126 THE BRAIN OF THE TIGER SALAMANDER 



neuropil of the reticular formation is less well developed than in the 

 medulla oblongata. 



THE MOTOR ZONE 



The motor zone of the cord is continuous with that of the medulla 

 oblongata, with no recognizable boundary; and throughout these 

 regions the peripheral motor neurons are mingled with co-ordinating 

 neurons, and they often resemble the latter so closely that they can- 

 not be distinguished unless their axons are seen to enter the motor 

 roots. 



THE SPINAL NERVES 



The upper spinal nerves are modified. The first usually has no 

 sensory root or ganglion. The arrangement of the motor roots of the 

 first and second pairs is exceedingly variable. The nervus accessorius 

 and the nervus hypoglossus are not separately differentiated. The 

 primordia of the former are represented in the lowermost vagal root- 

 lets, which emerge from the lateral aspect of the medulla oblongata; 

 and the primordia of the latter are in the first and second spinal 

 nerves, the ventral roots of which emerge at the ventral surface. 

 In one specimen the lowest vagal root was seen to emerge at the level 

 of the calamus scriptorius, but usually it is at a more rostral level. 

 The lowest root of the first spinal nerve usually emerges in the region 

 of the calamus, and the first root of this nerve at Variable distances 

 rostrally. 



The first spinal nerve of Salamandra as described by Francis 

 ('34, p. 159) agrees with that of Amblystoma. On page 161 he quotes 

 Goodrich, who has shown that the urodele hypoglossus innervates 

 muscles derived from the ventral outgrowths of the second, third, 

 and fourth myotomes and that "the hypoglossus of Amphibia and 

 Amniota may certainly be considered as homologous, although not 

 necessarily composed of the same segmental nerves." 



The neurons of the ventral horns of gray include tegmental ele- 

 ments, and motor cells which give rise to peripheral fibers ('44&, 

 fig. 10; van Gehuchten, '97). Both types may have very large, much- 

 branched dendrites, which in the larva ramify through almost the 

 entire cross-sectional area of the cord and may cross to the other side 

 in the ventral commissure. In the adult animal, internuncial connec- 

 tions within the dorsal and ventral zones and between these provide 

 for co-ordinated spinal reflexes; but all movements of the trunk and 

 limbs are subject to further control from bulbar and other higher 

 centers. The details of the structural apparatus by means of which 



