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The Cephalic Veins and Sinuses of Reptiles 
by elevating the floor of the sinus orbitalis, depressing the lower eyelid 
and retracting the adjoining skin. These functions probably explain 
the relatively strong development of this muscle, especially in the neigh- 
borhood of the posterior canthus. 
Slowing down of the heart-beat and vaso-motor adjustment of the 
arteries probably contribute to the reduction of the sinus orbitalis by 
limiting the supply of the blood which enters the sinus. 
The elasticity of the tissues is probably a factor of some importance 
in the reduction of the sinus orbitalis but such action is naturally limited 
to the early stages of the process. 
c. DISTENSION OF THE Sinus VesTIBuLI Nast. 
When the eye of the lizard is protruded, a sympathetic swelling may 
sometimes be observed in the region of the sinus vestibuli nasi. ‘The 
cause of this swelling is clearly indicated in certain forms (Sceloporus) 
by the occurrence of distinct phases corresponding to those observed in 
the sinus orbitalis. During the first phase the swelling is slow and 
gradual, but the second phase is marked by the arrival of a pulse-like 
wave which produces a conspicuous narrowing of the nasal opening. 
These observations show that the flooding of the sinus vestibuli nasi 
may be effected by the same mechanism that is used for the distension of 
the sinus orbitalis and other sinuses of the head. This sympathy does 
not always manifest itself, however. On the contrary, the intumescence 
at the external nasal opening may occur without a general flooding of 
other sinuses of the head, or vice versa, a general flooding of the cephalic 
sinuses and veins may occur without distension of the sinus vestibuli nasi. 
These facts may be readily understood by reference to the description of 
the sinus vestibuli nasi on page 12. It is there shown that the possi- 
bility of local control of the sinus is vested in the smooth muscle fibers 
of the trabecule and arteries. The fact that the sinus maintains its 
ordinary contracted state after obstruction of the vena jugularis interna 
may be explained by a higher tonus of the muscle fibers of the trabecule 
and greater constriction of the arteries. The distension of the sinus 
under ordinary conditions may be accounted for by relaxation of the 
smooth muscles of trabeculee and arteries. 
These arrangements for local control probably account for the ordi- 
nary changes which occur in the sinus vestibuli nasi. It is possible, 
however, that the sinus may be distended by contraction either of the 
m. constrictor ven jugularis interne or the m. protrusor oculi, and if 
