ANATOMY OF A 17.8 MM. HUMAN EMBRYO 57 



laterally between the vein and the oculomotor nerve to enter the 

 anlage of the lateral rectus muscle. 



A^. facialis. The motor root of the facial nerve (plate 3) issues 

 from the ventrolateral wall of the metencephalon {Meten.) and 

 passes to the geniculate gangUon {G.gn.) by which its fibers are 

 enveloped for a short distance. The sensory root (pars inter- 

 media, N.int.) arises from the geniculate ganglion and enters the 

 metencephalon immediately caudo-lateral to the motor root. 

 The mixed facial trunk which emerges at the caudo-ventral 

 border of the geniculate ganglion {G.gn.) represents the post- 

 trematic ramus of the first pharyngeal pouch. It takes a caudal 

 and lateral direction. Having given off the chorda tympani 

 (N.ch.tymp., pi. 3, ch.ty., pi. 2) it ends in several small branches. 



The chorda tympani nerve leaves the facial trunk at an acute 

 angle and extends cephalad, ventral to the auditory pouch, to 

 join the lingual branch of the mandibular nerve as described 

 above. From the geniculate ganglion a small nerve (N.pet.s.m.) 

 extends at first ventrally, and then bends sharply cephalad 

 medial to the mandibular nerve and the tubo-tympanal ridge. 

 It is the great superficial petrosal nerve. As it bends cephalad 

 it gives off eaudally a short twig (anastomotic with the tympanic 

 plexus) to meet the tympanic branch (N.tym.) of the glosso- 

 pharyngeal nerve: The nerve arising from this junction (the 

 anlage of the tympanic plexus) is the small superficial petrosal. 



N. acusticus. The ganglion acusticum and otic vesicle are 

 shown in plate 3. In plates 2 and 5 the otic vesicle has been 

 removed. The ganglion acusticum is partially differentiated into 

 cochlear and vestibular divisions. The vestibular part {G.ves.) 

 lies cephalad of the utriculo-saccular division of the otic vesicle 

 and lateral to the cochlear part of the ganglion, so that the 

 latter is mostly hidden from view in plate 3. Four nerve trunks 

 proceed from the vestibular division, a cephalic which divides 

 into two rami to supply the ampullae of the superior and lateral 

 semicircular canals ; a caudal which passes medial to the utriculo- 

 saccular division of the otic vesicle to supply the posterior semi- 

 circular canal; and two intermediate branches, the cephalic of 

 which extends to the developing utricle, the caudal to the saccule. 



