670 THE NERVOUS SYSTEM. 



soft fat and a plexus of thin-walled veins. In connexion with the spinal dura 

 mater there are no blood-sinuses such as are present in the cranial cavity, but it 

 should be noted that the veins in the epidural space, placed as they are between 

 the periosteum of the vertebral canal and tube of dura mater, occupy the same 

 morphological plane as the cranial blood-sinuses. Another feature which serves to 

 distinguish the spinal dura mater from the cranial dura mater consists in the fact 

 that it gives off from its deep surface no partitions or septa. 



The cylindrical tube of spinal dura mater does not lie quite free within the 

 vertebral canal. Its attachments, however, are of such a character that they 

 in no way interfere with the free movement of the vertebral column. On each 

 side the spinal nerve-roots, as they pierce the dura mater, carry with them into the 

 intervertebral foramina tubular sheaths of the membrane, whilst in front loose fibrous 

 prolongations more numerous above and below than in the thoracic region 

 connect the tube of dura mater to the posterior longitudinal ligament of the vertebral 

 column. No connexion of any kind exists between the dura mater and the posterior 

 wall of the vertebral canal. 



When the interior of the tube of spinal dura mater is inspected, the series of 

 apertures of exit for the roots of the spinal nerves is seen. These are ranged in 

 pairs opposite each intervertebral foramen. 



Viewed from the inside of the tube of dura mater, each of the two roots of a 

 spinal nerve is seen to carry with it a special and distinct sheath. When examined 

 on the outside, however, the appearance is such that one might be led to conclude 

 that both roots are enveloped in one sheath of dura mater. This is due to the fact 

 that the two sheaths are firmly held together by intervening connective tissue. 

 The two tubular sheaths remain distinct as far as the ganglion on the posterior 

 root, and then blend with each other. 



Cavum Subdurale. The dura mater and the arachnoid are closely applied 

 to each other, and the capillary interval between them is termed the subdural 

 space. It contains a minute quantity of fluid, which is just sufficient in amount to 

 moisten the opposed surfaces of the two bounding membranes. 



The subdural space in no way communicates with the subarachnoid space. 

 The fluid which it contains is led into the venous blood-sinuses around the arach- 

 noideal granulations (O.T. Pacchionian bodies), and thus gains exit. The subdural 

 space is carried outwards for a very short distance on the various nerves which are 

 connected with the brain and the spinal medulla, and it has a free communication 

 with the lymph-paths present in these nerves. In the case of the optic nerve the 

 sheath of dura mater is carried along its whole length, and with it the subdural 

 space is likewise prolonged to the back of the eyeball. .' 





 ARACHNOIDEA. 



The arachnoid is a very thin membrane, remarkable for its delicacy and trans- 

 parency, which envelops both the brain and the spinal medulla between the dura 

 mater and the pia mater. The cranial part of the arachnoid or the arachnoidea 

 encephali, except in the case of the longitudinal and the lateral fissures, does not 

 dip into the sulci on the surface of the brain. In this respect it differs from the 

 pia mater. It bridges over the inequalities on the surface of the brain. Conse- 

 quently, on the basal aspect of the encephalon it is spread out in the form of a very 

 distinct sheet over the medulla oblongata, the pons, and the hollow which lies 

 in front of the pons, and in certain of these regions it is separated from the brain- 

 surface by wide intervals. 



The spinal part of the arachnoid or arachnoidea spinalis, which is directly 

 continuous with the cranial arachnoidea, forms a loose wide investment for the 

 spinal medulla. This arachnoideal sac is most capacious towards its inferior part, 

 where it envelopes the inferior end of the spinal medulla and the collection of long 

 nerve-roots which constitute the cauda equina. 



As the nerves, both from the brain and the spinal medulla, pass outwards they 

 receive an investment from the arachnoid, which runs for a short distance upon 

 them and then comes to an end. 



