*2 50 INNEEVATION. [CHAP. x. 



the sacral canal, and is tied down in that situation by certain fili- 

 form processes, of which the central one is attached to the coccyx. 

 The spinal dura mater is evidently much more capacious than is 

 necessary merely to contain the spinal cord, and therefore it gene- 

 rally has a loose and flaccid appearance. During life it is kept 

 tense by the fluid which surrounds the cord. The dimensions of 

 the canal of the dura mater vary with those of the spinal canal. 

 It is wider, therefore, in the neck and loins, and narrow in the 

 back. In the lumbar and sacral regions it forms a wide sac around 

 the leash of nerves called cauda equina. 



The dura mater, in both cranium and spine, exhibits numerous 

 perforations for the transit of nerves from the contained centre to 

 the peripheral parts. In the spinal region each of these per- 

 forations is subdivided, by a vertical slip of the membrane, into 

 two foramina, which correspond to the anterior and posterior roots 

 of the spinal nerves, and extend on each side down to the lower 

 part of the sacral region. 



The blood-vessels of the dura mater are very numerous. Those 

 of the cranial membrane communicate freely with those of the bones. 

 Hence, when separated from the cranium, the external surface of 

 the dura mater has a rough appearance, from the number of blood- 

 vessels which have been torn. This membrane derives its supply of 

 arterial blood from the ophthalmic and ethmoidal arteries in front ; 

 in the middle, from the internal maxillary artery by the middle 

 meningeal, which enters the cranium by the foramen spinosuin, 

 and by small branches from the internal carotid, called the infer u 

 meningeal arteries. Posteriorly the vertebral, the occipital, and the 

 ascending pharyngeal supply branches which go by the name 

 posterior meningeal arteries. The arteries of the spinal dura mater 

 come from the deep cervical, the occipital, and the vertebral, in the 

 cervical region, from the intercostals in the back, and from the 

 lumbar arteries in the loins. These vessels pass in at the vertebral 

 foramina, and send branches to the bones as well as to the spii 

 membranes. 



The veins of the dura mater in the cranium constitute a remark- 

 able portion of the vascular system of that cavity. The venous 

 radicles collect the blood from the dura mater, and from the bones 

 of the skull ; large veins are formed on the former membrane, and 

 distinct canals in the osseous diploe, figured at page 107. These all 

 tend to certain venous channels, rigid canals, formed by the separa- 

 tion of two layers of the dura mater, which are lined by processes 

 of the inner membrane of the venous system, and are so placed as 



