62 The Cephalic Veins and Sinuses of Reptiles 
cannot be directly observed, but it may he safely inferred from the 
character of the result. 
During the first stage of orbital protrusion the sinus orbitalis is dis- 
tended until the blood-pressure in the sinus reaches a maximum for the 
existing arterial pressure. During the second stage the outlet of the 
sinus is closed by the posterior part of the m. protrusor oculi. A con- 
siderable amount of blood is forced out of the enlarged anterior end of 
the vena jugularis interna and much of this blood enters the sinus orbit- 
alis. At the same time the anterior part of the m. protrusor oculi presses 
against the wall of the sinus from below and behind. If the walls of the 
sinus were rigid outside of the territory affected by the muscle, contrac- 
tion of the latter would produce a simple rise of blood-pressure. But 
since the lateral wall of the sinus is composed of elastic tissues a different 
result must follow; for whenever the pressure exerted by the m. pro- 
trusor oculi exceeds the blood-pressure existing in the sinus orbitalis, 
the floor and posterior wall of the sinus are pushed in, the lateral wall 
is pushed outward to a corresponding degree and the amount of orbital 
protrusion is increased. 
More or less protrusion of the eyes under ordinary conditions may be 
readily produced by artificial pressure upon the roof of the mouth be- 
tween the anterior ends of the pterygoid bones (compare Figs. 4 and 5, 
Plate I), where the vena jugularis interna lies near the mucous 
membrane. 
Rathke, 66, assigns to the m. depressor palpebre inferioris of the 
crocodile the function of elevating the bulbus under certain conditions. 
Such a movement would assist in the protrusion of the eye of the lizard 
during the second stage, but I have not been able to discover any evidence 
that the lid muscle contracts at this time. Moreover, my experiments on 
Anolis show, that after the sinus orbitalis is flooded stimulation of the 
m. depressor palpebre inferioris causes simple depression of the lid, as 
under ordinary conditions. 
(2) Musculus Protrusor Oculi Accessorius (m. p. 0. a., Fig. 2, Plate 
III, and Fig. 1, Plate I1).—This muscle does not occur in Anolis, and no 
physiological observations have been made on any forms in which the 
muscle has been observed. In view of its position, however, there can 
be no doubt in regard to its function, which is the same as that of the 
anterior part of the m. protrusor oculi. In contraction it presses against 
the distended sinus orbitalis from below and behind, and thus by increas- 
ing the blood-pressure, augments the protrusion of the eye. 
(3) Musculus Temporalis (m. t., Fig. 1, Plate IT).—The contrac- 
