74 The Cephalic Veins and Sinuses of Reptiles 
(a) At a given time the venous pressure varies in different parts of 
the head. If all of the efferent blood-vessels of the head were obstructed, 
approximate uniformity of blood-pressure would soon be established. 
But after the obstruction of the vena jugularis interna the smaller blood- 
vessels of the posterior head region still continue to carry blood, the 
quantity of which is greatly increased on account of the obstruction of 
the larger vein. For this reason the venous pressure in the posterior 
part of the head is somewhat lower than elsewhere, and complete equali- 
zation of blood-pressure is impossible. For the same reason, also, the 
venous pressure can never quite equal the arterial pressure, although the 
difference decreases toward the anterior part of the head. 
(b) After the obstruction of the vena jugularis interna the blood- 
pressure rises in the veins until the acting efferent vessels of the head 
carry the same amount of blood as the afferent vessels. At this point a 
new mean pressure is established and the distension of blood-vessels 
reaches its limit. This condition of equilibrium is attained, under ordi- 
nary conditions, at the close of the first stage. It is due to an increase 
in the amount of blood transmitted by the acting veins, especially the 
vena jugularis externa, vena trachealis, perhaps, also, the vena spinalis. 
(c) Other things being equal, the mean pressure in the cephalic veins 
and sinuses is finally determined by the rate and force of the heart-beat, 
by the amount of dilation of the carotids and their branches, by the 
amount of constriction of the posterior arteries, and lastly, by the gen- 
eral arterial pressure, which is a resultant of all the preceding and other 
conditions, which need not be mentioned here. 
2. The second stage of blood-pressure, due to the contraction of the 
m. protrusor ocul and associated muscles, is of short duration. The 
most marked effects occur in the sinus orbitalis, but a wave of high pres- 
sure also sweeps through the connected veins and sinuses. The occur- 
rence of this wave indicates a blood-pressure which at least exceeds that 
already existing in the veins,—higher perhaps than that of the local 
arteries themselves. Since, however, the outlet of the sinus orbitalis is 
closed during this stage, the blood-pressure reaches a higher level in the 
sinus and its tributaries than in the other veins of the head. This seems 
to explain the relatively greater development of blood sinuses in the 
anterior part of the head, a fact already noted in the first part of this 
paper. ; 
3. In this connection a word may be said in regard to a modification 
of the swell mechanism which occurs in Chameleon vulgaris and Pla- 
tydactylus mauritanicus. In both of these. forms the m. protrusor oculi 
