104 



STRUCTURE OF THE VERTEBRATES 



in the adult to divide and reproduce themselves, although cut 

 ends of fibers may be regenerated. The "growth" of a muscle 

 as a result of exercise is not true growth in the sense that the 

 number of fibers is increased, but depends upon the individual 

 cells adding fluids. The muscle increases in size without an in- 

 crease of the elements of which it is made. The three types of 

 muscle tissue differ functionally and structurally. 



1. Striated or skeletal muscle forms the greater part of the 

 bulk of the body. Each individual fiber is very long, some be- 

 ing tw^elve or fifteen centimeters in length, and is syncytial in 

 structure. Under the microscope these fibers appear transversely 

 striped, light and dark bands of protoplasm alternating. The 

 fiber is surrounded by a very thin sheath of connective tissue; 

 and in the protoplasm are the numerous nuclei. These muscles 

 are voluntary in function, that is, they are under the control of 

 the central nervous system, and can be contracted at will. 



2. Smooth or visceral muscles are those which are involun- 

 tary in function. They contract slowly when stimulated and are 

 not under the direct control of the brain. The cells are small, 

 spindle-shaped, and each has a single nucleus. 



3. Cardiac muscle is found in the heart and combines the 

 functions of striated and smooth. It is involuntary in action, 

 but responds rapidly to stimuli. Cardiac muscle is usually de- 

 scribed as a syncytium, the protoplasm extending continuously 

 throughout the mass. Microscopically the tissue appears divided 

 into irregular cells which are striated, each with a single nucleus. 

 Physiologically the heart is a compact unit, nerve impulses trav- 

 eling rapidly over its entire area. 



E. Nerve Tissues are ectodermal in origin and are discussed 

 fully in the proper chapter. The individual nerve cell has a cell 



Fig. 45. A typical Nerve cell (Neurone) 



