84. The Cephalic Veins and Sinuses of Reptiles 
blood-vessels in the pre-ocular curtain of snakes increases greatly at the 
time of moulting, presumably as an aid to exuviation. If this is true, it 
is possible that a similar development may occur in the ordinary skin, 
and their occurrence might be expected also in the lizards. 
Such a growth of new capillaries offers a field for the application of 
one or both of the following theories: (a) According to Thoma, 96, 
the ‘‘ increase of blood-pressure in the capillary areas leads to the forma- 
tion of new capillaries.” Loeb, 93, on the other hand, claims that “ Die 
Abgabe von Aesten ist bestimmt durch innere Ursachen in den Zellen 
der Gefasswande oder durch Reizursachen, die von der Umgebung ausge- 
hend, diese Zellen treffen.” ” The physiological processes which oceur in 
the skin, particularly in the epidermis, during the early stages of exuvia- 
tion may act as such an external stimulus and lead to the formation of 
new capillaries. 
e. EXUVIATION BY MEANS oF BLOOD-PRESSURE. 
Thus far in the description of the moulting process it has not seemed 
necessary to invoke the aid of a special mechanism for the removal of 
the old stratum corneum. Indeed, in the case of the trunk and lmbs 
such a mechanism would seem to be superfluous. We may ask, therefore, 
what is the need of such a mechanism in the head? In answer it may 
be said : 
(a) The exuviation of the head is the most difficult part of the entire 
moulting process. This is due, first, to the rigidity of the skull prevent- 
ing those movements which assist in the moulting of other parts; second, 
to the close attachment of the skin to the lips and to the openings of the 
sense organs. 
(b) The exuviation of the head is the most urgent part of the entire 
process. In the later stages of moulting, as soon as the old stratum 
corneum begins to separate, it becomes a menace to sight, hearing, and 
touch, and its prompt removal from the neighborhood of the sense 
organs is absolutely necessary in order that they may retain their normal 
efficiency. The difficulties and dangers of this stage might evidently be 
reduced to a minimum by the seclusion of the moulting animal and by 
hastening the process of exuviation. If the animal does not conceal 
itself it is exposed to the attacks of enemies, if it depends on conceal- 
ment alone, it may suffer from lack of food. In either case, therefore, 
For a critical review of these histo-mechanical theories, see Mall, Am. 
Journ. Anat., Vol. V, pp. 231-253. 
