ORIGIN OF NERVES. 569 



canse a prominence on the surface. At the point of the calamus scrip- 

 torius the nuclei are in contact at the middle line, but a little higher 

 up are separated by the hypoolossal nuclei. 



The spinal accessory nerve arises by a long series of roots, the 

 upper of which are attached lo the side of the medulla, below those of 

 the pneumogastric, while the remainder arise from the cervical portion of 

 the spinal cord, as low down as the sixth or seventh pair of nerves. The 

 upper roots pass inwards to a nucleus which lies on each side at the back 

 of, and close to, the central canal, and is continuous, above, with the 

 nucleus of the pneumogastric nerve. The lower roots pass through the 

 lateral columns of the cord to the grey substance and curve forwards 

 into the anterior cornu. 



The hypoglossal nerve arises by a series of fine roots attached to 

 the furrow between the anterior pyramid and the olivary body. They 

 pass backwards, through the inner part of the olivary body, to reach 

 their nucleus, a long column of nerve cells, the lower part of which lies 

 in front of the central canal on each side, and, higher up, comes forward 

 to form a prominence on the floor of the fourth ventricle, close to the 

 middle line. 



THE MEMBRANES OF THE BRAIlSr AND SPINAL COBD. 



The cerebro-spinal axis is protected by three mrmhranes, named also 

 meninges. They are : — 1. An external fibrous membrane, named the 

 dura mater, which closely lines the interior of the skull, and forms a 

 loose sheath in the spinal canal ; 2. An internal areolo-vaseular tunic, 

 the pia mater, which accurately covers the brain and spinal cord ; and 

 o. An intermediate 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. 



THE DURA IvIATEE,. 



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

 sidrrable thickness. Its inner surface, turned towards the brain and 

 spinal cord, is smooth and lined with epithelium, which has been 

 generally regarded as constituting a parietal reflection of the arachnoid 

 membrane. The space between the dura mater and arachnoid, formerly 

 in like manner regarded as the sac of the arachnoid, has been conve- 

 niently 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 crauium it adheres to the inner surflice of the bones, and 

 forms their internal periosteum. The connection between the two 

 depends, in a great measure, on blood-vessels and small fibrous pro- 

 cesses, 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, 

 aud perforated by numerous foramina, through which the dura mater 

 is prolonged to the outer surface, being there continuous with the peri- 

 cranium. The fibrous tissue of the dura mater becomes blended with the 

 areolar sheath of the nerves at the foramina which give exit to them. 



