PLAIN MUSCULAR TISSUE. 



123 



muscles, where the fasciculi are short, the fibres may run 

 the whole length of each of the latter. 



Examined carefully with a good microscope each fibre 

 is seen to possess a very thin homogeneous sheath or envel- 

 ope, called the sarcolemma, within which lies the contrac- 

 tile portion of the fibre, b, which presents a striped appear- 

 ance as if composed of darker and dimmer alternating 

 bands (Fig. 53). During life this substance is very soft or 

 semifluid, but after death it rapidly solidi- 

 fies and death-stiffening, or rigor mortis, is 

 produced. Besides the contractile sub- 

 stance a number of oval nuclei, each sur- 

 rounded by a little unmodified protoplasm, 

 lie inside the sarcolemma. The latter is 

 im perforate except at one point where the 

 central portion (or axis cylinder, see Chap. 

 XII.) of a nerve-fibre penetrates it, and 

 ends in an expansion or end plate which is 

 in immediate contact with the striated sub- 

 stance. 



The larger blood-vessels of a muscle run 

 in the coarser partitions of the connective 

 tissue, and the finer ones lie close around 

 each fibre but entirely outside its sarcolem- 

 ma. 



Structure of the Unstriped Muscles. 

 Of these the muscular coat of the stomach 

 (Fig. 54) is a good example. They have no 

 definite tendons but form expanded mem- 

 branes surrounding cavities, so that they have 

 no definite origin or insertion. Like the 

 skeletal muscles they consist of proper contractile elements, 

 with accessory connective tissue, blood-vessels, and nerves. 

 Their fibres, however, have a very different microscopic 

 structure. They present no striation but are made up of 

 elongated cells (Fig. 55), bound together by a small quan- 

 tity of cementing material. Each cell is flattened in one 

 plane and tapers off at each end; in its interior lies an 

 elongated nucleus with one or two nucleoli. These cells 



FIG. 53. A small 

 piece of muscular 

 fibre highly mag- 

 nified. At a the 

 fibre has been 

 crushedand twist- 

 ed so as to tear 

 its contents while 

 the tougher sar- 

 colemma, e 1 s e- 

 where so closely 

 applied to the rest 

 as to be invisible, 

 remains untorn 

 and conspicuous. 



