THE WORK OF MUSCLES AND GLANDS 37 



the true contractile elements and there is the connective 

 tissue. The inactive substance forms a sheath enclosing 

 the rest and also partitions which subdivide the interior. 

 The arrangement is familiar in the cross-section of a piece 

 of meat. The subdivision which is apparent to the un- 

 aided eye is repeated on a microscopic scale until the 

 finest meshes of the connective tissue enclose the hair-like 

 individual fibers of the muscle. Each of these slender 

 fibers is a miniature muscle in principle. The function 

 of the connective tissue is often overlooked. While 

 this part of the muscle is entirely passive in character, 

 and scarcely to be considered alive excepting for a certain 

 power of renewal after injury, it is quite necessary to the 

 act of contraction. It may fairly be said to constitute a 

 harness through which all the numberless, minute contrac- 

 tile elements are enabled to unite their efforts. As the 

 end of a muscle is approached the connective tissue in- 

 creases in quantity at the expense of the typical contractile 

 material. In most cases there is an extension of the 

 muscle consisting of connective tissue only, and in a dense 

 form, which attaches the whole to the bone. This is the 

 tendon. It may be a long tough cord or it may form a 

 wide thin sheet. A muscle deprived of its connective tis- 

 sue would be simply a mass of unattached living fibers 

 which might slip about among themselves, but which could 

 not apply their combined tension to accomplish any ex- 

 ternal effect. 



The fiber of skeletal muscle is a modified cell. Its 

 length is exceptionally great for its width, perhaps a 

 thousand times as great. When it shortens it conforms 

 to the general principle laid down in Chapter I, that is, it 

 does not diminish in volume, but only in surface and, 

 therefore, in length. How the chemical process which 

 underlies the forcible shortening is made to contribute 

 energy to carry it out has proved one of the most difficult 

 problems of physiology. It cannot be dealt with here. 

 But the fact is to be emphasized that we are in the presence 

 of a mechanism somewhat like the steam-engine, inasmuch 



