THE STRUCTURE OF THE ELEMENTARY TISSUES. 91 



Development. (1.) Unstriped. The cells of unstriped muscle are 

 derived directly from embryonic cells, by an elongation of the cell, and 

 its nucleus; the latter changing from a vesicular to a rod shape. 



(2.) Striped. Formerly it was supposed that striated fibres were 

 formed by the coalescence of several cells, but recently it has been 

 proved, that each fibre is formed from a single cell, the process involv- 

 ing an enormous increase in size, a multiplication of the nucleus by fis- 

 sion, and a differentiation of the cell-contents. This view differs but 

 little from another, that the muscular fibre is produced, not by multi- 

 plication of cells, but by arrangement of nuclei in a growing mass of 

 protoplasm (answering to the cell in the theory just referred to), which 

 becomes gradually differentiated so as to assume the characters of a fully 

 developed muscular fibre. 



Growth of Muscle. The growth of muscles, both striated and non- 

 striated, is the result of an increase both in the number and size of the 

 individual elements. In the pregnant uterus the fibre-cells may become 

 enlarged to ten times their original length. In involution of the uterus 

 after parturition the reverse changes occur, accompanied generally by 

 some fatty infiltration of the tissue and degeneration of the fibres. 



IV. Nervous Tissue. 



Nervous tissue has usually been described as being composed of 

 two distinct substances, nerve-fibres and nerve-cells. The modern 

 view of the nature of nerve-tissue is, however, that it is composed 

 of one element alone, called the neuron or nerve unit, embedded in 

 and supported by a substance called neuroglia. This neuron consists of 

 a cell-body, a number of branching processes termed dendrites, and a 

 long process running out from it, the neuraxon, which becomes eventu- 

 ally a nerve-fibre. The nerve-cell and the nerve-fibre, are really parts of 

 the same anatomical unit, and the nervous centres are made up of these 

 units, arranged in different ways throughout the nervous system (fig 94A). 

 The different neurons do not unite anatomically with each other, but 

 form independent units. A further description of these structures will 

 be given later. 



Nerve-Fibres. 



While the nerve-fibre is really to be considered as a process of the 

 nerve-cell, it is convenient to describe it separately. 



Varieties. Nerve-fibres are of two kinds, medullated or white fibres* 

 and non-medullated or gray fibres. 



Medullated Fibres. Each medullated nerve-fibre is made up of 



