1885.] Fhysiology. 819 
amnion or the Reichert’sche deckschicht, Rauber, subzonal meni- 
brane, Turner, serous envelope, von Baer, and the true amnion, 
are formed, is cast off at birth or when incubation is completed. 
A highly vascular allantois is also developed in the majority of 
the species-of this group, and, as a rule, takes the most important 
share in the respiratory functions of the embryo, and in the 
development of the foetal part of the placenta whenever that 
organ is present.— 3. A. Ryder. [To be continued.) 
PHYSIOLOGY ®.' 
PFLÜGER’S “ AVALANCHE THEORY” OF NERVE-CONDUCTION.— 
M. Vulpian calls attention to the fact that in faradisation of vari- 
ous motor areas in the cerebral cortex of the dog, the effective 
strength of current which must be used varies definitely with the 
region stimulated. The minimal effective strength of current 
for the muscles of the face is less than that for the muscles of 
the fore-limb, and still less than for the muscles of the hind-limb. 
In general the strength of effective current must increase with 
the length of motor nerve tract. This result is readily explained 
if we consider that the nervous impulse started in the brain grad- 
ually diminishes in power as it passes along the motor nerve. 
But this assumption is in contradiction to the classic “ avalanche 
theory” of Professor Pflüger, according to which the nerve im- 
pulse accumulates energy in its progress, so that a certain strength 
of stimulus applied to a motor nerve produces a stronger con- 
traction the farther the place excited is removed from the muscle. 
It is this theory which Vulpian undertakes to test, and concludes 
with a denial of its adequacy. 
In Vulpian’s experiments dogs were used and so heavily chlo- 
ralized as to paralyze the reflex powers of the spinal cord. 
carefully isolated, without cutting, the external popliteal branch 
of the sciatic nerve and also the anterior tibial nerve, These 
nerves are in the dog of about the same thickness. Vulpian 
found that with a weak current strong flexion of the foot was 
Licut.—But three kinds of physiological change have been ob- 
served in the retina as an effect of the action of light: 1. The 
electrical currents discovered by Holmgren; 2. The bleaching 
of the visual purple in the outer segments of the retinal rods ; 
3. The movement of the pigment in the outermost or epithelial 
-1 This department is edited by Professor HENRY SEWALL, of Ann Arbor, Michigan, 
VOL, XIX.—NO. VIII, 54 
