CHAPTEE V 

 THE MUSCULAR TISSUES 



THE musculature of the body includes not only the skeletal 

 muscles, but certain portions of the wall of the hollow viscera, such 

 as the respiratory, alimentary, and urinary tracts. The skeletal 

 muscles contain the most highly developed type of muscle cell, the 

 visceral organs the lowest or least differentiated type ; the muscle 

 of the heart is intermediate between these, and stands in a class by 

 itself. 



Thus we distinguish threejfcypes of muscular tissue, the xolun-. 

 tary or striated, the involuntary smooth or plain, and the cardiac. 

 These types are all of mesoblastic origin, the first muscular tissue 

 to make its appearance being the " myotomes " from which the 

 striated fibres of the skeletal muscles are developed. Cardiac 

 muscular tissue next appears, the anlages of the visceral muscu- 

 lature being formed at a still later period. The completed devel- 

 opment of the three types of muscle may be said to correspond, in 

 a way, to the priority of their appearance, the smooth muscle cells 

 being the least and the striated cells the most altered or differen- 

 tiated from the primitive cell type. 



The physiological classification of muscle into the voluntary 

 and involuntary varieties, while it corresponds quite closely with 

 the striated and smooth types of muscle cell, is not exactly coinci- 

 dent with this histological division. Inasmuch as the terms vol- 

 untary and involuntary refer not to any structural peculiarity of 

 the cell, but rather to the form and mechanism of its nervous con- 

 trol, it is not to be expected that such a classification based upon 

 its physiological relation to the nervous system would be identical 

 with the histological types of muscle cell. Nevertheless, the stri- 

 ated type of muscle is found to be usually under voluntary, and 

 the smooth and cardiac types under involuntary control. 



SMOOTH MUSCLE TISSUE (plain, non-striated, or involun- 

 tary). This type of muscular tissue consists of small fusiform 



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