H 6 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 

 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 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. 



