96 The Cephalic Veins and Sinuses of Reptiles 
region, one directly in front of the anterior canthus, the other behind 
the posterior canthus. The plaster did not touch the eyelids themselves 
and their movements were not affected. At first the animal was shy and 
watchful and made no attempt to remove the plaster except by scratching 
with the foot. This movement was repeated several times, and on both 
sides of the head. Finally, after prolonged watching, the flooding of the 
sinus orbitalis was also observed. At first this occurred with the eyes 
open and alert, and the amount of protrusion was not large. Later, the 
sinus was flooded in a more marked way. As a preparation for this 
movement the animal became inattentive to its surroundings, then the 
eyes were closed and the head was drawn toward the opening of the 
shell. The enlargement of the orbital region then came on gradually 
and decreased in the same way, without any indication of a second stage. 
It is possible, however, that the regular movements were more or less 
modified or suppressed on account of the artificial conditions. After 
the reduction of the sinus orbitalis the eyelids were slowly wrinkled and 
distorted, as if by the contraction of a smooth muscle. 
The response to this experiment agrees entirely with the character of 
the swell mechanism of the turtle. On account of the absence of the 
protrusor muscles (protrusor oculi and protrusor oculi accessorius), the 
outlet of the sinus orbitalis cannot be closed and it is impossible to de- 
velop local high pressure in the anterior part of the head. Whether the 
pressure can be increased by the action of other muscles, I am unable to 
say. Such an effect would apparently follow the contraction of the m. 
temporalis, which supports the posterior membraneous wall of the sinus 
orbitalis. The elevation of the floor of the mouth against the flooded 
sinus palatinus would probably contribute to the same end by driving 
blood from that sinus into the sinus orbitalis. Under these conditions, 
however, the increase of blood-pressure must be more or less uniform 
throughout the head. 
Corresponding to the absence of the protrusor muscles we find also a 
modification of the arrangements for reducing the flooded sinus orbit- 
alis. The striated m. depressor palpebre inferioris is wholly wanting in 
Emys and its place and function have been assumed, in part at least, by 
the smooth orbital muscle, m. compressor sinus orbitalis, which is very 
strongly developed in the turtle. 
This muscle was first described by Bojanus, 19-21, Vol. I, p. 412, 
under the name musculus palpebralis. Stannius, 56, p. 172, recognized 
am. palpebralis superior and a m. palpebralis inferior. Later, the two 
parts were again united by Hoffmann, go, who, however, erroneously 
