NERVOUS SYSTEM. 307 



NERVOUS SYSTEM. 



The nervous system consists of a central part, or rather a series of connected 

 central organs, named the cerebro-spinal axis, or cerebro-spinal centre ; and of the 

 nerves, which have the form of cords connected by one extremity with the cerebro- 

 spinal centre, and extending from thence through the body to the muscles, sensible 

 parts, and other organs placed in functional relation with them. The nerves form 

 the medium of communication between these distant parts and the centre. One 

 class of nervous fibres, termed afferent or centripetal, conducts impressions towards 

 the centre, another, the efferent or centrifugal, carries stimuli from the centre to 

 the periphery. 



Besides the cerebro-spinal centre and the nervous cords, the nervous system 

 comprehends also certain bodies named ganglia, which are connected with the 

 nerves in various situations. These bodies, though of much smaller size and less 

 complex nature than the cerebro-spinal centre, agree, in some respects, with that 

 organ in their elementary structure, and to a certain extent also in their relation 

 to the nervous fibres with which they are connected ; and this correspondence 

 becomes even more apparent in the nervous system of the lower members of the 

 animal series. 



The nerves are divided into the cerebro-spinal, and the sympathetic nerves. The 

 former are distributed principally to the skin, the organs of the senses, and other 

 parts endowed with manifest sensibility, and the muscles. They are, for the most 

 part, attached in pairs to the cerebro-spinal axis, and like the parts which they 

 supply are, with few exceptions, remarkably symmetrical on the two sides of the 

 body. The sympathetic nerves, on the other hand, are destined chiefly for the 

 viscera and blood-vessels, of which the movements are involuntary, and the natural 

 sensibility is obtuse. They differ also from the cerebro-spinal nerves in having 

 generally a greyish or reddish colour, in their less symmetrical arrangement, and 

 especially in the circumstance that the ganglia connected with them are much more 

 numerous and more widely distributed. Branches of communication pass from 

 many of the spinal nerves at a short distance from their roots, to join, and in fact 

 to form, the sympathetic, which is thus seen to be merely an offset from the cerebro- 

 spinal centre, as indeed its mode of development (see EMBRYOLOGY) would also 

 appear to show. 



The nervous system is made up of a substance proper and peculiar to it, with 

 inclosing membranes, nutrient blood-vessels and supporting connective tissue. 

 The nervous substance has been long distinguished into two kinds, obviously differ- 

 ing from each other in colour, and therefore named the white, and the grey or 

 ineritious. 



When subjected to the microscope, the nervous substance is seen to consist of 

 two different structural elements, viz., fibres and cells. The fibres are found uni- 

 versally in the nervous cords, and they also constitute the greater part of the 

 nervous centres : the cells on the other hand are confined in a great measure to the 

 cerebro-spinal centre and the ganglia, and do not exist generally in the nerves 

 properly so called, although they are found at the terminations of some of the 

 nerves of special sense, and also interposed here and there among the fibres of 

 particular nerves ; they are contained in the grey portion of the brain and spinal 

 cord, and in the ganglia. 



