82 THE HUMAN BODY 



Not all histologists are agreed as to the details of structure of 

 the fibrils ; they are so small that only the highest powers of the 

 microscope can be used in studying them; they occur in ordinary 

 muscle surrounded always by sarcoplasm and in company with 

 many others. These circumstances combine to present to the eye 

 of the observer a more or less distorted picture. It is no wonder, 

 therefore, that differences of opinion as to the real structure of the 

 fibrils have arisen. 



Certain insects' muscles happen to be so constituted that the 

 fibrils can be separated one from another and isolated ones gotten 

 under the field of the microscope for study. When examined thus 

 singly and free from surrounding media which distort the view, 

 these fibrils are seen to be tiny cylinders divided at regular inter- 

 vals by transverse partitions, made, apparently, of delicate mem- 

 brane. Many biologists think it likely that the fibrils of ordinary 

 skeletal muscle have really this same structure; that the position 

 of the transverse membranes is indicated by faint dark lines in the 

 middle of the light bands and that the appearance of light and dim 

 bands of nearly equal width is an optical illusion due to the un- 

 favorable conditions of observation. Since the fiber as a whole 

 contains many fibrils and since the cross striations are regular 

 throughout the entire fiber it follows that all the fibrils of any 

 fiber must have their partitions at corresponding levels. The 

 fibrils are probably kept in place by an interfibrillar network of 

 some sort. 



The blood-vessels and nerve-fibers supplied to the skeletal 

 muscles are numerous. The larger blood-vessels run in the coarser 

 partitions of the connective tissue lying between the fasciculi and 

 give off fine branches which form a network between the individual 

 fibers but never penetrate the sarcolemma. 



Connected with each muscle-fiber is a nerve-fiber. The central 

 core of the nerve-fiber ends in an oval expansion (end plate) which 

 contains many nuclei and lies close under the sarcolemma, its 

 deeper side being in immediate contact and possibly continuous 

 with the striated contents. These nerve-fibers are motor or con- 

 cerned in exciting a contraction of the muscle-fiber. Other nerve- 

 fibers are connected with very peculiar bodies found scattered 

 throughout the muscle, but especially numerous near the tendons. 

 They are usually of a size just visible to the unaided eye and from 



