Part Five: The Muscular System
Biology of the Chicken, Part 5
The muscular systems of our Hank and Henrietta must truly be considered “the meat” of the series on the biology of the chick-en. Muscles, whether labeled white meat or dark, has been used as a source of protein by man since prehistoric times. In this article, I hope to give you a better understanding of the three muscle types included in the chicken musculature system, and how it relates to our very own system. I will also discuss the differences between white meat and dark meat.
Approximately 175 different muscles comprise about 75 percent of the weight of the chicken. All movement from the appendages to internal contractions of the intestines and vessels are controlled by the muscular system. Hank’s crow and Henrietta’s cluck would be mute without the muscular action of vocal cords. The modern broiler industry has taken advantage of the chicken’s musculature that was built to fly. By applying modern genetic selection, they have developed the breast muscles in particular to increase the amount of white meat so preferred by the consumer.
All animals have three types of muscles: smooth, cardiac and skeletal. Regardless of their type, all muscles provide some action of motion. Some muscles are involuntary and others take a conscious mental direction to react. The fibers of the muscles differ within the three muscle types depending on their individual job, strength or duration of work.
Smooth muscle, also called involuntary muscle, is the type of muscle that is found in the blood vessels, air passages, alimentary canal (food tube) and other internal organs. As its name implies these muscles are beyond the control of the will and directed by the autonomic nervous system (ANS). “Auto” as a prefix means self, and implies that the brain controls these muscles automatically. I will go into the nervous system in more detail in a future article.
Cardiac muscle is another type of involuntary muscle. As its name implies it is located in the heart and is specialized for doing a tireless and unending job. Structured differently than the other two types of muscle it must beat 24/7 without the rest afforded the other two muscle groups. The movement of blood cells from the tip of the comb to the tip of the toes is dependent on the contraction of this muscle.
Skeletal muscle is that which forms the shape of the bird and preforms all its voluntary movements. All skeletal muscle is attached to bone by a fibrous tissue called a tendon. Did you know that all skeletal muscles pull and never push? They accomplish this action by working in pairs. Muscles can only contract and then they must relax. Let’s consider Hank’s wing for an example. His largest skeletal muscle is the pectoral or breast muscle. When this powerful muscle contracts it provides the pull necessary for the wing to move down. The antagonistic (opposite) pull is done by the supracoracoideus muscle, and returns the wing back up. Interestingly, the point of attachment for both of these muscles is the keel. This reaffirms why the keel (breast bone) is so pronounced in the avian skeleton.
It is easy to see how skeletal muscles work in pairs. Try this for yourself. Make a muscle with your bicep by drawing your fist toward your shoulder like Popeye. Now, feel how hard that bicep muscle is. It has contracted and has pulled your arm toward you. While you are still flexed feel the tricep muscle directly under your arm. It is softer and relaxed. Now, extend (pull) your arm straight out. Feel how the bicep has softened and your tricep has contracted and hardened. This is also how all skeletal muscles for the chicken and other animals work.
Historically, Sunday chicken dinner has always perpetuated some minor conflict over who wants dark meat and who wants white. So what is the difference? It is all chicken, right? The truth is there are significant differences. Dark meat such as the leg and thigh are skeletal muscles that are used for sustained activity such as walking or running. Other species of poultry that commonly exhibit more flight (ducks, geese, guinea fowl) have dark meat throughout their bodies. More activity in a muscle increases its need for oxygen. Just like the hemoglobin in blood carries oxygen via our red blood cells, so too does myoglobin assist in transporting oxygen to muscle cells. Myoglobin tends to add the dark coloring to the active muscles and create what we call the dark meat. An advantage to choosing dark meat would be considerably more flavor than white. Disadvantages, however, include more fat content and a little tougher texture due to the muscles’ amount of activity.
White meat is the result of well rested muscles. The primary source of white meat in chicken and turkeys alike are the pectorals or breast muscles. Both domestic species tend to do more walking than flying. Commercial bred birds, in particular, have been produced to have larger breast muscles making them too heavy to fly. These little-used muscles have no need for a rich oxygen supply. Therefore, there is limited myoglobin to influence a darker presence in the muscle or meat. White meat is the average consumer’s preference. From nuggets to fingers, it is considered the “healthier” choice of the two meat types. It has a higher protein content and a lesser fat content than dark meat.
The muscular system of the chicken provides the overall motion for all the bird’s actions and systems. As consumers of chicken, we tend to be interested in the skeletal muscles we call “meat.” Here again, as we have seen in other systems, Hank and Henrietta’s heritage of once being birds of flight has influenced their importance. The development of the chicken’s seldom-used flight muscles has become a wealth of protein that feeds hungry nations. As for me, give me a good heritage chicken with a lot of dark meat and flavor and I will risk chewing it a little longer than a “nugget.”