THE STRUCTURE OF THE MOTOR ORGANS 



83 



their form have been named muscle-spindles. They are doubtless 

 sensory in function. Somewhat similar bodies (Golgi's tendon- 



FIG. 47. The muscular coat of the stomach. 



organs) are found in the tendons and are also richly supplied with 

 nerve-fibers. 



Structure of the Unstriped Muscles. Of these 

 the muscular coat of the stomach (Fig. 47) is a 

 good example. They have no definite tendons, 

 but form expanded membranes surrounding cavi- 

 ties, so that they have no definite origin or inser- 

 tion. Like the skeletal muscles they consist of 

 proper contractile elements, with accessory con- 

 nective tissue, blood-vessels, and nerves. Their 

 fibers, however, have a very different microscopic 

 structure. They present a slightly marked longi- 

 tudinal" but no cross striation and are made up of 

 elongated cells (Fig. 48), bound together by a 

 small quantity of cementing material. The cells 

 vary considerably in size, but on the average are 

 about ^ mm. (^ in.) in length. Each is flat- 

 tened in one plane, tapers off at each end, and pos- 

 sesses a very thin enveloping membrane; in its 

 interior lies an elongated nucleus with one or two 

 nucleoli. These cells have the power of shorten- 

 ing in the direction of their long axes, and so of diminishing the ca- 

 pacity of the cavities in the walls of which they lie. 



Fio. 48. Un- 

 muscle- 



