DIFFERENTIATION OF THE ANIMAL BODY. IIQ 



receive the stimuli from the outside world (Fig. 8 ; 6 and 14) ; and 

 they are able to produce response to these stimuli only because they 

 are connected with muscles (Fig. 2 5, n.m) . Without the nervous tissue, 

 the muscles' power of contracting could have no definite value to 

 the animal; without the nerves and the muscle the sensitiveness of 

 the end organs in the epithelium would amount to nothing. It 

 really takes them all to furnish a complete function. 



Nervous tissue consists of very complex and highly differentiated 

 cells probably the most highly specialized cells to be found in the 

 body. The cells vary very greatly, but they agree essentially in 

 the fact that the cell body (or ganglion) produces one or more out- 

 growths (or fibres) by which the ganglion is connected with other 

 ganglia, or with muscles, or with sensory epithelium. The ganglion 

 contains the nucleus of the cell and is the center of the activity of 

 the entire cell. The fibres are merely outgrowths which conduct 

 impulses to and from the ganglia. 



The ganglion cells are massed together in nerve centres, such as 

 the brain. The term ganglion is also applied to one of these groups 

 of cells. When several fibres are bound together they are called a 

 nerve. 



The ganglion cell and its various fibres are to be looked upon as a 

 unit. Fig. 25 shows the parts of such a unit. The whole nervous 

 system of an animal is made up of numerous units of this type more 

 or less joined together. 



Nervous tissue arises in the embryo from the ectoderm. It is so 

 highly specialized that its cells are probably incapable of division 

 after they are once differentiated. This means that all the nervous 

 cells are produced early in life, and that under ordinary circumstances 

 no new ones are formed. Probably all we can do with them is to 

 educate more highly those that we have. 



131. Organs. The tissues which have just been described are 

 rarely, if ever, pure and independent; but are associated with one 

 another in different ways. For example, connective tissue pene- 

 trates and binds together the nervous structures and the muscles. 

 Nerves penetrate into tissues of all kinds. Covering almost all 

 kinds of tissues we find epithelial cells. Whenever we have such a 

 mixture of different tissues, bound together for a common work, 

 as the heart or the stomach or the brain, we call it an organ. Organs 

 may be associated into systems of organs, as the nervous system, the 

 muscular system, the skeletal system. In such systems some type 

 of tissue and function is predominant in importance. The other 

 tissues present merely aid in this chief work. 



