62 THE BRAIN OF THE TIGER SALAMANDER 



with the localized nuclei of higher brains can be recognized. Farther 

 forward in the isthmus and peduncle the tegmental tissue of co- 

 ordination is much increased in amount and somewhat more dif- 

 ferentiated. In some of my former papers (e.g., '30, p. 76) the term 

 "nucleus motorius tegmenti" was used loosely (and inaccurately) 

 to include a tegmental zone defined topographically. This seemed to 

 be justified in the case of Necturus by the lack of localization of the 

 large motor elements which characterize this region; but this justifi- 

 cation is inadequate, both factually and morphologically — see the 

 discussion by Ariens Kappers, Huber, and Crosby ('36, pp. 653, 666). 



It is obvious that most of the tissue of the motor zone is concerned 

 with co-ordination of the action of the peripheral elements, so that 

 synergic groups of muscles are activated in appropriate sequence; 

 but, with the technic available, it has not been possible to analyze 

 this complex so as to reveal the mechanism employed. In the medulla 

 oblongata this organization is chiefly for local control of bulbar and 

 spinal reflexes, the intermediate zone participating. In the isthmus 

 and peduncle the number of peripheral elements is relatively small 

 and the co-ordinating apparatus larger, giving these areas control 

 over all motor fields spinal ward of them. This intrinsic motor ap- 

 paratus is supplemented by a segregated band of correlating tissue in 

 the intermediate zone, the subtectal dorsal tegmentum. In mammals 

 both these zones are further specialized into separate nuclei distrib- 

 uted in the tegmentum. 



The primary patterns of somatic movements are predetermined by 

 the course of central differentiation within the motor and inter- 

 mediate zones in premotile stages of development. After connection 

 with the peripheral musculature is established, each of these muscles 

 seems to exert some sort of distinctive reciprocal influence upon that 

 motor field of the central nervous system from which its innervation 

 is derived. The nature of this influence is unknown, but its reality is 

 well attested by experiments of Paul Weiss ('36, '41) and colleagues 

 upon "myotypic response" and "modulation." 



In later stages the primary motor patterns may be modified, or 

 "inflected," by sensory experience and practice. Influence of use or 

 some other functional factors seem to be essential for maintenance of 

 motor efficiency, as graphically shown by Detwiler's observations 

 ('45, p. 115; '46) on the behavior of decerebrate larvae of Amblys- 

 toma (to which further reference is made on p. 118). In young 

 larvae of stage 37, swimming movements may be perfectly executed 



