Henry L. Bruner 59 
muscles act upon the already distended sinus orbitalis. This effect is 
considered later. 
External evidence of the contraction of the muscles of the swell 
mechanism may be readily observed in the lizards. The distension of 
the superficial veins and sinuses produces external intumescence or swell- 
ing, which may become very pronounced in certain parts, especially in the 
region of the orbit. Evident movements also occur at the external nares, 
and in some forms these openings may be wholly closed by the distension 
of the sinus vestibuli nasi. 
Some of. the minor movements of this sort may be simple vaso-motor 
effects. More marked movements occur, however, which cannot be ex- 
plained in this way. Their occurrence was first observed in Anolis 
caroliniensis, the so-called chameleon of the southern United States. 
The movements began with a quiet swelling of the orbital region on both 
sides of the head, the enlargement increasing gradually and without ap- 
parent cause. Then came a spasmodic contraction of muscles, especially 
those of the lower jaw and pharynx, while at the same time the bulbus 
and eyelids were forcibly protruded, until the lids and loose skin about 
them were stretched to the utmost. At the maximum enlargement the 
diameter of the head across the orbits was increased by about one-third 
of the normal diameter. After this stage the parts were quickly restored 
to their usual condition and the swelling completely disappeared (com- 
pare Text Figs. 9, 10; and 11}. 
The conditions under which these movements occurred will be stated 
later; at the present time it is sufficient to say that they were repeatedly 
observed, so that the details could be carefully studied. At the time the 
movements were first noticed only the m. constrictor vene jugularis 
interne had been discovered. But the peculiar events of the second 
stage showed the participation of other muscles and finally led to the 
discovery of the mm. protrusor oculi and protrusor oculi accessorius. 
At the same time it was observed that the orbital swelling was accom- 
panied by similar, though less pronounced, effects in other parts of the 
head. 
These facts pointed to the existence of a complicated mechanism, con- 
‘sisting of muscles, nerves, and blood-vessels, all co-ordinated in a definite 
way to produce the observed results. On account of the character of 
these results the mechanism may be appropriately designated a swell 
mechanism. The nature and causes of the movements which it produces 
must now be studied in detail. For this purpose I shall use Anolis caro- 
limiensis as a type, but other forms will be frequently referred to. I 
begin with the protrusion of the eyes. 
