364 ANATOMY FOR NURSES [Chap. XIX 



(b) Cranial nerves pass to and from the brain through openings 

 in or between the cranial bones, and are distributed to various 

 organs. (See page 395.) 



(c) Spinal nerves pass to and from the cord to different parts of 

 the body. (See page 380.) 



(2) The sympathetic system consists of masses of nerve cell- 

 bodies, and the nerves connected with them. These masses are 

 termed ganglia and are found in the thoracic and abdominal cavi- 

 ties. (See page 375.) 



Properties of nerve tissue. — All of the organs included in the 

 nervous system are made up of nerve tissue, which is the most 

 highly specialized tissue in the body. It possesses the following 

 marked characteristics: (1) irritability or the power to respond 

 to stimulation, and (2) conductivity or the power to transmit the 

 stimulus or nerve impulse to the muscles, viscera, etc. Just as all 

 other tissues are composed of cells, so the structural unit of nerve 

 tissue is the nerve-cell or neurone. 



NEURONES 



Although the neurones vary considerably in size and in form, 

 there are certain structural characteristics which they all possess 

 in common. They consist of : — 



(1) The cell-body. 



(2) The cell-processes. 



These two parts make up a complete nervous entity called a 

 neurone, and the entire nervous system consists of neurones 

 supported by neuroglia^ in the central nervous system, and by con- 

 nective tissue in the nerve trunks. 



(1) The cell-body. — The cell-bodies vary as to size and shape, 

 but all varieties present certain common characteristics. A topical 

 cell-body consists of a mass of granular cytoplasm surrounding a 

 large, well-defined nucleus, it in turn containing a nucleolus, and 

 the whole mass of cytoplasm may in some cases be surrounded 

 by a cell-wall. 



From the angles of the cell-body are given off the processes or 



poles, and the number of processes corresponds to the number 



of angles. Each cell-body usually has one process and may have 



several more. 



' See page 373. 



