THE MAMMALIAN NERVOUS SYSTEM 51 



the dorsal, lateral, and ventral surfaces of the medulla oblongata. 

 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 sculptur- 

 ing in the floor of the fourth ventricle with that of the sheep and 

 of the dogfish (Section 23). The following references will aid 

 in the interpretation of the floor of the fourth ventricle : Cun- 

 ningham ('15), Figs. 477, 479, 482, pp. 542, 544, 550 respect- 

 ively; Morris ('14), Figs. 631, 640, 647, pp. 802, 814, 821 re- 

 spectively; Piersol ('13), Figs. 918, 948, 949, pp. 1067, 1097, 1098 

 respectively; Quain ('09), Fig. 150, p. 136; Rauber-Kopsch ('07), 

 Figs. 543, 544, pp. 465, 466; Reference Handbook of the Medical 

 Sciences, article Brain Anatomy, Vol. II, pp. 283-285; Sheldon 

 ('18), Chap. VII; Spalteholz ('09), Fig. 698, p. 630; Toldt ('04), 

 Figs. 1178-1181, pp. 768, 769; Weed ('14a). 



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 in- 

 terrupted in places in man and the sheep. In the latter cases 

 this column (the eminentia medialis, BNA) includes the tri- 

 gonum hypoglossi, the funiculus teres (see Fig. 1 1) 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 deeper. 

 In mammals some of these nuclei appear on the ventricular 

 floor, but most of them lie too deep to be located by surface 

 study. The ala cinerea (or trigonum vagi) is an eminence which 

 marks the position of the dorsal motor nucleus of the vagus 

 (general visceral efferent). The special visceral nucleus of the 

 IX and X nerves (nucleus ambiguus) is not visible from the sur- 

 face. Similarly the visceral motor nuclei of the VII and V 

 nerves lie too deep to be marked on the floor of the ventricle,. 



