1 8 NERVOUS SYSTEM OF VERTEBRATES. 



or the lobus facialis, owing to the relation which it bears to the 

 vagus and facialis nerves. The ridge in cyclostome fishes is small, 

 in selachians of moderate size, in ganoids (Fig. 3) relatively large 

 and in some bony fishes enormous (Fig. 4, L. vg.). In some 

 bony fishes, as in Carpiodes whose brain is shown in Figure 4, 

 it is the caudal part of this ridge which is greatly enlarged. In 

 other cases the cephalic portion is so large that it overtops all 

 other parts of the medulla oblongata, and the ridges of the two 

 sides may fuse in the median plane into a single mass, the lobus 

 impar. As will be seen later (p. 1 59) this ridge is the place of ending 

 of the sensory fibers of the visceral surfaces in the head and of 

 the organs of the sense of taste. The ridge would therefore 

 be appropriately named the lobus visceralis. In amphibians, 

 reptiles, birds and mammals the part of the brain corresponding 

 to this lobe, although recognizable microscopically, is much 

 smaller than in fishes and is not to be seen as a projecting ridge. 

 The name visceral sensory column will be used for this whole 

 structure. When it forms a projecting ridge the term lobus vis- 

 ceralis may be used for the whole, or lobus vagi and lobus facialis 

 for its two parts. 



, The upper part of the lateral wall of the myelencephalon is 

 formed by a thick ridge of gray matter which is continuous caudad 

 with the dorsal horn of the spinal cord and cephalad with the 

 cerebellum. This ridge is more or less prominent in all lower 

 vertebrates (see Figs. 2, 3) and its equivalent in mammals is to 

 be found in the acustic nuclei and the restiform bodies. 

 The ridge has been known as the tuberculum acusticum on 

 account of its relation to the eighth cranial or auditory nerve. 

 Since the ridge is the place of ending of all the cutaneous nerves 

 of the head and since the term tuberculum acusticum is applied 

 to a restricted portion of this region in mammals, it would be better 

 to call this the somatic sensory column. In selachians and ganoids 

 and to a less extent in cyclostome fishes a short part of this lobe 

 projects prominently dorsad just behind the cerebellum. Since 

 this is related wholly to nerves which supply the so-called lateral 

 line organs, it is called the lobus lineae lateralis. 



Most of the cranial nerves are connected with the myelen- 



