400 



THE NEKVOrrS SYSTEM 



as in the brain, but is separated from their periosteum 

 by a layer of fat and a network of blood vessels. It does 

 not send partitions into the cord as it does into the brain. 



The arachnoid is also not in close contact with the pia 

 mater, but forms a loose bag around it. The pia mater 

 closely invests the surface of the cord and, as in the brain, 

 supplies it with blood. 



The cord itself shows two fissures, a wide shallow anterior 

 one and a narrow deep posterior one, thus dividing it into 

 two right and left halves. The posterior fissure is not 

 a true groove but rather a partition of connective tissue. 



Each half of the cord gives off at intervals, from the top 

 and bottom, nerve roots which unite at a short distance 



Fig. 188 — Section of spinal cord showing nerve relations. 



from the cord into common nerve trunks. (See Fig. 188.) 

 These trunks or spinal nerves are arranged in pairs in the 

 cervical and dorsal regions and emerge through openings 

 between the vertebrae. At the lower end of the cord they 

 are crowded together in parallel bundles. There are, in 

 all, thirty one pairs of these nerves. Each spinal nerve 

 trunk splits up into many smaller nerves and these into 

 still smaller nerves which are distributed to various parts 



