MUSCLE-TISSUE. 263 



another muscle, or a group of muscles, is in that of extension. 

 Muscles which simultaneously work in such an opposite manner 

 are termed " antagonists." 



Muscle formations are of two varieties. In the tissue of 

 the unstriped, smooth, or involuntary muscle the Moplasson is 

 stored up, without much regularity, in comparatively small spin- 

 dles. In the tissue of the striped or voluntary muscle the bioplas- 

 son is distributed regularly in the shape of sarcous elements, in 

 relatively large spindles. 



1. SMOOTH OR UNSTRIPED MUSCLE. 



Smooth muscle is constructed of spindle-shaped plastids, 

 which are usually nucleated, and are held together in bundles 

 by means of a delicate cement-substance, surrounding each 

 spindle. Ensheathing the bundles there is a delicate layer of 

 fibrous connective tissue, the perimysium. The muscle nature 

 of these spindles was discovered by A. Kolliker in 1847. 



The spindles vary greatly in size. Very small spindles are 

 found in the skin and large arteries, medium-sized spindles in 

 the muscle-layers of the mucous membranes, and very large 

 spindles in the pregnant uterus. In all instances the separating 

 cement-substance can be stained brown by nitrate of silver, and 

 the brown line thus produced is pierced at right angles by deli- 

 cate light lines which correspond to the minute transverse 

 spokes interconnecting all spindles. The spokes are visible with 

 high powers of the microscope, without the addition of a re-agent. 

 The granules of the spindle are, as a rule, coarse, so as often to 

 conceal the central oblong or rod-like nucleus, which, however, 

 is easily seen in transverse sections. The granules not infre- 

 quently exhibit a partially regular arrangement, especially 

 toward their tapering ends, which produces an appearance 

 resembling the sarcous elements of striated muscles. The out- 

 lines of the spindle are smooth and regular when in a state of 

 rest ; but they become slightly scalloped by contraction, when 

 the body of the spindle is shortened and broadened. 



The bundles of smooth muscles of the skin are most abundant 

 in the region of the nipple and in the scrotum. They run in 

 oblique directions, interlacing sometimes at acute angles. The 

 oblique section of a bundle is characterized by short spindles ; 



