LOCOMOTOR SYSTEM 585 



elbow or stifle joints. This being so, it is evident that move- 

 ments of the upper part of the limbs are automatically followed 

 by movements of the lower parts. 



Flexion and Extension. — It is usual to picture muscles at 

 work from the appearance presented on the dissecting-room 

 table ; valuable as this is. it does not tell the whole story. It is 

 only when muscles function that the fundamental principle is 

 grasped that they are really never idle, and that even when not 

 engaged in performing the more active duties for which they 

 exist, they are still part of the animal machine, and engaged in 

 the work of muscle co-operation. Flexion, extension, abduction 

 and adduction, are not the exclusive function of individual 

 muscles. A muscle is not, for instance, exclusively engaged as a 

 flexor or extensor : it may be actively employed in flexion, or 

 passively employed in extension ; it may be actively engaged in 

 extending one joint, and actively engaged flexing another ; it 

 may be actively flexing one joint, and automatically flexing 

 its neighbour ; it may be passively employed in extending one 

 joint, and when active act as a flexor of another. Examples 

 of these will now be given, for if the machine at work is to be 

 correctly visualised, the ground-work for this is the facts above 

 stated. 



1. A muscle may be a flexor when contracting, and an extensor 

 when not active. A good example of this is the flexors of the 

 fore-limbs which, when not contracting, are maintaining the 

 elbow-joint firmly extended. Another is the biceps brachii 

 {flexor brachii), which when actively contracting flexes the elbow- 

 joint, and when passive is assisting, by means of a strip of tendon 

 to the extensor carpi radialis (extensor metacarpi magnus), in 

 keeping the knee firmly extended. 



2. A muscle may be engaged actively at one and the same moment 

 in extension and flexion. This is met with in the superficial 

 digital flexor (flexor perforatus) of the hind-limb, which extends 

 the hock-joint and flexes the pastern. 



3. A muscle may be actively engaged flexing one joint and 



of abduction and adduction of the limbs is very small. On the one hand, 

 it reaches its maximum in a ' cow-kicker, 'and on the other in a horse 

 that ' brushes ' from defective conformation, but especially in a ' weaver.' 



Rotation is a term limited to joints, and the only joints in the limbs of 

 the horse which can rotate are the shoulders and hip. Rotation of the 

 stifle is produced in the hock and carried out in the hip- joint. 



It is obvious that in all movements of flexion, extension, abduction, 

 adduction, and rotation, the act is not an isolated one ; the movements 

 must work at least in pairs, and sometimes the entire series is employed. 

 Take, for example, a horse ' cow-kicking ': the limb must first be flexed, 

 then forcibly extended, and at the same time abducted. To admit of 

 this there must be rotation of the hip, and to allow the limb to recover its 

 position adduction must follow. 



