Cytological Structure, Hypothalamic Nuclet, Functional Connections 73 
At the end of this period it was removed, dehydrated, embedded, 
and sectioned. 
The sections were stained with toluidin blue and erythrosin. 
Very satisfactory results were obtained, particularly with regard 
to the chromidial pattern, by staining the mounted sections for 
three minutes in a one per cent solution of erythrosin and then 
for seven minutes in a saturated solution of toluidin blue. De- 
Staining to the desired degree was accomplished as the tissues 
were passed through the series of alcohols preparatory to covering. 
MICROSCOPIC OBSERVATIONS 
The hypothalamic nuclei in the cat are made up mainly of 
small or medium-sized neurons. Although there is great sim- 
larity in the structure and morphology of the neurons in the 
different nuclei, it is fairly easy by careful study to identify these 
nuclei because of small areas of relatively undifferentiated gray 
matter and fiber tracts between them. The identity of most of 
the hypothalamic nuclei, however, could be established by their 
cytological structure even in the absence of such separation from 
neighboring nuclei. Variations in the cytological structure of 
the neurons of different hypothalamic nuclei have been pointed 
Out by Malone (1912, 1913, 1914) and others, but all of the 
neurons of the hypthalomic nuclei in the cat can be classified in 
four general categories on the basis of chromidial pattern and 
scneral morphology. These categories have been named accord- 
ing to the synaptic connections of their constituent neurons. 
Peripheral visceral efferent, central somatic efferent, centr al 
visceral efferent, and associational. 
The peripheral visceral efferent neurons (F ig. 1 A) are eon 
Which, although located centrally, effect functional connections 
with cells in the posterior lobe of the pituitary gland. They are 
