FORM FOLLOWS FUNCTION
MUSCLES move the bones and dictate where they will go and where they will stop. The skeleton is simply bone, and has no means of creating motion by itself. Although we are concentrating on the inner structure of the skeleton and its articulations and relationship, without muscles there is no movement. The musculature of the dog is a complicated subject, too lengthy for us to approach here. We are simply going to try to understand the structural limitations imposed by the composition of the juncture of the various bones, and leave the study of the muscles of the dog up to the individual student. TENDONS connect muscle to bone and are part of the muscle structure. LIGAMENTS are NOT part of the muscle structure. They are tough, fibrous bands developed at joints to connect bones to other bones. Other lig- aments form slings to hold tendons at the shoulder, wrist, and ankle joints. CANINE JOINTS There are “joints” (as in popular “joints” where people meet up), and then there are the joints formed within the body. Basically, a joint or articulation is a point where two or more bones meet and are united by fibrous, elastic or cartilaginous tissue or a combination of these tissues. There are three main types of joints: Fibrous (immovable); Cartilaginous (partially moveable); and Synovial (freely moveable). FIBROUS JOINTS are those that are stationery, such as the joints in the skull, which allow little to no movement. The bones are held together tightly by tough, fibrous, connective tissue. (See Figure 2.) The canine skull is actually made up of over 40 different bones, all tightly held together by this fibrous tissue. These various fibrous joints, also called “sutures” (see Figure 3A) serve to allow enough movement to absorb the shock of a blow as well as allow for growth at the edges of the bone. The fibrous joints range from those where a slight degree of compressibility is advantageous to those where a more extreme stability is desirable. A fibrous joint may have a considerable amount of intervening connective tissue. (See Figure 3.) CARTILAGINOUS JOINTS allow some movement as compared to a fibrous joint, but less movement than a synovial joint. Cartilaginous joints are formed when two or more bones are joined together entirely by cartilage. The joints formed between each vertebra in the spine are cartilaginous joints, allowing a smooth, frictionless movement of the spine. (See Figure 4.) The intervertebral disc is composed of fibrocartilage, which joins two vertebrae together. The center of the disc consists of a gel-like material. Another exam- ple of cartilaginous joints are the joints where the ribs meet the sternum. SYNOVIAL JOINTS are, by far, the most common classification of a joint within the canine body. They are highly moveable and all have a synovial capsule (collagenous structure) surrounding the entire joint, a synovial membrane (the inner layer of the capsule) that secretes synovial fluid (a lubricating liquid), and cartilage known as hyaline cartilage that pads the ends of the articulating bones (fibrous tissue enclosing a synovial cavity). The joints are lubricated for smooth action by synovial fluid and are stabilized by tendons and ligaments. The joints are the hinges of the body. A well-formed joint allows bones to act as levers; the bones move at angles to each other to produce movement. Internally, they have cushion-like padding called cartilage. Externally, they are held together by flexible ligaments. There are six types of synovial joints, which are classified by the shape of the joint and the movement available. The dog’s body has three basic types of joints that we are going to discuss: • BALL AND SOCKET such as the hip and shoulder joints. • HINGED such as the knees and elbows, which move in one plane (like a door). • GLIDING or PLANE such as the wrists and ankles. • CONDYLAR such as the stifle (knee) that resembles a hinge joint in movement, but differs in structure. (For the sake of simplicity, we will consider these to be hinged joints.)
Figure 2. Canine skull with some sutures accentuated.
Figure 3. Fibrous joint sutures in skull showing connective tissue.
Figure 4. Intervertebral Disc
Figure 4a. Placement of Invertebral Discs in Spinal Column and Sternum
86 | SHOWSIGHT MAGAZINE, MAY 2021
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