142 THE BRAIN OF THE TIGER SALAMANDER 



deep neuropil diffusely and here are in relation with terminals of the 

 optic and lemniscus systems; or ('2) they may descend into the 

 reticular formation of the medulla oblongata, where they act di- 

 rectly upon the motor nuclei and also upon the apparatus of bulbar 

 neuromotor co-ordination (including the cells of Mauthner). These 

 descending branches are accompanied by fibers of the spinal V root 

 and by other fibers from the tectum and tegmentum, which end in 

 the same field of the reticular formation (fig. 13, tr.t.h.p. and tr.teg.b.). 

 In larvae of early feeding stages, thick uncrossed fibers, which de- 

 scend from the tectum and subtectal areas into the bulbar reticular 

 formation, are especially clearly seen, and also others which take 

 similar courses after decussation in the ventral commissure. Some of 

 these fibers arise from neurons of the isthmus, which are in synaptic 

 connection with terminals of the secondary visceral-gustatory tract. 

 These connections of mesencephalic V fibers seem well adapted to 

 facilitate the feeding reactions, a conclusion which is supported by 

 observations on the cat by Corbin ('40) and the literature which he 

 cites. In Ambly stoma the field of reticular formation within which 

 the movements of the mouth and pharynx are organized receives the 

 descending mesencephalic V fibers, collaterals of V fibers, and fibers 

 of correlation from the nucleus of the f. solitarius, isthmic visceral- 

 gustatory nucleus, tectum, and the underlying dorsal tegmentum. 



PROPRIOCEPTIVE SYSTEMS AND CEREBELLUM 



Control of the course of muscular movement in process is insured 

 by a variety of sensory end-organs, including those in muscles, 

 tendons, joints, and the overlying skin. At the beginning of motility 

 in the embryogenesis of Amblystoma a single peripheral sensory 

 element (the transitory Rohon-Beard cells) may perform both extero- 

 ceptive and proprioceptive functions (Coghill, '14, Paper I, p. 199), 

 and this may be true of some spinal ganglion .cells in the adult, 

 though here special proprioceptive apparatus also is provided. The 

 Rohon-Beard cells are believed to be derived from a portion of the 

 neural crest which is incorporated within the neural tube; and the 

 mesencephalic nucleus of the trigeminus, as just described, has a 

 similar origin. The latter cells survive in the adults of all vertebrates, 

 in the service apparently of co-ordination of movements involved in 

 the feeding reactions. 



In the head the membranous labyrinth is the dominant organ of 

 this system, with participation of nerves of cutaneous and deep 



