86 The Cephalic Veins and Sinuses of Reptiles 
of these processes the old inelastic stratum corneum is gradually sepa- 
rated from the more flexible and elastic new layer. 
In the region of the external naris exuviation is effected especially by 
changes in the spongy body of the nasal vestibule (Figs. 2, 3, and 5, 
Plate II). The description given on pp. 68-70 indicates the variety of 
movements which has been observed here. These movements assist also 
in the removal of the stratum corneum of the vestibule itself, which is 
lined by the infolded epidermis. Since the spongy body of the nasal 
vestibule is under local control, these moulting movements may be exe- 
cuted also without simultaneous swelling in other parts of the head. 
The same movements of the spongy body are used for the removal of 
foreign substances which may enter the vestibule. : 
The mechanical stage of exuviation has not been followed from begin- 
ning to end and it is uncertain what the typical order of events may be, 
or if such an order exists. It is not improbable, however, that the 
actual removal of the old stratum corneum begins in the region of the 
orbit, where the great distension of the sinus orbitalis affects both the lids 
and the adjacent parts. After a beginning has been made each new 
movement adds more territory to that already gained, while between 
periods of high blood-pressure the process is hastened by scratching the 
head with the foot, by rubbing against a foreign body, perhaps, also, by 
licking the lips (Lacerta). In these different ways the old stratum cor- 
neum is gradually loosened and thrown off. 
According to yon Fischer, 82, some lizards enter water freely at the 
moulting periods and thus facilitate the removal of the exuvie. Trachy- 
dosaurus asper, for example, is said to remain quietly immersed for some 
time. In the case of those lizards which have the habit of sunning them- 
selves, the moulting is accelerated by such exposure and is made more 
difficult, or prevented altogether, by keeping the animals away from the 
sun. 
In healthy animals exuviation occurs at somewhat regular intervals so 
long as external conditions (food supply, temperature, and moisture) 
are favorable. According to Knauer, 79, the lizards of Austria moult, 
under favorable conditions, every month. Under adverse conditions the 
intervals become longer and in sickly animals the exuviation is omitted 
altogether. A corresponding variation occurs also in regard to the length 
of time required to complete the moulting process, which may be accom- 
plished in two days by a healthy Lacerta, while unfavorable conditions 
may lengthen the period to more than a week. 
