CHAPTER V. 



THE mus(;le tissues. 



The most oiitstan<linjj; functional feature of muscle tissue is the 

 capacity to contract and consequently it plays an important i)art 

 in all movements of an organism. Associated with the functional 

 features are intracellular thread-like structures, the myofibrillte, 

 which are considered to be the contractile elements. These are 

 embedded in a more fluid cytoplasm, the sarcoplasm. On the 

 basis of structural ditt'erences, smooth muscle, cardiac muscle, and 

 skeletal muscle are distinguished. Both cardiac and skeletal muscle 

 fibrilht have alternating dark and light cross-striations and are 

 often classified as striated muscle in contrast to the smooth muscle 

 in which such striations do not appear. 



SMOOTH MUSCLE. 



This ty])e shows a very close association with connective tissue 

 and is widely distributed through the vertebrates, occurring in the 

 wall of the alimentary tract, in the arteries and veins, and in numer- 

 ous other ducts. It is apparently the least differentiated type of 

 muscle, is involuntary, anrl appears widely in invertebrates in places 

 where, from vertebrate studies, we would expect to find skeletal 

 muscle. The histological unit is easily identified as the smooth 

 muscle cell which is fusiform in shape, though varying greatly in 

 length and breadth. 



Embryologically, mesenchyme cells in the region where smooth 

 muscle will develop begin to elongate. A multiplication of such 

 cells, called myoblasts, give rise to a network and finally sheets of 

 smooth muscle cells are formed. The reticular and loose fibroelastic 

 connective-tissue network associated with these differentiating and 

 later fully developed muscle cells is derived from mesenchyme cells 

 similar to those gi^'ing rise to the myoblasts. Even in the adult 

 vertebrate it is believed that smooth muscle cells may be derived 

 from undifferentiated mesenchyme cells in connective tissues. 



The myoblasts become more and more elongate with development 

 and appear still connected laterally by cytoplasmic processes, as 

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