170 
PROGRESS OF MICROSCOPICAL SCIENCE. 
sight of an external object. And since the changes depend on the 
greater or less absorption of light by the bottom, they must be 
regarded as true reflex acts, having their centre in the brain, and 
their starting-point in retinal impressions. The fundamental experi- 
ment in M. Pouchet's work is that in which he suppresses the chro- 
matic function by removing the ocular globe, or simply cutting the 
optic nerve. The blinded animal loses its power of changing colour 
according to the bottom. 
Having thus first established that the dilatation or contraction of 
the chromoblasts does not depend on local conditions produced in 
these elements at that point in the organism which they may occupy 
(as was previously thought), but is determined at a distance, by ante- 
cedent change of the elements of the central nervous system, it 
remained to find out by what route this transmission takes place, from 
the brain to the pigmentary cells of the periphery. 
The author made various sections of nerves, and he has demon- 
strated that the spinal cord is not the nervous conductor, nor yet the 
lateral nerve, to which it seemed natural to attribute a role in this func- 
tion. The trigeminus, on the other hand, has a direct action. Turbots 
taken from off a brown bottom, and, after section of the trigeminus, 
placed in a basin with sandy bottom, grow pale over their whole sur- 
face, except the face, which remains shaded, as if covered with a mask. 
Section of the spinal nerves gives results no less distinct. It confirms 
what has been said about the negative role of the cord. For the 
section of spinal nerves to influence the chromatic function, it is 
necessary that it be made below the point where they receive the 
thread of the great sympathetic. The result is a transverse dark band 
marking the region under the influence of mixed nerves receiving the 
cut sympathetic fibres. 
It is, then, the great sympathetic which governs the chromatic 
function. It forms the route of transmission for the influence going 
from the brain to the cutaneous chromoblasts. The disposition of 
this nerve in fishes, lying as it does in the same osseous canal with 
the principal artery and the principal vein of the body, does not allow 
of the section being made with advantage directly, as grave disorders 
ensue, which spoil the experiment. But the result of section of the 
mixed nerves, as above, sufliciently attests the influence in question. 
The author has not confined himself to fishes ; he has shown that 
the chromatic function also exists in some articulata ; more particu- 
larly in Palemon serratus. It may be demonstrated in the way that 
has been indicated for fish. Removal of the eyes, also, suppresses the 
function ; at least till these organs are regenerated. But M. Pouchet 
did not succeed in finding what route the nervous influence took in 
Crustacea, from the cerebral ganglions. 
M. Pouchet's observations establish, it will be seen, a series of new 
facts, which have, moreover, a remarkable character of generality. 
They open up an unexplored region, by revealing a series of reflex 
actions, of which the retina is the starting-point, and which irradiate 
over the whole system. 
These researches were prosecuted at Concarneau, in the laboratory 
