662 



NERVOUS SYSTEM OF THE HORSE 



The Fourth Ventricle 



The fourth ventricle (\'entrieuliis quartus) is the cavity of the rhombencepha- 

 lon; it communicates with the central canal of the spinal cord behind, and through 

 the aqueduct with the third ventricle in front. It is somewhat rhomboid in out- 

 line, elongated from before backward and narrowest behind. It is lined completely 

 by an epithelium (Ependyma) and contains a small amount of fluid. 



Its floor (Fossa rhomboidea) is formed by the medulla and pons and is marked 

 by three longitudinal furrows which converge behind. It is widest and deepest 

 a little in front of its middle. The posterior part narrows to a point at the opening 

 of the central canal, and on account of its appearance in man it has been termed the 



_. -- Columns of fornix 



Corpus striatum 



Chorioid plexus of 

 lateral ventricle 



Stria terminnlis 



Third ventricle 



Thalamus 



Fimbria 



Corpus genie, externum 



Pineal body 



Corpus genie, internum 



Corpora quadrigemina 



■Anterior peduncle of cerebellum 

 Middle peduncle of cerebellum 



Median sulcus 



Limiting sulcus 

 ~ Corpus restiforme 

 ■ — Tcenia 



- Obex 



_ T uherculum cinereum 



— Dorso-lateral sulcus 



J Median sulcus 



Facial eminence '' 



Fig. 504. — Brain Stem and Basal Gan(jlia of Horse, Dorsal View. 

 The cranial nerve-roots are designatetl by Roman numerals. 



calamus scriptorius. The median sulcus (Sulcus medianus) extends the entire 

 length of the floor and is deei)est toward the ends. The limiting sulci (Sulci 

 limitantes) begin on either side of the opening of the central canal and extend for- 

 ward as the lateral limits of the rhomboid fossa. Just beyond the middle of the 

 fossa they expand into a shallow depression, the anterior fovea (Fovea nasalis). 

 On eithei- side of the median sulcus and margined by the limiting sulcus is a slightly 

 rounded column, the eminentia medialis. Opposite the fovea this presents an 

 elongated prominence, the coUiculus facialis, so named because it overlies the bend 

 formed by the fibers of origin of the facial nerve. External to the limiting sulcus 

 is a long fusiform elevation, the area acustica, from which a band of fibers (Striae 

 acustica^) winds over the anterior end of the restiform body to the superficial 

 origin of the cochlear nerve. 



