42 The Cephalic Veins and Sinuses of Reptiles 
the eye. A thorough examination by means of sections led to the dis- 
covery of a special muscle for the obstruction of the vena jugularis in- 
terna. ‘This observation promised at least a partial solution of the 
problem. Further study, however, soon revealed the fact that the same 
muscle occurs in other Sauria, and afterwards it was found also in the 
Ophidia and Testudinata. 
These discoveries diverted my attention from Phrynosoma and led to 
a study of the wider significance of the peculiar mechanism. As a first 
step I undertook an investigation of the cephalic veins and sinuses and 
their relation to other organs of the head. In the course of this investi- 
gation it was found that the muscle for obstructing the vena jugularis 
interna is everywhere associated with enlarged cephalic veins and sinuses. 
In the Sauria other muscles are also discovered which assist in raising 
the blood-pressure in the distended vessels. It was, however, not to be 
supposed that this swell mechanism is extensively used for the ejection 
of blood. Its wide distribution pointed rather to the existence of an 
undiscovered function of fundamental importance in the life of its pos- 
sessors. A function which seems to meet the demands of the case was 
finally observed in the Sauria. 
In the following account of the swell mechanism I shall begin with 
the Sauria, which have been more thoroughly studied than the Ophidia 
and Testudinata. I shall describe first of all the mechanism for obstruct- 
ing the vena jugularis interna and raising the blood-pressure in the veins 
and sinuses of the head. 
I. DESCRIPTION OF A SWELL MECHANISM IN THE HEAD 
OF SAURIA. 
A. MUSCLES WHICH OBSTRUCT THE VENA JUGULARIS INTERNA 
AND RAISE THE VENOUS BLOOD-PRESSURE IN THE HEAD OF 
THE SAURIA. 
In the Sauria as a group the mechanism for raising the blood-pressure 
in the veins and sinuses of the head includes three muscles which must 
first of all receive adequate description. These muscles I designate as 
follows : 
a. Musculus constrictor vene jugularis interne. 
b. Musculus protrusor oculi. 
c. Musculus protrusor oculi accessorius. 
a. THE MuscuLus ConstricroR VEN@® JUGULARIS INTERN #. 
1. ANATOMICAL RELATIONS.—The m. constrictor vene jugularis in- 
terne, which was first described by the writer of this article, 98, is a 
