84 MUSCULAR TISSUE. [CH. VI. 



CHAPTER VI. 



MUSCULAR TISSUE. 



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

 contraction, 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 volun- 

 tary 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 trans- 

 versely 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 differences 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 physio- 

 logical classification into voluntary and involuntary is shown in 

 the following table : 



1. Transversely striated muscular fibres : 



a. In skeletal muscle . . . . VOLUNTABY. 



b. In cardiac muscle . . . . \ 



2. Plain muscular fibres : I TVVOTTTNTARV 



In blood-vessels, intestine, uterus, f " OLU1 TA - K Y. 

 bladder, etc J 



