218 J. B. JOHNSTON, 
elements in the nucleus of the fasciculus longitudinalis. Thus the 
corpus mammillare not only carries out reflexes but serves certain 
functions of coördination as well. 
Decussations occur in the hypothalamus in two regions, im- 
mediately behind the chiasma and in the corpus mammillare. De- 
cussations or commissures behind the chiasma occur in all Ver- 
tebrates but there appear to be several different tracts crossing 
here which are not homologous throughout the Vertebrate series. 
In Mammals there have long been known two commissures in this 
position, the commissures of MEYNERT and of GUDDEN, the former 
dorsal to the latter. Attempts have been made to homologize the 
decussations found in various lower Vertebrates with these com- 
missures. The destinations of the fibres crossing in these com- 
missures in Mammals are not well known. GUDDEN traced the 
commissure of MEYNERT into the midst of the pes pedunculi. KôL- 
LIKER gives nothing further regarding it and EDINGER (96b) says: 
“Ihr Anfang und Ende sind nicht genügend sicher bekannt.” The 
fibres of GUDDEN’s commissure have been traced by various authors 
to the corpus geniculatum mediale and externum, the thalamus and 
the posterior corpus quadrigeminum. The comparison of these 
commissures with decussating bundles in lower Vertebrates has evi- 
dently rested mainly on the position of the latter bundles at their 
point of crossing. 
The most work upon these decussations has been done by 
EpINGER. He describes (92) in Selachians and Amphibians a 
decussatio transversa and a decussatio postoptica, both behind the 
chiasma, the former dorsal to the latter. The decussatio transversa 
he traces in some Selachians to the dorso-caudal part of the mid 
brain, and in other Selachians and Amphibians to the tectum. This 
he homologizes with MEYNERT’s commissures in Mammals. In the 
Vorlesungen (5th ed., p. 117, 134) he traces the decussatio trans- 
versa to the most caudal part of the mid brain, perhaps the ganglion 
isthmi. Here, contrary to his statement in ‘92, he uses “GUDDEN’sche 
Kreuzung” as synonymous with “Decussatio transversa”. On p. 267 
of the same work he traces the commissura inferior of GUDDEN to 
the posterior corpus quadrigeminum, which is in agreement with 
his results on Selachians and Amphibians. For Reptiles EDINGER’s 
description (99) is somewhat more definite. He describes an end- 
nucleus for the decussatio transversa dorsal and medial to the geni- 
culatum laterale and not sharply defined from the nucleus diffusus. 
