156 PRINCIPLES OF ANIMAL BIOLOGY 



occupy the form of a loose spiral in the sheath and perhaps to other pecu- 

 liarities in the shape of the myonemes. Relaxation of the myonemes 

 permits the elastic stalk to extend to its full length. 



Muscle. — In multicellular animals, while a large proportion of the 

 cells are capable of slight contraction, certain cells are highly developed 

 for purposes of contraction. As in the Protozoa, cells of metazoa which 

 function for strong contraction have developed within themselves fibrillar 

 structures capable of great contractility. Cells containing such struc- 

 tures are muscle cells. Muscle cells rarely occur singly in organs or tissues 

 but usually as plates or bundles. These groups of muscle cells are 

 called muscles. Two types of muscle cells have been developed. Cells 

 of one of these kinds are simple in structure, capable of only relatively 

 slow contraction; those of the other kind are complex and are capable of 

 rapid contraction. The former are known as smooth muscle cells and the 

 latter as striated. 



Smooth muscle is composed of cells each of which is provided with a 

 nucleus. The cytoplasm contains well marked longitudinal fibrils. 

 These cells (Fig. 116) have the form of slender spindles with unbranched 



Fig. 116. — Smooth musole cells. 



tips or in certain organs the tips may be branched. They are capable 

 of strong but relatively slow contraction and relaxation. In higher 

 animals they occur in organs in which rapid contraction is not required, 

 for example in the walls of the digestive tract, urinary bladder, gall 

 bladder, arteries and veins, and in certain glands and their ducts. Often 

 these cells are arranged in layers which are apphed to the organs in differ- 

 ent directions, as transversely and lengthwise, respectively, giving rise to 

 circular and longitudinal muscles. Smooth muscle is under the control 

 of lower nervous centers and is therefore not directly subject to the will. 

 For this reason it is sometimes called involuntary muscle. Striated 

 muscle, except heart muscle, is under the control of higher nervous 

 centers, and since its contraction may be initiated by an act of the will 

 it is called voluntary muscle. 



Striated muscle differs from smooth muscle in the character of its 

 fibrilla) which are striated and in respect to the number of nuclei within 

 the cell. Striated muscle, except the heart muscle of the vertebrates, 

 is composed of much elongated parallel cells each containing numerous 

 nuclei which are scattered among the fibrillar. Striated muscle cells 

 originate embryologically from cells each of which has a single nu- 



