NERVOUS TISSUE. 97 



membrane and the sarcoplasm, which has been spoken of 

 above, can be observed. 



It seems that the same hypothesis (MacCallum) is applica- 

 ble here as was suggested for the development of heart muscle. 

 It simplifies the conception of striated muscle very greatly to 

 consider the fibril bundles and the membranes bounding the 

 compartments of sarcoplasm as derived from the primitive net- 

 work found in the muscle cells of young embryos. The later 

 stages in this development of heart muscle and voluntary 

 muscle differ somewhat on account of the differences in the 

 adult tissues. But since the beginning of the differentiation is 

 the same, the development of the power of contraction must 

 run a somewhat similar course. If this be so, it is conceivable 

 that the contractions in definite directions begin when the 

 irregular network of the primitive cell becomes strengthened 

 in these directions by an accumulation of the substance of the 

 network to form fibril bundles. Why this should take place 

 first around the periphery of the cell is not clear. It is true, 

 however, not only in the development of heart muscle and 

 voluntary muscle cells, but also in the evolution of the heart 

 muscle cell in lower animals. 



IV. NERVOUS TISSUE. 



The essential constituents of the nervous system are nerve 

 cells and nerve 'fibres. Formerly the latter were considered as 

 separate elements, but now are recognized generally as processes 

 of the nerve cells. It is a characteristic feature of nerve cells 

 that at least one process proceeds from each. Usually there are 

 many of such processes, one of which always becomes a nerve 

 fibre, the so-called axis-cylinder process, Deiter's process, or 

 axone. The rest are known as protoplasmic processes or den- 

 drites. Independent nerve fibres do not exist in the animal 

 organism. They are in every case in connection with cells. 

 Thus the nerve cell, axone, and dendrites together form a nerve 

 unit which is known as the neurone (Waldeyer). The nervous 

 system is made up of such units. 



7 



