64 The Cephahe Veins and Sinuses of Reptiles 
2. SECONDARY CAUSES AND CONDITIONS WHICH AFFECT THE DISTENSION 
OF THE SINUS ORBITALIS. 
I have shown that the distension of the sinus orbitalis is largely an 
effect of the contraction of certain muscles (m. constrictor ven jugularis 
interne, m. protrusor oculi, m. protrusor oculi accessorius, and the bucco- 
pharyngeal muscles). There are also certain other factors which affect 
the distension of the sinus. 
(1) Acceleration of the Heart's Beat—In Anolis there is a marked 
increase in the number of cardiac pulsations during the distension of the 
sinus orbitals. For example, in a specimen showing 112 beats per 
minute before protrusion began, the number of beats increased as disten- 
sion increased, until the rate reached a maximum of from 135 to 149 
pulsations per minute. The acceleration is not noticeable at the very 
beginning of distension but follows after a short interval. The maxi- 
mum rate is attained approximately at the moment of maximum disten- 
sion. If the stage of distension is prolonged, the rate of pulsation con- 
tinues about the same but the beats become feeble, probably on account 
of exhaustion. After the stage of high pressure passes, the heart-beat 
becomes slower, but symptoms of exhaustion may be noticed for a short 
time. 
The more rapid cardiac action undoubtedly raises the blood-pressure 
in the cephalic arteries and facilitates the distension of the sinuses and 
veins of the head. If other conditions remained uniform after obstruc- 
tion of the vena jugularis interna, the amount of blood sent to the head 
would gradually decrease so long as the distension of the cephalic vessels 
continued. But when the heart begins to beat faster, the blood-pressure 
is diminished in the veins near the heart, the intake of blood from the 
posterior veins is increased and a larger amount is sent to the head at 
the expense of the posterior parts. 
The acceleration of the heart-beat is presumably a reflex effect, due 
to stimulation of the cardio-accelerator center of the medulla. The ori- 
gin of the stimulus has not been determined, but it may be accounted 
for, perhaps, either by the accumulation of impurities in the blood of 
the brain, or by the rise of blood-pressure within the cranial cavity. The 
first mode of acceleration occurs in the mammals (Foster, 94). The 
second method has apparently not been observed in this group, in which 
high blood-pressure in the cranial cavity produces an exactly opposite 
effect, the heart being slowed down by vagus inhibition. This last- 
mentioned reflex may account for the slowing of the heart of the lizard 
