TENDONS 



115 



and motor end-organs respectively. These endings will be further de- 

 scribed under Peripheral Nerve Terminations. The sympathetic or 'ac- 

 cessory fibers' (Fig. 129) relatively sparse and delicate, and in close 

 relationship to the motor fibers and endings terminate in special 'end- 

 plates/ close-meshed networks of generally oval outline. Boeke (Anat. 

 Anz.,44, 15-16, 1913) sug- 

 gests that they may me- 

 diate the maintenance of 

 muscle tone. 



Cardiac muscle is sup- 

 plied only with sympa- 

 thetic motor fibers. These 

 terminate on the muscle 

 fibers in brushes of fibrils, 

 but without highly spe- 

 cialized endings. Sensory 



FIG. 129. MOTOR END-PLATE ON AN INTERCOSTAL 

 MUSCLE FIBER OF A YOUNG RABBIT. 



The motor nerve fiber m is accompanied by an 

 accessory (sympathetic) fiber, a/. (After Boeke, 

 Anat. Anz., 44, 15, 1913.) 



fibrils from the vagus are 



distributed to the cardiac 



endomysium. Each smooth 



muscle cell, likewise, is supplied with a sympathetic fibril, ending in 



minute knobs or plates. 



According to Malone (Amer. Jour. Anat., 15, 1, 1913), the three types 

 of muscle are innervated by three histologically distinct types of nerve 

 cells, representing specific functional differences. The cells supplying 

 heart muscle are from the standpoint of size and granular (chromophilic) 

 content, intermediate between those supplying smooth and those supply- 

 ing skeletal striped muscle. (See Fig. 139 below.) 



TENDONS 



A tendon, taken as a whole, is invested by a dense fibro-elastic mem- 

 brane, the peritenoneum (peritendineum), or vagina fibrosa. Where 

 tendons play in bony grooves this may be modified into a tendon sheath, 

 the peritenoneum acquiring a mucous cavity, when it becomes known as a 

 vagina mucosa. Septa from the external peritoneum penetrate the 

 mass and divide the tendon imperfectly into irregular columns, the ter- 

 tiary bundles. These are further divisible into more regular aggregations 

 of fibrils, completely enveloped by an internal peritenoneum, and are the 

 tendon fasciculi. These correspond to the muscle fasciculi enveloped by 



