This runs down the anterior and outer side of the upper arm, and is attached to the proximal tendon of the extensor metacarpi radialis longus, a little below the outer condyle of the humerus.
It is a brief extension of the limb at the knee-joint, due to a simple contraction of the extensor muscle, elicited by a tap or other short mechanical stimulus applied to the muscle fibres through the tendon of the muscle.
If the sensory nerves of the extensor muscle be severed, the "jerk" is lost.
Stretching of the muscles antagonistic to the extensors - namely, of the flexor muscles - reduces the jerk by inhibiting the extensor spinal nerve cells through the nervous impulses generated by the tense flexor muscles.
The motor cells of the extensor muscles, when preoccupied by cerebral influence, appear refractory.
On the anterior or extensor surface of the limb a powerful tendon (7 in fig.
2), that of the anterior extensor of the phalanges (corresponding to the extensor communis digitorum of the arm and extensor longus digitorum of the foot of man) passes down over the metacarpal bone and phalanges, to be inserted mainly into the upper edge of the anterior surface of the last phalanx or pedal bone.
There is also a much smaller second extensor on the outer side of this in each limb, the lateral extensor of the phalanges.
In the fore-leg the tendon of this muscle (which corresponds with the extensor minimi digiti of man) receives a slip from that of the principal extensor, and is inserted into the first phalanx.
A strong ligamentous band behind the metapodium, arising from near the upper extremity of its posterior surface, divides into two at its lower end, and each division, being first connected with one of the paired upper sesamoid bones, passes by the side of the first phalanx to join the extensor tendon of the phalanges.
14, The podophyllous or laminar 7, Tendon of anterior extensor membrane, and of the phalanges.
There is a patella, intercalated in the tendon of the femori-tibialis or extensor cruris muscle.
Consequently only a few of the original extensor muscles have been preserved, but these are much modified into very independent organs, notably the extensor metacarpi radialis longus, the ext.