40 ESSENTIALS OF PHYSIOLOGY. 



to the active tissues of the body. The consideration of the mental 

 processes and the emotions is the province of the sister science of 

 Psychology. 



The nervous system consists of the brain and spinal cord (spinal 

 medulla), with the nerves by means of which these central organs are 

 connected with the other structures of the body. Microscopic examina- 

 tion of the brain and spinal cord shows that they are built up of 

 nerve cells and nerve fibres, supported by a special form of con- 

 nective tissue called neuroglia. 



The nerves contain nerve fibres only, held together by ordinary 

 connective tissue. Every nerve fibre, however, is a process of a nerve 

 cell ; and if a nerve -fibre is divided, the part which is no longer in 

 connection with the nerve cell undergoes degeneration. The histo- 

 logical unit of the nervous system is therefore the nerve cell with 

 its processes, and this unit is known as a neuron. 



The Neuron. Every neuron consists of a nerve cell, known as a 

 cyton, and its processes. In the simplest form the cyton is bipolar, 

 with one process connected with each pole. Examples of this type 

 are found in the spinal ganglia of fishes and in the ganglion on the 

 auditory nerve in man. A modification of this form occurs in the 

 human spinal ganglia, the two processes becoming fused together for 

 a short distance in the course of development, so that the cell is 

 histologically unipolar, though physiologically it is still a bipolar cell. 

 A third form is multipolar, there being more than two processes 

 connected with each cell. One of these processes is unbranched, and is 

 known as the axon ; the others are branched, and are called dendrons. 

 The neurons which enter into the structure of the brain and spinal 

 cord belong to the multipolar type, varying however in the shape of 

 the cyton, the numb'er and character of the dendrons, and the length 

 of the axon in different regions. 



The cyton, although differing in shape and size in different regions, 

 is always distinguished by certain definite characteristics. It is usually 

 large as compared with other cells, varying in diameter from 20 to 100 /A. 

 It possesses a large, spherical nucleus which contains little chromatin, 

 and within the nucleus a well-marked nucleolus. The cell substance 

 is distinguished by the presence of certain bodies known as Nissl 

 spindles and also by having delicate fibrils running through it. The 

 Nissl spindles are granular aggregations in the form of minute spindles, 

 as the name implies, are arranged more or less concentrically in relation 

 to the nucleus, and stain well with methylene blue or toluidin blue- 

 They appear to consist mainly of nucleoproteiri, and also contain some 

 iron. They are found in all parts of the cyton and in the basal parts 



