126 HISTOLOGY 



The size of the muscle fibers is subject to considerable variation. They 

 are said to enlarge at a uniform rate throughout the body until birth, when 

 their diameter is about twice as great as in embryos of four months. After 

 birth the fibers of certain muscles become much coarser than those in 

 others. Thus the gluteal muscles have large fibers (av. diam. 87.5/0 

 and the ocular muscles have small ones (av. diam. 17.5 /A), as determined by 

 Halban (Anat. Hefte, Abth. i, 1894, vol. 3, pp. 267-308). He finds that 

 the diameter of the adult fibers in general is about five times greater than 

 at birth. As a result of exercise the diameter of muscle fibers in rats may 

 show an average increase of 25% according to Morpurgo (Arch. f. path. 

 Anat., 1897, vol. 150, pp. 522-554). He states that the enlargement of the 

 muscle takes place without an increase in the number of its fibers, but merely 

 through the thickening of existing elements. The fibers which grow most 

 are those which originally were thinnest, and which act as a reserve 

 material with great capacity for growth. The enlargement of fully formed 

 fibers apparently takes place through an increase in the sarcoplasm, with- 



Transition zone. 



Nucleus tendon. Q Z 



FIG. 117. BRANCHED STRI- FIG. 118. LONGITUDINAL SECTION OF A PART OF A MUSCLE 



ATED MUSCLE FIBER FROM FIBER FROM A HUMAN INTERNAL INTERCOSTAL MUSCLE. SHOW 



THE TONGUE OF A FROG. ING ITS TRANSITION TO TENDON. X 750. 



out multiplication or thickening of the fibrils. After injury striated mus- 

 cle gives slight evidence of regeneration, but it has been thought that 

 latent myoblasts may become active. A proliferation of nuclei toward 

 the injured ends of the muscle fibers has been recorded, but repair is chiefly 

 through the production of connective tissue. 



Longitudinal sections of skeletal muscles may be easily recognized 

 by the presence of unbranched striated fibers, bounded by well-defined 

 membranes, associated with which are the flattened peripheral nuclei. The 

 striations Q and / are visible under low magnification. In a few situa- 

 tions, striated muscle fibers branch (Fig. 117). Branching has been re- 

 ported toward the place where the muscle fibers of the tongue are inserted 

 into the mucous membrane, and where the facial muscles end in the sub- 

 cutaneous tissue. The way in which the fibers connect with tendon has 



