44 



GENERAL PAET. 



In the first case we have to do with flat, spindle-shaped, or band- 

 shaped elongated cells, and with layers of such cells. They react 

 slowly to nervous stimuli : they enter the condition of contraction 

 gradually, and remain, contracted for some time. The contractile 

 substance appears for the most part to be homogeneous, but it is 

 sometimes longitudinally striated. The smooth muscles have the 



widest distribution amongst the 

 Invertebrata ; but they are also 

 found in vertebrates, in the walls 

 of numerous organs (vessels, ducts 

 of glands, intestinal wall) (fig. 35). 

 Cross-striped muscle consists 

 of cells, more frequently of multi- 

 nucleated so-called primitive bun- 

 dles. It is characterised by the 

 partial or complete transforma- 

 tion of its protoplasm into a cross- 



FIG. 35. a, smooth muscle fibres isolated. I, 

 piece of an artery (after Frey) ; 1, outer 

 connective tissue layer ; 2, the middle layer 

 formed of smooth muscle fibres ; 3, non-nu- 

 cleated inner layer. 



Cf 



- 

 warn iraiHumiiamra HUB',, .' , 



.. . 





! 



FiG.3G. a, Primitive fibre. i,cross-striped 

 muscle fibre (primitive muscle bundle) 

 of La cert;i with nerve termination. 



striped substance, consisting of 

 special doubly refracting elements 

 (sarcous elements) connected to- 

 gether by a simply refracting inter- 

 mediate substance (fig. 36, a, b~). 

 Physiologically, this form of mus- 

 cular tissue is characterised by the energetic and considerable 

 contraction which immediately follows its excitation, a property 

 which renders it especially suitable for the carrying out of powerful 

 movements (muscles of vertebrate skeleton). 



In the simplest cases the cross-striped fibrilhe are produced by the 

 deeper parts of the myoblasts, which form a continuous flat surface 

 epithelium (muscle epithelium) above the layer of delicate fibres 

 (Medusa? and Siphonophora) (fig. 34 b). In the higher animals they 



