Nerves, Spinal Coed, Brain and Other Ganglia 81 



gray matter of the locus perforatus posticus, and the tegmenta of the 

 crura cerebri. The ' frontal wall ' or upper boundary is the lamina 

 cinerea, and the ' roof ' or dorsal boundary, an epithelial layer continuous 

 with the epithelium lining the chamber. At the ' anterior ' (or upper) 

 end the third ventricle communicates with the first and second ventricles, 

 and at the ' posterior ' end it communicates through the aqueduct of 

 Sylvius (aqueductus cerebri), a very small tubular aperture through the 

 mid-brain, with the fourth ventricle (ventriculus quartus). 



The fourth ventricle is the cavity under the cerebellum (the hind- 

 brain), its ventral wall being the medulla, and its dorsal wall, a thin 

 epithelial membrane. 



The aqueduct of Sylvius and the third and fourth ventricles (except 

 for the dorsal walls or ' roofs ' of these latter) are surrounded by layers 

 of gray matter, forming the nuclei of the third, fourth, sixth, and twelfth 

 cranial nerves, and the secondary nuclei of the eighth nerve, and of 

 the afferent portions of the ninth, tenth and eleventh nerves. The nuclei 

 for the fifth nerve and the efferent portions of the ninth, tenth and 

 eleventh lie in the immediate neighborhood. 



GROSS DETAILS OF THE BRAIN. 



Certain details of the external appearance of the medulla, mid-brain 

 and fore-brain are of importance both as indicating structural details and 

 as points of topographical reference. 



On the ventral side of the medulla the olives (olivae), the pyramids 

 (pyramis), and the decussation of the pyramids (decussatio pyra- 

 midum), are noticeable [Fig. 68]. On the dorsal side the cuneate 

 tubercules, the clava, the funiculus gracilis, and the funiculus cune> 

 atus appear [Fig. 69A]. Above the medulla the pons appears on the ven- 

 tral side, and the cerebellum on the dorsal, with middle and superior 

 cerebellar peduncles (brachia pontis and brachia conjunctiva) joining 

 it to the ends of the pons and to the brain stem behind the pons [Figs. 

 68 and 69]. 



Conspicuous on the floor and side walls of the fourth ventricle (between 

 the stem and the cerebellum) are the striae acusticae (or stria medu- 

 lares) crossing the area acustica, the eminentia teres (colliculus 

 facialis) and the beginning of the Sylvian aqueduct (Aqueductus 

 cerebri) [Fig. 69 A]. 



The chief features of the mid-brain are the corpora quadrigemina on 

 the dorsal surface (two pair, ' inferior ' and ' superior ' [Fig. 69A] and the 

 pineal body (corpus pineale) above them [Fig. 66]. The ventral aspect 

 shows the crura (singular crus) cerebri (fedunculi cerebri) with the 



