104 The Cephalic Veins and Sinuses of Reptiles 
more primitive function of a branchial muscle. Associated with this 
primitive relation of the constrictor muscle, we find a single accessory 
muscle: the m. protrusor oculi. In other forms the same muscles are 
present but the epibranchial cartilage is wanting and the constrictor 
muscle is more strongly developed (Phrynosoma). In a few forms the 
moulting mechanism becomes still more efficient by the addition of a 
m. protrusor ocul accessorius (Monitor). 
The complicated moulting mechanism of the Sauria is associated with 
movable eyelids and an external auditory depression, both of which ren- 
der the moulting more difficult. At the same time we find great activity 
and intelligence demanding the prompt removal of the exuvie. In re- 
sponse to such demands, apparently, the moulting mechanism of the 
Sauria has come into existence. 
The absence of the m. constrictor ven jugularis interne in Chameleon 
and Platydactylus is undoubtedly a result of degeneration. In Platy- 
dactylus such a result is probably to be correlated with the union of the 
eyelids and the consequent removal of one of the chief difficulties of 
exuviation. In Chameleon exuviation has been made easier by a close 
union of the head and trunk and the disappearance of the external audi- 
tory depression, changes which give greater flexibility to the skin of the 
posterior region of the head. The absence of the entire moulting 
mechanism in Rhineura is apparently to be explained, in part, by the loss 
of eyes and external auditory depression, in part, by the burrowing habit, 
which doubtless facilitates the removal of the old stratum corneum. 
Degeneration of the moulting mechanism on a larger scale has probably 
occurred in the Crocodilia. The only surviving portion of the mechan- 
ism in this group seems to be the spongy body of the nasal vestibule, a 
structure of purely local importance. The loss of the major part of the 
mechanism is to be explained in this case by a radical change in the mode 
of renewing the stratum corneum. According to Gegenbaur, 98, the 
epidermis of the Crocodilia shows the same characteristics that we find 
in higher forms, the superficial strata wearing away gradually, while they 
are renewed in the same way from the stratum Malpighii. 
A history similar to that just outlined might probably be written of 
the mammals. This group is probably descended from reptilian or pro- 
reptilian forms which were provided with the moulting mechanism. It is 
a matter of special interest, therefore, that modern mammals are furnished 
with a spongy body which is similar, both in structure and position, to 
that which occurs in the nasal vestibule of reptiles. The conclusion is 
almost irresistible that the two structures are homologous; in other words, 
