320 THE MUSCULAR SYSTEM. 
The biceps is an extremely variable muscle. Its chief anomalies are due to an increase or 
diminution in the number of origins. A third head of origin is common (10 per cent), and 
usually arises from the humerus, between the insertions of the deltoid and coraco-brachialis. 
Two or even three additional heads may be present at the same time. The long head of the 
muscle may be absent, or may take origin from the bicipital groove. The muscle may have an 
additional insertion into the internal condyle of the humerus, or into the fascia of the forearm. 
The brachialis anticus (m. brachialis) is a large muscle arising from the 
lower two-thirds of the front of the humerus, and from the intermuscular septum 
on each side. Extending from the insertion of the deltoid above, it ends below in 
a strong tendon inserted into the rough inferior surface of the coronoid process of 
the ulna. The outer part of the muscle arising from the external epicondylic 
ridge and external intermuscular septum forms a slip more or less separate, which 
may be partially fused with the brachio-radialis muscle. 
The brachialis anticus is almost wholly concealed from view by the biceps in 
front, the brachio-radialis externally, and the brachial vessels internally. It covers 
the elbow joint, and forms part of the floor of the hollow of the elbow. 
The triceps (m. triceps brachi) is the only muscle on the back of the arm. 
It arises by three heads, an outer and an inner head, from the humerus, and a 
middle or long head from the scapula. (1) The middle, long or scapular head (caput 
longum) begins as a strong tendon attached to the axillary border of the scapula 
just below the glenoid fossa. This gives rise to a fleshy belly which occupies the 
middle of the back of the arm. (2) The outer head (caput laterale) is attached 
to the outer border of the humerus from the insertion of the teres minor to the 
musculo-spiral groove, and to the back of the external intermuscular septum. Its 
fibres are directed downwards and inwards over the musculo-spiral groove and 
the inner head of the muscle to the tendon of insertion. (3) The inner head (caput 
mediale) arises by fleshy fibres from a triangular area on the back of the humerus, 
extending upwards to the insertion of the teres major and downwards to the 
margin of the olecranon fossa. It also arises on each side from the intermuscular 
septa. 
The three heads of origin are inserted by a common tendon, broad and mem- 
branous, into the upper end of the olecranon process, and into the deep fascia of 
the forearm on either side of it. The long and outer heads join the borders of the 
tendon of insertion, and the inner head is attached to its deep surface. 
The long head of the triceps is concealed at first by the teres major and minor, 
and by the deltoid muscle. Along with the outer head it conceals the musculo- 
spiral nerve and superior profunda artery, and covers the inner head of the muscle. 
The inner is the deep head, and is only visible at the lateral borders of the muscle. 
The sub-anconeus is a small muscle occasionally present, which consists of scattered fibres 
arising from the lower end of the humerus beneath the triceps, and inserted into the posterior 
ligament of the elbow joint. 
NERVE SUPPLY. 
The following nerves supply the muscles of the arm :— 
Muscles. Nerves. Origin. 
Coraco-brachialis | | fics a 
Biceps Ee : . | Musculo-cutaneous |;C. 5. 6. | 
Brachialis anticus | | | | SONG: | 
Tee anticus | C. (5) 6. | 
a . | sculo-spirs 1 
Outer head | . | Musculo-spiral he (6) ie 8. 
Middle and inner heads / eh RNR | 
ACTIONS. 
(1) The chief action of these muscles is on the elbow joint, producing along with other muscles 
flexion and extension. The flexor muscles are much more powerful than the extensors, 
