38 Mitchell^ on a Pulsatile Muscular Organ. 
microscope has shown me_, without entering into any specula- 
tions; but if we sought for a reason why this insect is furnished 
with such an unusual organ^ I think it might be found in 
the slow pulsation of the dorsal vessel and languid circulation 
in the body^ rendering some additional force necessary to 
impel the blood to the extremity of its long and slender 
limbs. 
[The foregoing is an abstract only of Lieut. Mitchell^s 
communication, which was accompanied by figures. But we 
have not thought it necessary to insert more than the main 
points in his paper, seeing that he is quite mistaken in sup- 
posing that he was the first to discover the curious organ he 
attempts to describe. 
It has been long well known that the circulation of the 
blood in the legs of certain insects is aided by the action of 
muscular fasciculi, situated near the articulation of the tibia 
with the femur. This fact was discovered by M. Belm in 
1835, in the young of Notonecta ; and the same condition 
has since been noticed in Corixa, Plea, Naucora, Nepa, and 
Ranatra. M. Behn conceived the organ in question to be of 
a special kind, whilst M. Leon Dufour conceives that the 
movement in question is due to the ordinary muscles of the 
leg. The latter observer, moreover, denies the existence of 
the currents described by M. Behn, and whose statement is 
now supported by the independent observations of Lieut. 
Mitchell. 
A similar phenomenon has been observed by M. Verlohren 
in the feet of Tettigonia and of the larvse of Ephemera, con- 
trary to the experience of Lieut. Mitchell. Degeer also 
speaks of pulsations resembling those of an artery in the 
legs of a species of Ornithomyia. 
M. Behn describes the double current noticed by Lieut. 
Mitchell as running in opposite directions on the two sides of 
the limb; and states that the movements of these currents 
coincide with those of the pulsatile organ, and are apparently 
independent of the contractions of the dorsal vessel. It will 
thus be seen that, although the phenomenon has been long 
well known, there are still some disputed points connected 
with it. For this reason, and because the subject is one of 
considerable interest, and perhaps not familiar to many 
microscopical observers, we have thought right to publish 
the sum of Lieut. Mitchell's brief communication, with the 
above comments ; for the greater part of which we are in- 
debted to a note in the third volume, p. 226, of M. Milne 
Edwards^s invaluable ' Lectures on Physiology. '^Eds.] 
