, a, b, c). 

 One of these apparently is connected with the pars inferior, the other with the 

 pars superior. 



The cochlear ganglion cells at an early stage become closely associated with 

 the developing cochlear duct and, as the latter forms a spiral, are carried 1 along 

 with it. They thus come to form an elongated group of cells extending through- 

 out the entire length of the cochlea (whence the name, spiral ganglion) (Fig. 

 512, j-n; Fig. 513, a-.c). Consequently, the peripheral processes of these cells, 

 which terminate in connection with the hair cells of the organ of Corti, are com- 

 paratively short. The central processes are naturally longer and form the 

 cochlear nerve root which is twisted like a rope in part of its course (Fig. 513, c) . 



The Middle Ear. — The cavity of the middle ear develops from the upper 

 (dorsal) part of the first inner branchial groove. The epithelial lining of the 

 cavity is thus of course derived from entoderm; and the other structures 

 (auditory ossicles, etc.) from the adjacent mesoderm. 



It has been stated elsewhere that the mesoderm in the first and second 

 branchial arches gives rise, among other things, to certain skeletal elements. 

 In the first arch there develops a rod of cartilage, known as' Meckel's cartilage, 

 Which extends from the symphysis of the lower jaws to the region of the upper 

 part of the first inner branchial groove (p. 200; Figs. 174, 177, 180). The 

 proximal end of the cartilage becomes constricted to form two masses which 

 constitute the anlagen of the malleus and incus (Figs. 173 and 174). In' the 

 second arch there develops a rod of cartilage which forms the lesser horri of the 

 hyoid bone, the stylohyoid ligament, and the styloid process (Figs. 174, 177, 

 i'8o) . In close relation to the dorsal end of the styloid process, in the mesoderm 

 destined to give rise to the periotic capsule, a mass of cartilage appears which 

 is destined to give rise to the stapes (except the base?). It has not been fully 

 determined whether the stapes is actually a derivative of the cartilage of the 

 second arch or of the mesenchyme near its dorsal end. It has been suggested 



