This article continues our discussion from the previous issue of the fore assembly of the dog.
In the average dog, the shoulder blade (scapula) is described as “well laid back” at about a 45-degree angle to the ground, and is laid against the upper front section of the chest (thorax) (A) with the lower forward part of the scapula at about the first rib or frontal opening of the chest (B). (See Figure 1.)
The 90-degree angle formed by the shoulder joint enables the dog to stand well over its forelimbs due to the return of an upper arm of the correct length to the elbow, which in the average dog is set in a line under the withers. (See Figure 2.)
The layback of the shoulder blade determines the forward reach of the front leg. The junction of the shoulder blade to the upper arm at the ball and socket joint is referred to as the point of shoulder. This shallow cavity at the bottom of the shoulder blade forms a very shallow socket for articulation with the head of the upper arm (humerus). (See Figures 3A, 3B, 3C.)
It is important to note that the ball and socket joint formed where the shoulder blade (scapula) and the upper arm (humerus) meet is not a simple ball and socket type of joint, but is designed for a more sliding movement. This sliding action allows for freer movement in the front assembly and has far less force placed upon it than that produced by the hindquarters. This difference in action is what allows for more lateral movement of the front legs (paddling, winging, etc.) than can be found in the rear assembly. The corresponding ball and socket joint of the pelvis and the hind limb has a very deep cavity and an extremely stable joint that allows it to perform well, even with the stronger force placed upon it by the propulsion of the hindquarters. At the bottom end of the shoulder blade is a beak-like protrusion of bone—the “scapular tuberosity.” (See Figure 4.) This tuberosity fits into a corresponding area on the head of the humerus and restricts the forward movement of the humerus and hence the forearm, thus limiting the reach of the forefoot, depending upon the layback of the shoulder blade as shown in Figures 3A-C.
PHYSICAL EXAM OF THE FOREQUARTERS
It is extremely important to understand that although the rear assembly is attached via the articulation of the upper thigh (femur) at the hip (pelvis) and the pelvis is fused to the sacrum of the spine, the forelimbs of the dog have no skeletal attachment to the chest (thorax), only a muscular attachment. The shoulder blade is bound to the body by several broad, flat muscles. At the lower end of the shoulder blade is the upper arm (humerus), which is fairly free in movement, though it is attached to the chest (thorax) by several muscles. The layback of the shoulder blade determines the front reach of the dog. The angulation can be determined when physically examining a dog and depends on certain “landmarks” or points where the bones may be felt through the coat, skin, muscles, and fat of the dog being examined.
On the shoulder assembly, the easily felt landmarks are the spine of the scapula (A) and the “notch” formed where the shoulder blade meets the upper arm (B). The point of shoulder is actually the top of the upper arm (C). (See Figure 5.)
The upper arm is approximately equal or somewhat longer in nearly all but the dwarf breeds. The body of the upper arm (humerus) makes approximately a 90-degree angle with the spine of the scapula as it returns back to articulate with the leg bones at the point of the elbow. From the point of shoulder, the angle between the blade and the upper arm is perceived to be approximately 90-degrees. By perceived, I mean that using the landmarks
available to us when physically examining a dog, the optimum layback of the shoulder blade for most breeds is a 45-degree angle to the ground. To place the leg up under the body, then a 90-degree angle formed by the juncture of an upper arm of the correct length to the shoulder blade would place the leg back under the body with the elbow approximately beneath the highest point of a well laid back shoulder blade. (See Figures 2 & 6.)
Ideally, the highest part of the shoulder blade lies just below the level of the first and fourth vertebrae, with the spine of the shoulder blade pointing to the highest part of the blade.
In the majority of breeds, the upper arm is the largest bone in the fore assembly and, even though most standards call for an upper arm length as long as the shoulder blade, it is, for most breeds, actually longer than the shoulder blade. Again, the point of the shoulder usually referred to in measuring body length is actually the upper end of the upper arm. (See Figure 6.)
To help put all of this together in a manner much more straightforward than I am able to do in a few pages, I would encourage you to purchase The Dog Anatomy Workbook, edited by Andrew Gardiner and illustrated by Maggie Raynor. This is a sophisticated “coloring book” that has to do with canine anatomy. I found the illustrations on pages 54 & 55 and the explanations of the forelimb muscle functions on pages 70 & 71 to be quite enlightening. I especially like the “muscle lines” diagram of the forelimb muscles on pages 80 & 81. If you are like me, your eyes start to glaze over when trying to study the muscles of the canine body. As an artist, I am very visual and I’ve found this book to be an excellent source of information.
As always, if you have any comments or questions, or to schedule a seminar on structure and movement, contact me at firstname.lastname@example.org.