NERVOUS TISSUE 65 



The sarcolemma is apparently produced from embryonic connective 

 tissue. 



The cardiac muscle cells are at first spindle-shaped embryonic cells 

 which elongate more and more. In further differentiation their protoplasm 

 exhibits faint striations which pervade the cell as it grows in the great increase 

 in size. The rhythmic contractions begin long before the striations appear. 



IV. NERVOUS TISSUE. 



Nervous tissue has usually been described as being composed of two 

 distinct substances, nerve fibers and nerve cells. The modern view of the 

 nature of nerve tissue is, however, that the nerve cell and the nerve fibers 

 are to be considered together as one unit, called the neurone. The neurone 



FIG. 83. Diagram Showing the Arrangement of the Neurones or Nerve Units in the 

 Architecture of the Nervous System. (Raymdn y Cajal.) A, Pyramidal neurone of 

 -cerebral cortex; B, anterior horn motor cell of spinal cord; D, collateral branches of A; E, 

 medullary neurone with ascending axone; F, spinal-ganglion neurones; G, sensory axones 

 of F; I, collaterals of F in the cord. 



is embedded in, and supported by, a substance called neuroglia. This neurone 

 consists of a cell body, a number of branching processes termed dendrites, 

 and a long process running out from it, the neuraxone, or axone, which be- 

 comes eventually a nerve fiber. The nerve cell and the nerve fiber are parts 

 of the same anatomical unit, and the nervous centers are made up of those 

 units, arranged in different ways throughout the nervous system, figure 83. 

 5 



