THE VASCULAR SYSTEM. 157 



into the precava. The subclavian -vein returns the blood 

 from the arm, and accompanies the subclavian artery. 

 The external jugular veins are large vessels lying on either 

 side of the neck beneath the platysma myoides muscle. 

 When the skin is removed, the veins are plainly seen 

 through this very thin muscle. The external jugular is 

 formed ventral to the angle of the mandible by the 

 union of the internal and external 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 injection, but may be filled by a gelatin mass. 

 They cannot be dissected by the beginner. The superior 

 longitudinal sinus 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 proceed laterad and ventrad in the substance of 

 the tentorium. The lateral sinus emerges from the ten- 

 torium 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 jugular foramen. The inferior petrosal sinus, col- 

 lecting blood from the base of the brain, lies in the 

 groove ventral to the petrosal bone, and unites with the 



