66 The Cephalic Veins and Sinuses of Reptiles 
alis and to keep the lower lid depressed. When the muscle relaxes, the 
blood enters the palpebral space and elevates the lid. This fact was 
observed long ago by Weber, 77.’ 
(b) The distension of the sinus orbitalis under ordinary blood-pres- 
sure is prevented by the striated muscles of the orbit. By raising the 
tonus of these muscles, therefore, the distension of the sinus may be 
more or less prevented even under extraordinary blood-pressure. ~This 
explains the difficulty sometimes experienced in attempts to flood the 
sinus orbitalis by artificial compression of the venz jugulares interne. 
Whether all of the bulbus muscles are concerned in producing these 
results must be left undecided. Retraction of the bulbus is generally 
supposed to be a special function of the m. retractor ocuh, but according 
to Weber, 77, this muscle is assisted by the m. bursalis. It is evident, 
moreover, that the recti and obliqui might play a similar role. 
(4) Musculus Compressor Sinus Orbitalis—This muscle, which was 
first described by Leydig, 72, is a sheet of smooth fibers which arises in 
the median part of the orbit and extends in a meridional direction around 
the bulbus and into the eyelids (m. c. s. 0., Fig. 2, Plate III, Fig. 1, 
Plate II). The muscle les everywhere outside of the sinus orbitalis, 
for which it forms an almost complete covering. Delow the bulbus the 
muscle lies on the dorsal side of the m. depressor palpebre inferioris, but 
it is weakly developed in the region of this muscle, which seems to some 
extent to take the place of the smooth muscle in its relation to the sinus 
orbitalis. In the neighborhood of the canthi, where the smooth muscle 
is very strong, it includes both meridional fibers and a second layer which 
is composed of vertical fibers (Fig. 2, Plate III). The smooth muscle also 
reaches into the membrana nictitans and envelopes the sinus membrane 
nictitantis. 
At the proximal border of the lower eyelid the smooth muscle divides 
into two parts, one of which passes between the tarsus and the con- 
junctiva, while the other runs through the trabecule of a great lymph- 
sinus which hes next to the cutis plate of the hd. A similar lymph- 
sinus occurs in the upper eyelid, where also the smooth muscle is well 
developed. 
® According to Weber, the closing of the eye may be more or less acceler- 
ated by two other factors: (1) Elasticity of the tissues of the lower eyelid. 
When the m. depressor palpebre inferioris relaxes, the folded parts of the 
lid, including both conjunctival and cutis plates, tend to unfold and elevate 
the lid. (2) Retraction of the bulbus causes a rise of blood-pressure in the 
sinus orbitalis and thus hastens the elevation of the lid. This would evi- 
dently tend to depress the upper eyelid also. 
