CHAPTER XXIII 

 THE NERVOUS SYSTEM 



A. ITS TISSUES AND DEVELOPMENT 



THE nervous system is readily divisible into two anatomical 

 portions, the central and the peripheral. 



The central nervous system (cerebro-spinal axis) includes, as 

 its more important gross divisions, the cerebrum or telencephalon, 

 its large basal nuclei (optic thalmi, etc.) or diencephalon, the crura 

 cerebri and corpora quadrigemina or mesencephalon, the pons 

 Varolii and cerebellum or metencephalon, the medulla oblongata 

 or myelencephalon, all of which lie within the cranial cavity and 

 are collectively called the brain, and the spinal cord or myelon 

 which is contained within the medullary cavity of the vertebral 

 column. 



The peripheral nervous system includes the cranial and spinal 

 nerve trunks with their cerebro-spinal ganglia, and the sympa- 

 thetic nerve trunks and ganglia, together with their peripheral 

 nerve endings, the motor and sensory end organs. These portions 

 have been already described in Chapters VIII and IX. 



Though the above anatomical divisions are macroscopically 

 distinct and are of great convenience in description, it must be 

 borne in mind that the histological elements, the cell units called 

 neurones, are not confined to any one gross division, but may, as a 

 nerve cell with its many processes, be traced in direct anatomical 

 continuity through several such gross divisions. Some neurones, 

 for example, whose cell bodies lie in the posterior root ganglia of 

 the spinal nerves may be followed throughout a peripheral nerve 

 trunk on the one hand, while on the other it sends a process cen- 

 tralward which enters the spinal cord and passes all the way to the 

 medulla oblongata. 



The central nervous system is said to consist of grey and white 

 matter, the grey matter being composed chiefly of nerve cells with 



