THE MAMMALIAN NERVOUS SYSTEM 53 



Note that immediately behind the cochlear nucleus the 

 tsenia turns abruptly to the lateral margin of the medulla ob- 

 longata, thus forming the lower boundary of a wide expansion 

 of the fourth ventricle, the lateral recess, which extends dorsally 

 over the cochlear nucleus. 



51. The fossa rhomboidea. Next remove the membranous 

 roof of the fourth ventricle of the sheep and examine carefully 

 the dorsal, lateral, and ventral surfaces of the medulla ob- 

 longata. Identify all the parts named on Figs. 11 and 12. 



In the human brain also remove the membranous roof of the 

 fourth ventricle and compare its external form and the sculp- 

 turing in the floor of the fourth ventricle with that of the 

 sheep and of the dogfish (Section 23). The following refer- 

 ences will aid in the interpretation of the floor of the fourth 

 ventricle: Cunningham ('15), Figs. 477, 479, 482, pp. 542, 544, 

 550 respectively; Morris ('14), Figs. 631, 640, 647, pp. 802, 814, 

 821 respectively; Piersol ('16), Figs. 918, 948, 949, pp. 1067, 

 1097, 1098 respectively; Quain ('09), Fig. 150, p. 136; Rauber- 

 Kopsch ('12), Figs. 113, 114, pp. 99, 100; Reference Handbook 

 of the Medical Sciences, article Brain Anatomy, Vol. II, pp. 

 283-285; Sobotta ('11), Figs. 659-661; Spalteholz ('09), Fig. 

 698, p. 630; Toldt ('04), Figs. 1178-1181, pp. 768, 769; Weed 

 C14a). 



52. In all of these cases (dogfish, sheep, embryonic and 

 adult human) there is a deep median sulcus in the floor of the 

 fourth ventricle. Laterally of this is a slender somatic motor 

 column, seen as a continuous ventricular ridge in the dogfish, 

 but interrupted in places in man and the sheep. In the latter 

 cases this column (the eminentia medialis, B N A) includes the 

 trigonum hypoglossi, the funiculus teres (see Fig. 11) and the col- 

 liculus facialis which is sometimes called the eminentia abdu- 

 centis and below which are the VI nucleus and root-fibers of the 

 VII nerve. In the midbrain the III and IV nuclei also belong 

 in this column, and throughout its length the longitudinal 

 medial fasciculus runs immediately below the ventricular floor 

 (see Section 92). 



53. Visceral motor column. In the dogfish the visceral 

 motor nuclei of the cranial nerves form a longitudinal column 

 lying laterally of the somatic motor column and somewhat 



