96 PHYSIOLOGY OF MUSCLES AND NERVES. 



bones. The tendons of some pass into soft structures, 

 such as the muscles of the face. In this case also the 

 dift'erent muscles exercise a mutual power of extension, 

 though it is but slight, and they thus effect a definite 

 balanced position of the soft parts, as may be observed 

 in the position of the mouth-opening in the face. If 

 the tension of the muscles ranged on both sides is not 

 equal, the mouth opening assumes a crooked position. 

 This happens, for example, when the muscles of one 

 half of the face are injured ; and it thus appears that in 

 this case the elastic tension is too weak to allow of the 

 retention of the normal position of the mouth. 



In muscles attached to bones the elastic tension is, 

 however, much greater, a circumstance which naturally 

 exercises an influence on their action durinor contrac- 

 tion. 



4. As yet attention has only been paid to one kind 

 of muscle-fibre, that which from the very first we dis- 

 tinguished as striated fibre. There is, however, as we 

 have seen, another kind, the so-called sviooth muscle- 

 fibre. These are long spindle-shaped cells, the ends of 

 which are frequently spirally twisted, and in the centre 

 of which exists a long rod-shaped kernel or nucleus. 

 Unhke striated muscle, they do not form separate mus- 

 cular masses, but occur scattered, or arranged in more 

 or less dense layers or strata, in almost all organs.* 

 Arranged in regular order, they very frequently form 

 widely extending membranes, especially in such tube- 

 shaped structures as the blood-vessels, the intestine, 



' An instance of a considerable accumulation of }-'mooth muscle- 

 fibres is afforded by the muscle-pouch of birds, which, with the ex- 

 ception of the outer and inner skin coverings, consists solely of these 

 fibres collected in extensive layers. 



