58 The Cephalic Veins and Sinuses of Reptiles 
The closing of the jugular vein is also made easier by valves within the 
vein at this point. They are valves of the ordinary form, which are used 
under normal conditions to prevent a reversal of the blood current. When, 
however, the constrictor muscle contracts, they serve as a cushion which 
assists in blocking the lumen of the vein. Similar valves have been ob- 
served at this poit in several genera, including Lacerta, Agama, Monitor, 
and Moloch. 
The effect of the obstruction of the vena jugularis interna upon the 
circulation of blood in the head is naturally very marked. Under ordi- 
nary circumstances the vein probably receives nine-tenths of all the 
blood from the cranium, face, and jaws. When the vein is closed by the 
constrictor muscle, blood accumulates in the distal portion of the vein, 
its tributaries are distended and the venous blood-pressure rises. Within 
the cranium the large sinus-like veins are flooded, but on account of the 
rigid brain case the enlargement of these veins soon reaches its limit. In 
the extracranial sinuses, on the other hand, the distension increases as 
the blogd-pressure rises and the total enlargement may be very great. 
The most conspicuous effects occur in the sinus orbitalis, whose distension 
is facilitated both by its direct connection with the vena jugularis interna 
and by the great extent of its drainage territory. From the sinus orbit- 
alis the area of high blood-pressure spreads into the antorbital veins and 
into the vena supratemporals, and thus completes the invasion of the 
head. These effects may be modified more or less by tonic contraction 
of certain orbital muscles, which thus tend to prevent the enlargement 
of the sinus orbitalis and to hasten the distension of the veins and other 
sinuses of the head. 
The contraction of the m. protrusor oculi and m. protrusor oculi 
accessorius produces effects which are limited chiefly to the anterior part 
of the head. The first muscle, acting either alone or with the second, 
causes a rise of blood-pressure in the sinus orbitalis and elevates the 
bulbus. At the same time the sinus orbitalis and its tributaries are more 
or less distended. The precise effects may vary, however, under different 
conditions. A strong tonic contraction of the orbital muscles might pre- 
vent distension of the sinus orbitalis, while it would accelerate distension 
of the tributaries of the sinus. Relaxation of the orbital muscles is fol- 
lowed by gradual distension both of the sinus orbitalis and its tributaries. 
The contraction of the protrusor muscles is not favorable, however, under 
any conditions, for the greatest distension of the sinus orbitalis, on 
account of the pressure which they exert upon the wall of the sinus. 
The most striking effect of their contraction is to be observed when the 
