THE HEART AND VASCULAR SYSTEM. 207 



The internal jugular vein is formed by the lateral and the 

 superior petrosal sinuses, descends at the outer side of the com- 

 mon carotid behind the anterior border of the sterno-mastoid 

 muscle, and joins the subclavian vein to form the innominate. 

 At its junction with the subclavian the left internal jugular vein 

 receives the thoracic duct and the right internal jugular vein the 

 right lymphatic duct. 



THE SINUSES OF THE DURA MATER. These are venous 

 channels analogous to veins between the layers of the dura mater. 

 They are destitute of valves, follow no vessels, and their inner 

 coat is continuous with the lining membrane of the veins. They 

 are fifteen in number, divided into two sets, those at the back 

 and upper part of the skull five in number : 



Superior longitudinal, Lateral sinuses (2), 



Inferior longitudinal, Occipital sinus. 



Straight sinus, 



And those of the base five also : 



Cavernous (2), Superior petrosal (2), 



Circular, Transverse (anterior occipital, 



Inferior petrosal (2), Leidy). 



The superior longitudinal sinus arises at the foramen caecum, 

 passes backward along the margin of the falx cerebri to the 

 crucial ridge of the occipital bone, where it terminates in the 

 torcular herophili (confluence of the sinuses). 



The inferior longitudinal sinus passes in the "free margin of 

 the falx cerebri. 



The straight sinus passes backward from the junction of the 

 tentorium with the falx cerebri to enter the confluence of the 

 sinuses. 



The lateral sinuses pass horizontally outward from the con- 

 fluence of the sinuses, torcular Herophili, along the temporal 

 bone to the jugular foramen, where they terminate in the internal 

 jugular vein. 



The occipital sinus, the smallest of all the sinuses, com- 

 mences at the margin of the foramen magnum on either side and 

 passes backward to the confluence of the sinuses. 



The cavernous sinuses pass from the sphenoidal fissure along 

 either side of the sella turcica to the apex of the petrous portion 

 of the temporal bone, where they join the petrosal sinuses. They 

 are crossed by fibrous bands or offsets of the dura mater, and 

 inclose the pathetic, motor oculi, abducens*, and ophthalmic 

 ncrvts, and the internal carotid artery, from which they are 

 separated by the lining membrane. 



