318 THE RHOMBENCEPHALON. 



nucleus alas cinereae, the nucleus tractus solitarii and the nu- 

 cleus ambiguus, three nearly parallel columns of cell-bodies a half- 

 inch in length, are in part covered by the ala cinerea (Fig. 96). 



The superior and inferior foveae and the sulcus limitans fossae 

 rhomboideae in which they are located represent the lateral sul- 

 cus, which in the embryo separates the ventral from the dorsal zone 

 of the rhombencephalon. 



Area Postrema. Below the ala cinerea and between it and the 

 taenia ventriculi quarti, there is a small fusiform strip of the ven- 

 tricular floor which Retzius has called the area postrema. An 

 oblique stria separates it from the base of the ala cinerea. 



The area acustica occupies the lateral angle of the ventricular 

 floor (Fig. 86). It is partly in the superior triangle, but chiefly 

 in the inferior. Inclosed between the ala cinerea and the taenia, 

 its apex points downward, and its base looks upward and is crossed 

 by the medullary striae. A slight eminence, the tuberculum 

 acusticum, makes the lateral angle of the acustic area most prom- 

 inent. Beneath the acustic area are the vestibular nuclei of the 

 auditory nerve; also the lateral part of the cochlear nucleus, which 

 is found in the acustic tubercle (Fig. 96). 



ORIGIN OF CEREBRAL NERVES. 



According to Sommering, there are twelve pairs of cerebral 

 nerves (nervi cerebrales), but to this must be added the nervus 

 intermedius (pars intermedia) which, though associated with 

 the facial nerve in the facial canal, is of itself a true mixed nerve. 

 The first, second and eighth cerebral nerves are purely sensory; 

 six of them, the third, fourth, sixth, seventh, eleventh and twelfth, 

 are purely motor; while the. fifth, the intermediate, the ninth and 

 tenth are mixed nerves and contain both efferent and afferent 

 fibers. 



Cerebral Nerves, Nervi Cerebrales (Figs. 86 and 96). 



1. Olfactory (nn. olfactorii) special sense of smell. 



2. Optic (n. opticus) special sense of sight. 



3. Oculomotor (n. oculomotorius) motor. 



4. Trochlear (n. trochlearis) motor. 



