THE FROG 41 



the muscle contracts one of the bones remains more or less still and 

 the other is drawn towards it. The tendon attached to the relatively 

 fixed point is termed the origin, and that attached to the movable 

 bone the insertion. Usually a muscle has but one origin and one 

 insertion, but it may have two, as, for example, in the biceps, which 

 receives its name from the fact that it has a double origin. The 

 tendons are not always united to bone. The distal tendon of the 

 gastrocnemius is called the tenclo Achillis, and in the frog it passes 

 over the place occupied by the heel in ourselves, and is inserted into a 

 sheet of connective tissue spread out over the sole of the foot. A 

 connective tissue sheet serving for the attachment of muscle or 

 tendon is termed an aponeurosis, and the particular one just described, 

 from its position, is known as the aponeurosis plantaris. 



The entire muscle is enclosed in a connective tissue sheath, the 



n. 



FIG. 12. Striped muscle fibres (m.) from the tail of a larval axolotl, 

 showing their nuclei (.). X 560. From a photograph (Dendy). 



perimysium, and within this are to be found a number of bundles of 

 fibres or fasciculi, each enclosed by a sheath, the endomysium, 

 continuous with the external one. The individual fibres composing 

 the fasciculi have quite a fair diameter, as much as i mm., and 

 may in some cases reach a length of 120 mm. (nearly five inches). 

 They are circular or polygonal in cross-section, and as a rule un- 

 branched, although branched fibres are present in the tongue 

 muscles. Each fibre is enclosed by a very thin sheath, the sar co- 

 lemma, immediately beneath which are numerous nuclei irregularly 

 distributed. The interior is filled by a semi-fluid substance, the 

 sarcoplasm, consisting of a large number of small fibrils parallel 

 with one another, and a certain amount of other material. These 

 fine fibrils are the fibrillse or sarcostyles, and give the whole fibre 



