204 THE BRAIN OF THE TIGER SALAMANDER 



horizontal glomeruli seem to be especially adapted for summation of 

 the discharge from the nucleus. If one of these neurons is experienc- 

 ing sul)liminal activation, a collateral discharge from an active 

 neuron may impinge upon it in a horizontal glomerulus, and the sum 

 of the two excitations may be sufficient to fire the element. In a simi- 

 lar way, terminals of intrinsic fibers in the vertical glomeruli may re- 

 inforce the action of the extrinsic fibers. The glomerular activities 

 may reinforce (or inhibit) those of the spiral, or conceivably they 

 may act independently if the habenular component is inactive. 



The observed structure seems to favor the supposition that, what- 

 ever may be the functions performed by this complex, they are of 

 generalized type. The apparatus which patterns the specific reflexes, 

 both somatic and visceral, is located elsewhere in the brain stem. 

 The intrinsic structure of the interpeduncular field seems to preclude 

 any well-defined local representation of different functions within it. 

 The nucleus apparently acts as a unit, and any functional specificity 

 that may exist would be in terms of differential excitation at the 

 various forms of synaptic junction or of the functions of the motor 

 fields into which its efterent fibers discharge. The ventral bulbar tract 

 descends in the primary motor column of Coghill, and this suggests 

 that it is in some way concerned with mass movements of the mus- 

 culature of the trunk and limbs. The tegmental and dorsal bulbar 

 efferent tracts form an aftatomical unit, and they probably are simi- 

 larly related physiologically. These fibers discharge into a motor 

 field within which the reflexes of the head are organized, notably 

 those concerned with feeding. The interpeduncular complex seems to 

 be so organized as to act as a whole without intrinsic localization of 

 function. What part, then, does this complex play in patterned be- 

 havior? 



In the first place, it is a noteworthy feature of the interpeduncular 

 system that it is connected "in parallel," as the electrician would say, 

 with the major systems of fore-and-aft conduction of the brain stem. 

 The olfactory, optic, and lemniscus systems on the afferent side and 

 the strio-pedunculo-bulbar systems on the efferent side, with their 

 related centers of adjustment, seem to provide the apparatus which 

 patterns overt behavior. The interpedunculo-habenular system is in 

 addition to this apparatus and in some way ancillary to it. 



In noncorticated vertebrates the activators of the motor zone re- 

 ceive their nervous excitations mainly from three regions which are 



