434 C. JUDSON HEltRICK 



in the caudal end of the pars ventralis thalami from which the 

 tractus thalamo-spinalis arises. This is the tractus thalamo- 

 peduncularis. An unmedullated tract passes from the rostral 

 end of the pars dorsalis thalami into the hypothalamus, the trac- 

 tus thalamo-mammillaris. A much larger tract of unmedullated 

 fibers passes from the middle and rostral part of the pars dorsalis 

 thalami to the lateral forebrain tract, and thence to the cerebral 

 hemisphere. This is the precursor of the tractus thalamo-cor- 

 ticalis of mammals and may receive the same name here, though 

 its termination in the hemisphere cannot be given more precisely 

 than to say that it reaches its lateral parts. 



The pars dorsalis thalami is evidently a receptive center for 

 somatic (exteroceptive) impulses. These come from the spinal 

 cord and oblongata by way of the lemniscus and from the tectum 

 mesencephali by fibers associated with the lemniscus. Optic 

 impulses are also received directly by collaterals from the tractus 

 opticus ending in the corpus geniculatum laterale. This con- 

 nection has been frequently described in fishes, where the genic- 

 ulate body is very small. Johnston mentions it in Acipenser 

 ('01, p. 57) though the lateral geniculate body is not here separable 

 from the nucleus anterior of the pars dorsalis thalami, a condition 

 which prevails to some extent in Amphibia also, where in young 

 larvae the optic connection is confined to the posterior part of the 

 thalamus. In adult Amblystoma the lateral geniculate body 

 covers more than two-thirds of the pars dorsalis thalami and in 

 the frog it has extended forward so as to cover almost its whole 

 lateral surface. I have observed collaterals from the tractus 

 opticus ending in relation with these cells. From this it follows 

 that the pars dorsalis thalami receives optic fibers along with 

 those of the other exteroceptive senses — tactile, somaesthetic and 

 acoustic — received from the lemniscus, and that all of these sen- 

 sory elements may be represented in the sensory radiations which 

 enter the telencephalon from this part by way of the lateral fore- 

 brain tract. 



From the preceding description it appears that in Amblystoma 

 the massive lateral wall of the diencephalon on each side is divided 

 longitudinally into four parts which bear simple and obvious rela- 



