The skeletal system is the basis of support for penguins. It binds muscles together and it provides protection for the internal organs. It allows penguins to stand upright and prevents them, basically, from becoming a ball of mush. Penguin skeletons are evolutionarily designed for flight, despite the fact that penguins don't fly. However, penguin bones are slightly more dense than the average avian skeleton. And if you've been to the histology section, you know that the skeleton is a living and dynamic system.

    I'm not going to enter a detailed discussion of muscular attachments, but I will point out the basic components of the penguin skeleton and how they are important to the daily functioning of a penguin. Exposure will be cranial to caudal. There is one important definition that I will develop at this point. Articulation: the point, area, etc. where one bone meets and interacts with another bone. (verb: articulate, articulates).

    The image above is obviously the skull of a penguin -- a King penguin to be exact. There are several features you should know about the basic penguin skull, and they are all easily visible on this picture. What you know as the bill or "mandible" is really composed of two separate portions. One is connected directly to the premaxilla; this is known as the maxillary process. The inferior part of the mandible is known as the dentary. Contained within the maxillary process is the nasal aperture; this is the site through which scents and air enter the olfactory system of the penguin.

    As we move down the list of terms on the first image, we come across three terms for portions of the skull: the frontal portion, the parietal portion, and the occipital portion. They correspond to portions or lobes of the penguin brain. The zygomatic arch is a thin strip of bone that forms the inferior base of the "eye socket." The mesethmoid is a sinus that is lined by the same tissue that lines the respiratory system -- pseudostratified columnar epithelium.

    The figure to the left demonstrates the fundamental aspects of the bony skeleton. Although the bill or beak appears to be part of the skeletal system, it is not. As you know, the penguin bill is a keratinous structure, not hydroxyapatite. Its thick and compact molecular structure allows it to persist. The bill is connected to the pre-maxilla of the penguin. The pre-maxilla forms a portion of the cranium. The cranium contains one of the more important organs of the penguin: the brain, the coordinating center of the penguin.

    Caudal to the skull are the cervical vertebrae. Cervical vertebrae do not directly articulate with the base of the skull, however. They are united via several ligaments, which are dense, tough connective tissue. There are several cervical vertebrae; other than the first, each vertebra articulates with a vertebra above and below it. The cervical vertebrae quickly change morphology and become known as the thoracic vertebrae. Thoracic refers to the thorax or chest. This is where the "rib cage" begins. The head of each rib articulates with a thoracic vertebra. The numerous ribs form a protective shield for the internal organs.

    The most elaborate thoracic bone is the most ventral: the sternum and keel. Note that this bone is unusually large. The pectoralis muscles attach to the keel of the sternum and extend to the humerus of the upper limb. Why is this sternum so large? Birds fly, and in order to fly, they need particularly large and powerful pectoralis muscles. In the case of penguins, they do not fly; however, they do swim and also require large pectoralis muscles. Sterna and keels of penguins are large as well. The pectoralis is the primary depressor muscle, and it runs ventromedially to the keel from the humerus. The primary elevator of the penguin is the supracoracoideus which runs dorsomedially from the humerus to the articulation of the clavicle and the coracoid bone. (These muscles are analogous to the pectoralis major and the latissiums dorsi in human beings.) In penguins, this entire "flight" structure is known as the pectoral girdle.

    The limbs of the penguin are similar in skeletal structure, blood supply and muscular compartmentalization. Each of the extremities (limbs) is composed of a singular bone (humerus=upper limb; femur=lower limb) proximally, two intermediate bones (radius and ulna=upper limb; tibiotarsus and fibular=lower limb), metacarpals (upper limb) or metatarsals (lower limb) and phalanges distally. In penguins, one can not usually see the femur (thigh) since it is buried below the feathers. The last structure is the pubis. The pubis of a penguin is an interesting structure when it is compared to the pubis of a dinosaur. That of the penguin points toward the head; that of the dinosaur points to the tail. This evolutionary change seen in birds (and some advanced dinosaurs) is called the reverse pubis.