CHAPTEK VI 



MUSCULAR TISSUE 



MUSCLE is popularly known as flesh. It possesses the power of con- 

 traction, and is, in the higher animals, the tissue by which their 

 movements are executed. The muscles may be divided from a 

 physiological standpoint into two great classes, the voluntary muscles, 

 those which are under the control of the will, and the involuntary 

 muscles, those which are not. The contraction of the involuntary 

 muscles is, however, controlled by the nervous system, only by a 

 different part of the nervous system from that which controls the 

 activity of the voluntary muscles. 



When muscular tissue is examined with the microscope, it is 

 seen to be made up of small, elongated, thread-like structures, which 

 are called muscular fibres ; these are bound into bundles by connective 

 tissue, and in the involuntary muscles there is in addition a certain 

 amount of cement substance, stainable by nitrate of silver, between 

 the fibres. 



The muscular fibres are not all alike; those of the voluntary 

 muscles are seen by the microscope to be marked by alternate dark 

 and light stripings or striations ; these are called transversely striated 

 muscular fibres. The involuntary fibres have not got these markings 

 as a rule. There is one important exception to this rule, namely, in 

 the case of the heart, the muscular fibres of which are involuntary, 

 but transversely striated. There are, however, histological differ- 

 ences between cardiac muscle and the ordinary voluntary striated 

 muscles. The unstriated involuntary muscular fibres found in the 

 walls of the stomach, intestine, bladder, blood-vessels, uterus, and 

 other contractile organs are generally spoken of as plain muscular 

 fibres. 



From the histological standpoint there are, therefore, three 

 varieties of muscular fibres found in the body of the higher 

 animals: two of them are transversely striated, and one is not. 

 The relationship of this histological classification to the physiological 



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