78 HISTOLOGY 



secretion of the cell and a substance particularly sensitive to several 

 chemical or physical laws which rule that, when warmer, the substance 

 must absorb water, and that when such a substance absorbs water it 

 must become thicker and shorter. 



Then, assuming that the plasma of the muscle cell, which surrounds 

 all the fibrils in it, is filled with some secretion substance that gives heat 

 when combined with oxygen, and, assuming that the motor nerve impulse 

 causes oxygen to unite with this substance, we can imagine the sudden 

 warming of the watery plasma by the rapid oxidation of the heat secre- 

 tion and the sudden shortening of the fibrils by the absorption of water. 



This explanation does away with all unknown vital factors, except 

 a secretory activity of protoplasm that enables it to produce the easily 

 oxidizable material, and a nerve stimulus that can cause oxygen to sud- 

 denly unite with this substance. The secretion power and the nerve 

 stimulus remain to be explained. 



Another but less satisfactory explanation of more remote causes is 

 Schaefer's theory whereby the isotropic or light substance is supposed 

 to retire into a series of longitudinal channels in the anisotropic sub- 

 stance, thus swelling and shortening the latter. This process involves 

 electrical and chemical changes. It is not as clear as the first. 



The cell-membrane which surrounds a muscle cell is known as the 

 sarcolemma. In a large number of forms this membrane is reenforced 

 by the closely applied bodies of connective-tissue cells, in which case 

 there has been no change of name and the entire covering is still called 

 the sarcolemma. In most cases it is practically impossible to demon- 

 strate a sarcolemma apart from this connective tissue. 



A muscle cell is always in bodily connection with a nerve cell which 

 controls it. The organ by which this contact is effected is known as a 

 motor nerve ending. 



The muscle cells are of several shapes (Fig. 79). Spindle shape and 

 tapering to pointed ends (D), elongated cylinders with blunt ends (B), 

 cubical cells arranged end to end (A) are the shapes usually found, 

 while epithelial cells with the base flattened out and converted into 

 muscle (C) and branching cells are some of the unusual forms. The 

 spindle-shaped, cubical, and epithelial cells are usually mononuclear, 

 while the cylindrical are nearly always multinuclear cells or syncytia. 



In many small and simply organized animals the muscle cells are 

 placed apparently at random through the tissues of the body or in layers 

 on its surface. At the point where the animal attains to developed 

 powers of movement, however, these scattered fibers become localized 

 into groups which are placed in positions where their force can be exerted 

 to the best advantage. Such groups are called the muscles. They can 

 operate to move the body in two principal ways. The first is without 



