|
Facial and Masticatory Muscles |
Photos and text by Adam Hartstone-Rose Introduction: As a quadrupedal predator, the cat can pronate and supinate its forelimb for grasping prey and climbing trees [1]. It is digitigrade, walking on its “fingers” and the “balls” of its paws, which are the distal ends of the metacarpals. It walks mainly on its four lateral, or ulnar, digits (rays 2-5) with the first digit markedly reduced, though it still possesses a claw [2]. All five digits have fleshy, furless pads on their underside for walking. The balls of the paws also have fleshy pads, as does the small proximal wrist bone, the pisiform, though this pad probably has little or no contact with the ground during locomotion. The forelimb has relatively long antebrachium, though the humerus is only slightly shorter than the ulna. The cat’s manus is not particularly elongated compared to those of ungulate mammals, but the metacarpals are proportionally longer than those of many plantigrade animals. The shoulder and arm (brachium) show marked adaptations for cursorial (distance walking and running) locomotion. For example, the vertical orientation of the scapula, the highly mobile shoulder joint which lacks a clavicular attachment, and the short, powerful, arm flexor musculature all are cursorial adaptations. However, the highly muscular forearm (antebrachium) and dexterous manus suggest that the cat is adapted for behavior other than optimal cursoriality. More about this will be discussed in the forearm lab. Dissection Technique: The first step in this dissection is the removal of the skin. If you have already dissected the shoulder, then chances are you have already skinned the brachium. If you have not, then begin by retracting the skin with forceps, nicking it with a scalpel or scissors, making sure not to damage the tissues below, and loosening the skin from the underlying muscles by breaking the loose connective tissue with a blunt probe. The skin overlying the brachium of a cat, like that over the torso and abdomen, is loosely connected to the underlying musculature and should not require much effort to remove. Before many of the arm muscles can be observed, the pectoral muscle must be reflected. Cleanly cut the muscle a centimeter or two from its insertion (close to the elbow) and reflect both ends. Besides the examination of the muscles of the brachium, this dissection allows you to observe the neurovasculature of the arm as well. With some care, the veins, arteries and nerves of the brachial plexus can be preserved. Most of these structures lie in the axilla (arm pit) and the medial aspect of the arm between the major flexors and extensors [3]. As is true with many of the delicate structures throughout the body, these structures are embedded in fat and fascia. These should be carefully removed through blunt dissection with a probe, forceps and, if necessary, pointed scissors. Cutaneous Muscles: Unlike the panniculus carnosus of the torso and abdomen, the brachium of the cat is not deeply invested with cutaneous musculature. Thus, no particular care is necessary when removing the skin over this region. Musculature: In contrast to the shoulder region, or especially the antebrachium, the brachium has relatively few separate muscles (Tab. 1). Though these muscles are not particularly numerous, they are relatively powerful, especially the elbow extensor, the triceps. The muscles of the brachium can be broken up into two main groups: the elbow extensors and the elbow flexors. Table 1. Adapted from [4].
Conclusion: By far the largest muscle of the cat brachium is the triceps, particularly its long and lateral heads. This is in contrast to the state found in humans in which the flexor group muscles are larger than the extensor group muscles of the arm. There are several possible adaptive explanations for why cats apparently need to extend their elbow with more force than they need to flex it; however, the truth is that this is the most common state found in quadrupeds, and humans are probably in the minority of mammals in our muscular arrangement. The cat’s muscular proportions are logically explained by considering the force necessary to do the two different actions: the only energy necessary for flexing the elbow is to raise the antebrachium and the manus, whereas the extension of the elbow is a weight bearing and locomotive action in quadrupedality.References: 1. Turner, A. 1997. The Big Cats and Their Fossil Relatives. New York: Columbia Univ. 2. Vollmerhaus, B. & Roos, H. 2001. Konstruktionsprinzipien an der Vorder- und Hinterpfote de Hauskatze (Felis catus). Ant. Histol. Embryol. 30:89-105 3. Ozudogru, Z., Aksoy, G., Soyguder, Z. & Ozmen, E. 2003. Veins of the thoracic limb of the Van cat. Anat. Hist. Embryol. 32:116-123 4. Reighard, J. & Jennings H.S. 1935. Anatomy of the Cat. New York: Henry Holt and Co.
|
Artwork: Weil, from Stubbs' 1776
"Anatomy of the Horse."
Background free from Eos Development, with
slight color modification.
