does not seem a _conditio sine qua non_ for psychosis--so far
as we may trust the rather hazardous inferences which study of the
behaviour of fish, &c., allows. And the difference between higher and
lowlier animal forms in respect of the fore-brain as a condition for
psychosis becomes more marked when the Arthropoda are examined. The
behaviour of some Insecta points strongly to their possessing memory,
rudimentary in kind though it may be. But in them no homologue of the
fore-brain of vertebrates can be indisputably made out. The head ganglia
in these Invertebrates may, it is true, be analogous in function in
certain ways to the brain of vertebrates. Some experiments, not
plentiful, indicate that destruction of these head ganglia induces
deterioration of behaviour such as follows loss of psychical functions
in cases of destruction of the fore-brain in vertebrates. Though,
therefore, we cannot be clear that the head ganglia of these
Invertebrates are the same structure morphologically as the brain of
vertebrates, they seem to hold a similar office, exercising analogous
functions, including psychosis of a rudimentary kind. We can, therefore,
speak of the head ganglia of Arthropods as a brain, and in doing so must
remember that we define by physiological evidence rather than by
morphological.

_Cerebral Control over Lower Nervous Centres._--There accrues to the
brain, especially to the fore-brain of higher Vertebrates, another
function besides that of grafting psychical qualities upon the reactions
of the nervous system. This function is exhibited as power to control in
greater or less measure the pure reflexes enacted by the system. These
pure reflexes have the character of fatality, in the sense that, given a
particular stimulus, a particular reaction unvaryingly follows; the same
group of muscles or the same gland is invariably thrown into action in
the same way. Removal of the fore-brain, i.e. of that portion of the
central nervous organ to which psychosis is adjunct, renders the nervous
reactions of the animal more predictable and less variable. The animal,
for instance, a dog, is given over more completely to simple reflexes.
Its skin is touched and it scratches the spot, its jaw is stroked and it
yawns, its rump is rubbed and it shakes itself, like a dog coming out of
water; and these reactions occur fatally and inopportunely, for
instance, when food is being offered to it, when the dog normally would
allow no such