146 ELEMENTS OF MAMMALIAN ANATOMY. 
nal maxillary veins. The external maxillaries are united 
by a large transverse vein. The two tributaries of the 
external jugular are the cephalic and the internal jugular. 
The former is the superficial vein on the lateral aspect of 
the arm, and the latter is the small vein returning blood 
from the brain. 
A system of tube-like spaces, called sinuses, whose walls 
are formed by the dura mater lined with epithelium, takes 
the place of the large venous trunks within the cranial 
cavity. These sinuses are usually not filled by a starch in- 
jection, but may be filled by a gelatin mass. They cannot 
be dissected by the beginner. The superior longitudinal 
simus extends in the median line beneath the roof of the 
skull, from the ethmoid region to the tentorium cerebelli, 
where it bifurcates to form the lateral sinuses, which pro- 
ceed laterad and ventrad in the substance of the tentorium. 
The lateral sinus emerges from the tentorium just caudad 
to the petrosal bone, where it follows the groove to the 
jugular foramen. This groove may be seen in a bisected 
or unroofed skull. 
The superior petrosal sinus lies in the angle formed by 
the junction of the tentorium and the petrosal bone, and 
unites with the lateral sinus just before it reaches the jugu- 
lar foramen. The inferior petrosal sinus, collecting blood 
from the base of the brain, lies in the groove ventral to 
the petrosal bone, and unites with the lateral sinus at the 
jugular foramen. The union of these sinuses at this 
foramen forms the internal jugular vein, which extends 
deep beneath the muscles of the neck with the carotid 
artery. It joins the external jugular opposite the shoulder- 
joint. At the base of the skull the internal jugular gives 
off a large transverse anastomosing vein to the vertebral, 
so that the blood from the vein may return partly by the 
latter vessel. 
