36 THE BRAIN OF THE TIGER SALAMANDER 



primarily with synergic activation of large masses of muscle, notably those con- 

 cerned with locomotion and conjugate movements of the eyes. Control of the 

 movements directly involved in seizing and swallowing food is believed to be 

 chiefly in the isthmic tegmentum, and this apparatus matures later n ontogeny. 



The chief synaptic connections of the great motor pool of the peduncle are in the 

 intermediate, deep, and periventricular layers of neuropil. External to these is a 

 ventrolateral band of dense neuropil extending from the root of the III nerve for- 

 ward along the entire length of the peduncle. In former papers this has been termed 

 "area lateralis tegmenti" and "nucleus ectomamillaris," but both these names now 

 seem to me inappropriate ('4'-2, p. 233). This band receives at the anterior end all 

 the terminals of the large basal optic tract (figs. U, 94) and, at the posterior end, 

 terminals of the secondary and tertiary ascending ^'isceral sensory and gustatory 

 tracts (figs. 8, 23). The terminals of these two systems of fibers are intimately inter- 

 laced, and among them are dendritic arborizations of tlie underlying cells of the 

 peduncle into which optic, olfactory, and visceral systems of nervous impulses con- 

 verge and through which the combined effect is transmitted in the outgoing motor 

 pathways. These seem to be the primary components of this neuropil, and to them 

 are added axonic terminals from a wide variety of sources, the most notal)le of which 

 are sketched in figure '23 (compare Necturus, '34, p. 103 and fig. 4). 



The peduncular dendrites, which arborize within this neuropil, include some 

 from the nucleus of the oculomotor nerve (fig. 24; '42, p. 275), so that here fibers of 

 the basal optic tract coming directly from the retina may synapse with peripheral 

 motor neurons — one of the rare examples of a two-neuron arc witli only one synapse 

 between the peripheral receptor and the effector, though even here there are at 

 least two additional synapses in this arc within the retina. Other peduncular 

 neurons may transmit retinal excitations downward through the ventral tegmental 

 fascicles to all lower motor fields. 



At its anterior end this neuropil is connected by fibers conducting in both direc- 

 tions with the dorsal (mamillary) part of tlie hypothalamus, as in Necturus 

 ('346, fig. 3), and also with its ventral (infundibular) part (fig. 23; "42, pp. 226, 227). 

 This is an extension of the visceral-gustatory tract into the hypothalamus, and it 

 may be that some of the visceral fibers pass without interruption through the 

 peduncular neuropil to the hypothalamus, though this has not been satisfactorily 

 demonstrated. In some fishes such a direct connection is evident and large; in 

 mammals the course of the ascending visceral-gustatory path is still uncertain. 



Other connections of this neuropil, as shown in figure 23, include terminals of the 

 olfacto-peduncular tract from the anterior olfactory nucleus, probably the nervus 

 terminalis (observed in Necturus) fibers from the tectum, pretectal nucleus, dorsal 

 and ventral thalamus, and terminals of the fasciculus retroflexus (p. 202 and fig. 20). 

 These terminals of fibers from surprisingly diverse sources are all closely interwoven 

 with one anotlier and with the terminal dendrites of peduncular neurons, a unique 

 arrangement occurring, so far as known, only in Amphibia. I have indulged in the 

 following speculations upon its possible physiological significance. 



In the first place, it is clear that this curious tissue is either an undifferentiated 

 primordium of a number of structures which are separately differentiated in more 

 specialized brains, or else it is a retrograde fusion of several such structures. The 

 former supposition seems more probable, for the phylogenetic history of one of its 

 components is easily read ('25, pp. 443-49). In Necturus there are no cell bodies 

 directly associated with this neuropil; in Amblystoma a few cells have migrated 

 out of the gray layer to its border (fig. 24) ; and in the frog the optic component of the 

 neuropil is separate and surrounded by a spherical shell of cell bodies, which is a true 

 basal optic nucleus (Gaupp, '99, p. 54, fig. 19). This nucleus attains large size and 



