~2 
Henry L. Bruner 8 
The above facts indicate the close relation of exuviation to the general 
well-being of the lizard. In the head region this relation is sufficiently 
important to warrant the development of a special mechanism to expedite 
the removal of the old stratum corneum. But the moulting of other 
parts is no less important in the long run. On this point Knauer, 79, 
p- 496, says: 
“Unschwer kann man sich tiberzeugen, dass der Hautwechsel bei 
Lurehen und Kriechthieren durchaus kein nebensichlicher Akt ihrer 
Lebensthitigkeit, vielmehr ein ganz unerlislicher Vorgang im Lebens- 
prozess dieser Thiere sei, einerlei, ob nun Verhinderung der Hautung 
die Ursache des bald eintretendes Todes oder die Consequenz vorherge- 
gangener Storung der eigentlichen Lebensthiatigkeit ist.” 
II. DESCRIPTION OF A SWELL MECHANISM IN THE HEAD 
OF OPHIDIA. 
A. MUSCULUS CONSTRICTOR VENA JUGULARIS INTERNZ®. 
In Tropidonotus natrix the special mechanism for elevating the blood- 
pressure in the head includes a single muscle, the m. constrictor ven 
jugularis interne (m. c. 7. 1., Text Figs. 12 and 13), which is located at 
the point where the vena mandibularis enters the jugular vein. In the 
snake the muscle has no relation to the skeletal parts; it consists of stri- 
ated fibers, chiefly circular or spiral in direction, but with an irregular 
layer of longitudinal fibers imbedded in the wall of the vein inside of 
the spiral fibers. In a specimen with a total length of 59 cm. the muscle 
envelopes the jugular vein for a distance of about two millimeters, about 
one-half of which lies in front of the mouth of the vena mandibularis. 
The middle portion of the muscle includes five layers of spiral fibers, in 
addition to the longitudinal fibers. 
The muscle surrounds also the terminal parts of the veins which enter 
the vena jugularis interna at this point: vena mandibularis, vena ceso- 
phagea, and vena cervicalis. On the vena mandibularis it extends forward 
beyond the mouth of the vena maxillaris and envelopes also the posterior 
part of the latter vein. 
Different physiological conditions of the m. constrictor jugularis in- 
terne have been observed in the different specimens used in the prepara- 
tion of this description. In one individual the jugular vein is closed, the 
muscle fibers are much thickened and the entire muscle forms a compact 
band about the vein, whose walls are also thickened and folded. In 
