2l6 ANATOMY AND PHYSIOLOGY FOR NURSES 



and five single sinuses. It is a peculiarity of the cerebral 

 circulation that the venous blood is returned through these 

 canals or sinuses. 



The cerebral sinuses are formed by a separation of the 

 dura mater into layers and are unyielding structures. 

 They are lined by the same smooth membrane that lines 

 the veins. 



As has been said, there are fifteen sinuses; their names 

 are given below, and their arrangement should be studied. 



Lateral (2). Circular. 



Superior petrosal (2). Superior longitudinal. 



Inferior petrosal (2). Inferior longitudinal. 



Cavernous (2). Transverse. 



Occipital (2). Straight. 



All these eventually discharge their blood into the in- 

 ternal jugular veins. 



The spinal system of veins is considered together with 

 the sinuses, as there is an extensive communication of 

 vessels along the spinal column, their structure resembling 

 that of a ladder; these all discharge through the inter- 

 vertebral foramina into the several regions of the spine 

 as follows : In the cervical region, into the vertebral veins ; 

 in the dorsal region, into the intercostal veins; in the 

 lumbar region, into the lumbar veins. These veins are 

 not provided with valves, which accounts for the serious 

 conditions that result from injuries and inflammation of 

 the spine. 



THE SPINAL CORD 



The spinal cord does not occupy the whole area of the 

 spinal canal; the space is filled up with a reddish, fatty 

 substance, and with the associated ramification of veins. 



The Membranes of the Cord. The dura mater does 

 not adhere to the vertebra, therefore it is not called the 

 internal periosteum, as in the skull; if this were the case, 



