586 A MANUAL OF VETERINARY PHYSIOLOGY 



automatically flexing its neighbour. The muscles which flex the 

 stifle-joint automatically flex the hip. 



4. A muscle may in flexing one joint flex several others. This 

 is met with in the flexors of the fore-limbs, which flex knee, 

 fetlock, pastern, and foot, at one and the same contraction. 



5. A muscle may be passively engaged extending one joint and 

 actively employed in flexing another. An example of this is the 

 tibialis anterior and peroneus tertius (flexor metatarsi), which are 

 passively engaged by means of the cord which runs through its 

 substance in maintaining the stifle extended, and when that 

 joint is flexed is actively engaged in flexing the hock. 



6. Two parallel muscles, with a common tendon of origin, may 

 act in opposite senses. This is met with in the tibialis anterior, 

 peroneus tertius, and anterior long digital extensor (flexor metatarsi 

 and extensor pedis) ; the one extends the foot, the other flexes 

 the hock. 



7. A muscle may both passively and actively be an extensor only. 

 An example of this is the gastrocnemius muscle, which, whether 

 the foot be on or off the ground, is an extensor of the hock. 



It is possible for a muscle to be so anatomically situated as to 

 appear to act as an extensor of one joint and a flexor of another, 

 yet when the muscle is acting in the living animal it may be found 

 to act only as a flexor, and never as an extensor. Sherrington 

 has shown that this is the case with the semitendinosus muscle 

 of the cat, which is so situated as to act as a flexor of the knee 

 (stifle) and an extensor of the hip joint ; but experimentally it 

 can be shown that the muscle always flexes the knee, and never 

 extends the hip, for the reason that the hip flexor contracts at 

 the same moment, and neutralises its action on that joint. 



It has already been pointed out (pp. 391, 402, and 458) that 

 even when the muscles of the body are apparently not acting 

 they are in a condition of slight contraction, which is necessary 

 for the maintenance of the standing attitude, and that it is only 

 during the time the animal's weight is actually off its legs that 

 the muscles can be considered to be at rest. 



When a muscle contracts its antagonist is relaxed, and the 

 nervous channel through which this is effected has been described 

 under the head of ' reciprocal innervation ' (see p. 456). Where 

 muscles act on a single joint as a flexor and extensor the reciprocal 

 nervous mechanism is easily understood, as the two muscles are 

 direct antagonists. But when muscles are not exclusively 

 devoted to one joint — in fact, affect two or more — the nervous 

 mechanism is not so simple, for the reason that flexion at one 

 joint may lead to extension at another. An example of this has 

 already been given ; the semitendinosus muscle is only permitted 

 \o act as a flexor of the knee (stifle) of the cat, and not as an. 



