Rachel Page Elliott - Dogsteps: A New Look

Dedication

To my husband, whose patience made it possible.

Chapter 6: Understanding Fronts An Inside Look

Function Influences Form

Here is a variety of dog developed through selective breeding to ferret pests from underground. The elbows are set above the line of the sternum (lower chest), a feature that frees the elbows for digging, while the dogs weight rests mainly on his chest. The long gradual upward slant of the sternum helps him to slide more easily over rocks and rootsa handy arrangement in case a speedy retreat is called for.

Three Comparative Types of Front Assemblies

Though countless variations in structure continue to occur through genetic mixtures, specific types of dogs have survived through centuries of selective breeding to meet our needs and preferences. These skeletal diagrams illustrate three basic kinds of front assemblies. While each indicates a good position and slope of the shoulder blade, distinguishing features reveal differences in angulation of the shoulder joint, set of the humerus and placement of the elbow in relation to the chest wall. The following page shows illustrations of these particular types showing not only structure of the shoulders, but also differences in outline of the croups and rear assemblies.

A. Greyhound

This fore assembly is designed primarily for speed, with wide-angle at the shoulder joint, a sharply descending humerus and elbow set below the sternum. The sharply descending croup lends flexion in bringing the hind legs well forward under the body for maximum propulsion and rear extension when galloping

B. GermanShepherd/Retriever

Strong fronts are needed for herding, retrieving, carrying, tracking, draft work and other purposes. This calls for a humerus long enough to set the elbow back under the highest point of the shoulder blade to provide maximum area for muscle attachment. A good fore assembly balanced with normal angulation in the rear quarters contributes to even movement and ease over fences.

C. Corgi/Basset

The achondroplastic structureseen in numerous dogs bred down in sizeshows limbs foreshortened with ones frequently bowed.

This type of farm dog will drive a herd of cows and should be able to dodge kicking heels because of its low frame.

Cineradiography Casts New Light on a Much Discussed Subject

There seems to be more misunderstanding about the front structure of a dog than of any other part of its anatomy, probably because the bones making up the fore assembly are not visible to the naked eye, nor clearly pictured through feel. In the Introduction of this edition I mentioned that my search for answers to questions that have long puzzled me led to a study of bone and joint motion at Harvard Universitys Museum of Comparative Zoology, where specially designed equipment makes possible the simultaneous photographing and fluoroscoping of dogs as they move on a treadmill.

The technique is known as cineradiography . Taken on 16mm film, the results are similar to moving X-rays, which reveal the dogs remarkable skeletal machinery from the side as well as from underneath the body.

Dogs representing a variety of type and conformation were used in the survey. Except at the gallop, the subjects moved on the treadmill at average trotting speed. In addition to the cineradiography, a number of still X-rays were taken of dogs in standing position, rather than lying on their side, to show the natural setting of the shoulder assembly as it bears the weight of the trunk. The radiographic findings take the guesswork out of what really goes on and challenge some long standing notions.

Tracing from single 16mm movie frame of a Collie moving at a moderate trot. Dotted lines indicate free excursion of the shoulder assembly as the blades swing from their upper edges. At a moderate trot, the excursion (lift and fall) of the blades is about 30 degrees.

At the gallop, the excursion is much greater.

The Myth and Facts about Shoulder Structure

There is a human tendency to accept what appears in print as having withstood some incredible truth test. A case in point is the long held belief that a 45-degree slope of the shoulder blade is necessary for providing maximum extension or reach of the forelimb, and this is written into many breed standards. I have often wondered about the illusiveness of this detail, for I have never found it myself, and it was not until given the opportunity to observe skeletal action through cineradiography that I realized why such a position would be a mechanical impossibility to the dogs function. A 45-degree slant, or layback, would be workable if the blade were a stationary bone with a more or less fixed joint from which the upper arm moved forward and back. Actually, this is not the case.

What we have failed to recognize is the great mobility of the shoulder blade as partner to the action of the upper arm, which serves as a lever in lifting and transporting the central body forward as smoothly as possible.

At a moderate trot, the blade swings from its upper rim with an excursion, lift and fall of about 30 degrees. At the gallop, the excursion is much greater. ( See previous drawings. ) As the assembly lifts, it also swings inward, leaving the supportive role of the chest wall. If the motion began from a 45-degree position, the scapula would lift too high and the medial component of the shoulder musculature would force the shoulder to bear directly on cervical structures. (F.A. Jenkins).

Radiographic studies show that as a dog stands with the forelegs in natural pose, a blade that sets about 30 degrees off a vertical planemeasured up the scapular ridgeis within normal limits for the average well-built dog. We must try to visualize the anatomical limitations within which the shoulder and upper arm are held against the rib cage and the way in which chest shape and muscling influence the position of both. The combined factors not only influence shoulder excursion but also explain why a dog tends to single track or move with a wider footfall.

The humerus is a strong bone shaft, normally longer than the scapula, that slopes down and back into the elbow. Its position and relationship with the shoulder blade varies with functional type and may be correct or faulty within members of the same breed. The differences influence angulation and placement of the elbow against the chest wall and, as previously stated, are reflected in particular gaiting styles and in the quality of performance.