LESS. XI COVERINGS OF C. N SYSTEM 453 



another and with the cerebro-spinal nerves. (See 

 Fig. 142.) 



Nerves are made up entirely of nerve-flbres, the 

 structure of which is somewhat different in the cerebro- 

 spinal and in the sympathetic systems. (See p. 459 ) 

 Nerve centres, on the other hand, are composed of 

 nerve-cells mingled with nerve-fibres. (See p. 466.) 

 Such nerve-cells are found in various parts of the brain and 

 spinal cord, in the sympathetic ganglia, and also in the 

 ganglia belonging to spinal nerves as well as in certain 

 sensory organs, such as the retina and the internal ear. 



2, The Investing Membranes of the Cerebro-Spinal 

 System. — The brain and spinal cord lie in the cavity of 

 the skull and spinal column, the bony walls of which 

 cavity are lined by a very tough fibrous membrane, 

 serving as the periosteum of the component bones of this 

 region, and called the dura mater. This is composed of a 

 thick layer of densely interwoven fibres of connective 

 tissue, with which a small amount of elastic tissue is mixed. 

 The brain and spinal cord themselves are closely invested 

 by a very vascrdar membrane of fibrous connective tissue, 

 called pia mater. The numerous blood vessels supplying 

 these organs run for some distance in the pia mater, and 

 where they pass into the substance of the brain or cord, 

 the fibrous tissue of the pia mater accompanies them to a 

 greater or less depth. 



Between the pia mater, and the dura mater, lies another 

 delicate membrane, called the arachnoid membrane. 

 These three membranes are connected with each other at 

 various points, and the arachnoid, which is not only very 

 delicate, but also less regular than the other two, divides 

 the space between the dura and pia mater into two spaces, 

 each containing fluid, and each more or less lined by a 

 delicate epithelium. The space between the dura mater 

 and the arachnoid, often called the subdural space, is 

 nowhere very large ; but the space between the arachnoid 

 and the pia mater, often called the subarachnoid 



