252 INNERVATION. [CHAP. x. 



cranium, insured not merely by the inextensible nature of the 

 principal venous canals, but also by the free anastomosis that exists 

 between them, and by the numerous points at which they commu- 

 nicate with the veins of the cranium, and through these with the 

 superficial veins of the scalp. 



The spinal veins are extremely numerous and complicated. A 

 very intricate venous plexus surrounds the dura mater on its lateral 

 and posterior surfaces, embedded among the lobules of soft fat by 

 which the exterior of that membrane is invested. This plexus, less 

 intricate in the dorsal than in the cervical and lumbar regions, com- 

 municates very freely with a plexus of veins which surrounds the 

 exterior of the vertebral larninse and processes (the dorsi-spinal veins 

 of Dupuytren) . In front of the dura mater and situate between 

 the outer edge of the posterior common vertebral ligament and 

 the pedicles of the vertebrae, we find two remarkable venous si- 

 nuses which extend the whole length of the vertebral column from 

 the occipital foramen to the sacrum. They are the longitudinal 

 spinal sinuses of Willis. In calibre they present many inequalities, 

 being dilated at one part and constricted at another, according to 

 the number and size of the vessels which communicate immediately 

 with them. The sinuses of opposite sides are parallel to each other, 

 and communicate by transverse branches, which pass beneath the 

 posterior common ligament. These transverse branches are of va- 

 riable calibre, like the sinuses themselves, and are dilated at their 

 middle ; at which point they receive veins which emerge from the 

 spongy texture of the bodies of the vertebrae (basi-vertebral veins 

 of Breschet*). At the highest part of the vertebral canal, the 

 spinal sinuses communicate with the internal jugular veins ; in the 

 neck, they communicate with the deep and superficial vertebral 

 veins ; with the intercostal veins in the dorsal region, and with the 

 lumbar ones in the loins. 



The arachnoid is the serous membrane of the cranio-spinal cavity. 

 By its parietal layer it adheres to the dura mater, both of the cra- 

 nium and spine, and by its visceral layer to the brain and spinal 

 cord, with the intervention of the pia mater. The space between 

 these two layers is the arachnoid cavity. In most regions, an intei 1 - 

 val exists between the visceral layer of the arachnoid and the pia 

 mater, which is called the sub-arachnoid cavity. This space may be 

 demonstrated by driving air or coloured liquid beneath the visceral 

 layer of the arachnoid. In the spine the connexion of the arachnoid 

 and pia mater is very loose, being effected by some long filaments 

 * See his very beautiful illustrations of the venous system. 



