THE DURA MATER, 



181 



THE MEMBRANES OF THE BRAIN" AND SPINAL CORD. 



The cerebro-spinal axis is covered by three membranes, named also meninges. 

 They are : 1. An external fibrous membrane, named the dura mater, which lines 

 the interior of the skull, and forms a loose sheath in the spinal canal ; 2. An internal 

 areolar and vascular tunic, the pia mater, which closely covers the brain and spinal 

 cord; and 3. An intermediate non-vascular membrane, the arachnoid, which lies 

 over the pia mater, the two being in some places in close connection, in others 

 separated by a considerable space. 



Some authors describe only two meninges, considering the arachnoid and pia mater to 

 constitute one (leptomeninx), and the dura mater the other (jjachym-eninx). 



THE DURA MATER. 



The dura mater is a very strong dense inelastic fibrous tunic of considerable 

 thickness ('5 mm. or more in the cranium, less in the spinal canal). Its inner 

 surface, turned towards the brain and spinal cord, is smooth and lined with 



Fig. 128. SECTION THROUGH THE PLACK 



OF EXIT OF A SPINAL NERVE-HOOT 

 THROUGH THE DURA MATER. (Key 



and Retzius.) 



a, bundles of the nerve-root becoming 

 collected into a single bundle as they 

 emerge ; b, dura mater ; c, arachnoid ; 

 d, a reticular lamella of the arachnoid 

 reflected along the nerve-root ; s, sub- 

 dural space ; s', s', subarachnoid space. 



epithelium (endothelium), which 

 was formerly regarded as a parie- 

 tal reflection of the arachnoid 

 membrane, this having been 

 generally looked upon as a 

 serous membrane. The space 

 between the dura mater and 

 arachnoid was formerly in like 

 manner regarded as the sac of 

 the arachnoid, but is now con- 

 veniently termed the subdural 

 space. The outer surface of the 



dura mater is connected with the surrounding parts in a somewhat different manner 

 in the cranium and in the spinal canal. 



In the cranium it adheres to the inner surface of the bones, and forms their 

 internal periosteum. The connection between the two depends, in a great measure, 

 on blood-vessels and small fibrous processes, which pass from one to the other ; and 

 the dura mater, when detached and allowed to float in water, presents a flocculent 

 appearance on its outer surface, in consequence of the torn parts projecting from it. 

 The adhesion between the membrane and the bone is more intimate opposite the 

 sutures, and also at the base of the skull, which is uneven, and perforated by 

 numerous foramina, through which the dura mater is prolonged to the outer surface, 

 being there continuous with the pericranium. The fibrous tissue of the dura mater 

 becomes blended with the areolar sheath of the nerves at the foramina which give 

 exit to them. 



In leaving the skull, the dura mater is intimately attached to the margin of 



