THE ANATOMY OF TEN TO TWELVE MM. PIG EMBRYOS 1 23 



hypophyseal lobe. (3) The mesencephalon is undivided but its cavity becomes the 

 cerebral aqueduct leading caudally into the fourth ventricle. (4) The melcncephalon 

 is separated from the mesencephalon by a constriction, the isthmus. Dorso- 

 lateral!)- it becomes the cerebellum, ventrally the pons. (5) The elongated 

 myelencephalon is roofed over by a thin non-nervous ependymal layer. Its ventro- 

 lateral wall is thickened and still gives internal indication of the ncuromcrcs. 

 The cavity of the metencephalon and myelencephalon is the fourth ventricle. 



Cerebral Nerves. Of the twelve cerebral nerves all but the first (olfactory) 

 and sixth (abducens) are represented in Fig. 115. For a detailed description 



Accessory gang. 1 

 Accessory gang. 2 



Myelencephalon 



C erv. gajig. 2 





Gang, nodosum 



X. 12 



Fig. ii6. — Dissection of the head of a 15 nun. pig embryo from the right side to show the accessory 

 vagus ganglia with peripheral roots passing to the hypoglossal nerve. 



of these nerves see Chapter XII. (2) The optic nerve is represented by the optic 

 stalk cut through in Fig. 115. (3) The oculomotor, a motor nerve to the eye 

 muscles, takes origin from the ventro-lateral wall of the mesencephalon. (4) 

 The trochlear nerve fibers, motor, to the superior oblique muscle of the eye, arise 

 from the ventral wall of the mesencephalon, turn dorsad and cross at the isthmus, 

 thus emerging on the opposite side. From the myelencephalon arise in order 

 (5) the trigeminal nerve, mixed, with its semilunar ganglion and three branches, 

 the ophthalmic, maxillary, and mandibular; (6) the n. abducens. motor, from the 

 ventral wall to the external rectus muscle of the eye; (7) the u. facialis, mixed, 



