CHAPTEE XLI 



THE CENTRAL NERVOUS SYSTEM 



WE already know sufficient from our preliminary study of nerve- 

 centres to be aware that the central nervous system is divided into 

 the two main parts called the brain and spinal cord. 



Fig. 445 shows the general arrangement of the cerebro-spinal 

 axis, and some anatomical details concerning the membranes that 

 envelop the brain and cord may here conveniently be added. 



Membranes of the Brain and Spinal Cord. The Brain and Spinal Cord are 

 enveloped in three membranes (1) the Dura Mater, (2) the Arachnoid, (3) the Pia 

 Mater. 



(1) The Dura Mater, or external covering, is a tough membrane composed of 

 bundles of connective tissue which cross at various angles, and in whose interstices 

 branched connective-tissue corpuscles lie : it is lined by a thin elastic membrane, 

 on the inner surface of which is a layer of endothelial cells. 



(2) The Arachnoid is a much more delicate membrane, very similar in structure 

 to the dura mater, and lined on its outer or free surface by an endothelial mem- 

 brane. 



(3) The Pia Mater of the cord consists of two layers between which numerous 

 blood-vessels ramify. In that of the brain only the inner of the two layers is repre- 

 sented. Between the arachnoid and pia mater is a network of fibrous tissue 

 trabeculae sheathed with endothelial cells : these sub-arachnoid trabeculae divide up 

 the sub-arachnoid space into a number of irregular sinuses. There are some similar 

 trabeculae, but much fewer in number, traversing the sub-dural space, i.e., the space 

 between the dura mater and arachnoid. 



Pacchionian bodies are growths from the sub-arachnoid network of connective- 

 tissue trabeculae which project through small holes in the inner layers of the dura 

 mater into the venous sinuses of that membrane. The venous sinuses of the dura 

 mater have been injected from the sub-arachnoidal space through the intermediation 

 of these villous outgrowths. 



In the chapters preceding this one we have seen how all-per- 

 vading nervous action is ; in connection with circulation, respiration, 

 secretion, peristalsis, etc., the way in which such functions are 

 regulated by nervous activity has taken up a considerable amount of 

 space. Some of the facts there described will be better understood, 

 or be seen in a clearer light, if the student turns back to them and 

 studies them once more after he has grasped what we are going to 

 consider in the chapters that follow this on the physiology of the 

 central nervous system. 



It would also be advisable, before he begins this subject, that he 



