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Brachium of the Pig (Sus scrofa)

Text by Lazaro Gonzales, Jr.

General Dissection:

The tensor fasciae antebrachii muscle and triceps muscle (long and lateral head) are visible from the dorsal aspect of the brachium. Tensor fasciae antebrachii is difficult to view as it is a very thin fascia covering the triceps. It is easily destroyed in the skinning of the pig. I separated the heads of the tricep by inserting a blunt probe between them and tore away the fascia that connects them. Brachialis can be seen from a lateral or medial aspect of the brachium. I then moved to the ventral side of the brachium where I reflected pectoralis superficialis in order to view the musculature deep to it. Deep to pectoralis superficialis is tricep (medial head), biceps brachii, and coracobrachialis.

Muscles of the Brachium:

The mammalian triceps can have upt to five heads. The tricep in the fetal pig has three heads: long head, lateral head, and medial head [1]. Two of the tricep heads are on the dorsal side of the brachium. The long head forms the posterior border of the brachium and can be seen on both ventral and dorsal sides of brachium. Anterior to this muscle (dorsal aspect) is the lateral head. Depending on the development of the fetal pig the definition between these two heads may vary. The third head (medial head) is on the ventral side of the brachium deep to pectoralis superficialis. Dorsally, brachialis is distal to the lateral head of the tricep. Ventrally, it is visible distal to pectoralis superficialis. Upon reflection of pectoralis superficialis, biceps brachii and coracobrachialis are visible. Biceps brachii is superior and superficial to subscapularis and the medial head of the tricep. Coracobrachialis is immediately distal biceps brachii followed by brachialis.

Muscles of the Brachium [1, 2, 3]:

Muscle	Origin	Insertion	Function	Comments
Tensor fasciae antebrachii	Edge of latissimus dorsi	Antebrachial fascia; insertion of triceps		Very thin covering over triceps
Triceps: long head	Caudal border of scapula	Olecranon process	Extends forelimb
Triceps: lateral head	Proximal half of humerus	Olecranon process	Extends forelimb
Triceps: medial head	Proximal half of humerus	Olecranon process	Extends forelimb
Brachialis	Proximal portion of the humerus	Proximal portion of the ulna	Flexes lower arm	Elbow flexor
Biceps brachii	Glenoid area	Radius	Flexes forelimb	Elbow flexor
Coracobrachialis	Coracoid process	Humeral shaft	Adducts arm	Weak adductor; reinforces shoulder; elbow flexor

Comparative Anatomy:

The beaver had shorter thicker musculature in the brachium. This most likely due to its swimming locomotion. Muscles of the brachium in the rabbit were much smaller relative to its body size due to its reliance on the hindlegs to propel itself when hopping, which is its main form of locomotion. The mammal whose brachial musculature most closely resembles the musculature of the fetal pig is the cat. Both mammals are quadrupedal and depend on all four limbs equally for locomotion. They are cursorial, which is an explanation for these similarities. Much like other artiodactyls the pig has adaptations for running. This is also true of the cat.

References:

1. Odlaug, T.O. 1980. Laboratory Anatomy of the Fetal Pig. Wm. C. Brown Company Publishers, Dubuque, Iowa.

2. Walker, W.F. 1980. Dissection of the Fetal Pig. W.H. Freeman and Company, San Francisco, CA.

3. Smith, D.G. 1998. A Dissection Guide and Atlas to the Fetal Pig. Morton Publishing Company, Englewood, CO.

Artwork: Weil, from Stubbs' 1776 "Anatomy of the Horse."
Background free from Eos Development, with slight color modification.