CHAPTER X. 



THE NERVOUS TISSUES. 



SECT. 1. INTRODUCTORY. 



ciassifica- THE organs which compose the nervous system of the 

 tion of nerve- higher animals may be classed as 1st central organs, 

 organs. ^^ ^ ^ Q brain and spinal cord and the various 



peripheral or sporadic ganglia ; 2nd conducting structures or nerves 

 which are engaged in bringing into communication the central organs 

 with, 3rd, end-organs wherein the fibres of certain of the nerves 

 (afferent) commence, and those of certain others (efferent) terminate. 



Grey and ^he large nerve-centres are composed of grey and 



white matter white matter, properly so called. In the grey matter 

 of the nervous re side the nerve-cells which are the characteristic ele- 

 ments of the central organs, and which for the most 

 part certainly have a connection direct or indirect with nerve- 

 fibres. 



The white matter is composed of nerve-fibres, making their way 

 to and from the grey matter, and only very exceptionally contains 

 nerve-cells. Both grey and white matter are supported by a connec- 

 tive-tissue framework termed the neuroglia ; both are supplied with 

 blood-vessels which penetrate from the surrounding membranes, 

 though the grey matter is much more vascular than the white ; 

 in both we can trace the commencement of lymphatic vessels. 



Nerve-cells. 



Nerve-cells are irregular masses of protoplasm, possessed of a well- 

 marked nucleus and nucleolus, and sending out one or more processes. 

 The protoplasm of the cell is often somewhat pigmented (greyish) ; in. 

 the nerve-cells of the ganglia of Aphrodite aculeata it has been shewn 

 to be tinged of a red colour, due to the presence of haemoglobin. That 

 part of the protoplasm which immediately surrounds the nucleus is 

 granular, while, in most cases, that which is disposed at the periphery 

 of the cell exhibits a striated appearance which seems to be similar 

 to, and indeed continuous with, that often exhibited by the axis 

 cylinders of the nerves. It is beyond the province of this book to 



