Origin, insertion, extrinsic, intrinsic
The proximal, less mobile end of a limb muscle is called the origin; the distal, more mobile end, the insertion. Extrinsic muscles connect the trunk or neck to the limb: they move the limb relative to the body, and if the limb is fixed by the body's weight, they move the body relative to the limb. Intrinsic muscles lie entirely within the limb and move its joints.
Who carries the trunk
The forelimb has no bony joint with the thorax: running animals have lost the clavicle as a functional bone, to gain a mobile shoulder and a laterally flattened chest. The serratus ventralis, attached by digitations to the ribs and the cervical vertebrae and inserted on the medial face of the scapula, carries the weight of the trunk; the pectorals support the limb from the sternum, the trapezius lifts and pulls the shoulder blade forward, and the brachiocephalicus pulls the limb forward or, with the limb fixed, moves the neck. The forelimbs carry about 60 % of the static weight.
The arm: triceps and biceps
The triceps brachii, the strongest muscle of the forelimb, attaches to the olecranon and extends the elbow; in the dog it has four heads, although the name says “three”, and the biceps has a single head: the names come from human anatomy. The biceps flexes the elbow and extends the shoulder; the deltoid flexes the shoulder. In the greyhound, the deep pectoral is the heaviest muscle of the forelimb, and the latissimus dorsi and the deep pectoral produce most of the mechanical work of acceleration.
The engine: the hip extensors
The hindlimb is built for propulsion: the pelvis is joined by bone to the spine through the sacroiliac joint, the limb is longer and more angulated. Running quadrupeds are described as having “rear-wheel drive”: the torque generated at the hip pushes the center of gravity forward. The biceps femoris and the adductor are the largest hip extensors, the semitendinosus and the gluteals follow; the quadriceps extends the stifle through the patella and the patellar ligament, and the rectus femoris also flexes the hip.
The gastrocnemius extends the hock through the common calcaneal tendon, which is not a single tendon: its core comes from the gastrocnemius and the superficial digital flexor, with contributions from the biceps femoris, semitendinosus and gracilis. That is why a dog could stand even with the gastrocnemius and the superficial digital flexor paralyzed; the superficial digital flexor is, despite its name, first of all a weight-bearing muscle.
The spine: the epaxial muscles
Above the spine lie the epaxial muscles, in three longitudinal systems (iliocostalis, longissimus, transversospinalis), which extend the spine; the longissimus runs from the pelvis to the head and is the longest muscle of the body. At the gallop, flexion and extension of the spine greatly lengthen the effective stride, compensating for the relatively short limbs of carnivores. The elastic nuchal ligament continues the supraspinous ligament and helps carry the head; the cat does not have one.
Breathing
The diaphragm, a muscular dome that completely separates the thorax from the abdomen, is the main muscle of inhalation. The ribs being curved, the muscles that pull them forward (the scalenes, the serratus dorsalis cranialis, the external intercostals) widen the thorax, and those that pull them back (the internal intercostals, the serratus dorsalis caudalis, the external abdominal oblique) exhale; the external oblique performs the “abdominal press”. The dog loses heat mainly by panting, not by sweating.
The head: the bite and the face
The temporalis, massive and elastic in carnivores, starts in the temporal fossa and on the sagittal crest and attaches to the coronoid process of the mandible; the masseter starts on the zygomatic arch and attaches to the lateral face of the mandible. Both close the mouth; the only masticatory muscle that opens it is the digastricus. The muscles of the face (the platysma, which retracts the lips and bares the teeth; the orbicularis oculi; the levator nasolabialis) and the ear muscles, grouped into rostral, lateral and caudal, are innervated by the facial nerve. Under the skin of the trunk lies the cutaneous muscle, the one that “shakes” the skin.
What the muscle layer shows and what it does not
The drawing shows the superficial muscles visible from the side, simplified and without the deep layers; the semimembranosus (on the medial face of the thigh, a hip extensor in weight bearing), the adductors, the deep muscles of the neck and the flexors/extensors of the digits are not drawn, but appear in the text where they matter. The origins of the three vasti of the quadriceps were not found on any page we read; they are described only as “from the femur”.