Shoulder

Brachium

Antebrachium

Facial and Masticatory Muscles

Thorax and Heart

Brain

Body Wall

Hip and Thigh

Shank and Foot

Digestive System

Urinary and Reproductive Systems

Hip and Tail

Antebrachium of the Cat (Felis catus)

Photos and text by Adam Hartstone-Rose

Introduction:

Cats use their highly dexterous and manipulative forepaws (or manus) for hunting and feeding as well as in terrestrial and arboreal locomotion.  Each paw has four clawed digits that come in contact with the ground (rays 2-5).  The first digit (the equivalent of the human thumb, ray 1, or pollex), also has a claw, but is shorter than the other four digits and is suspended above the ground during regular digitigrade locomotion (walking on the toes and not walking on the whole foot, like humans do).  Cat paws also have a small fleshy nub on the opposite side from the first digit.  This pseudo-digit is a furless, textured adaptation of the pisiform bone.Cat’s claws are retractable, meaning that during locomotion they are sheathed in flesh and out of contact with the ground.  This allows for silent movement.  During attack, various muscles (flexor digitorum longus and the lumbricals) flex the distal phalanges (the bone attached to the claw) extruding the sharp claws [1].Most of the muscles of manual manipulation are found not in the distal extremity (manus), but in the forearm or antebrachium.  The dissection of the manus and antebrachium of the cat is fairly labor-intensive and reveals more named muscles than almost any other region of anatomy.

Dissection Technique:

The skin can be removed as a glove or sock in one whole tube rolled distally, or a slit can be made either on the ventral or dorsal surface of the antebrachium.  Either way, if the skin was removed from the brachium in a previous dissection, begin with the free end of the skin, and separate the skin from underlying connective fascia with a blunt tool or finger.  The skin is much more tightly adhered to the underlying muscles in the forearm (and even more so in the paw) than it is on either the thorax or brachium.  After the skin is removed from the underlying fascia, a tight fascial binding covers the entire specimen from the elbow to the digits.  It is this fascia (the fascia antebrachii) that is tensed by the brachial muscle tensor fascia antebrachii (see arm dissection).

This fascia is extremely difficult to remove and its removal will likely take more dissection time than the identification of the muscles.  Besides being tightly adhered to the deeper muscles (which makes the muscles easy to damage during its removal), one reason the fascia is difficult to remove is that not only does it superficially wrap the entire antebrachium, but slips of it penetrate between the antebrachial muscles and wrap each of them individually before adhering to the radius, ulna and interosseus membrane.  This penetrating fascia is especially thick where it divides the flexor and extensor groups of muscles.

Forearm musculature:

The muscles of the antebrachium are all long, strap-shaped muscles that attach either to different locations on the wrist or continue into the manus to move the digits (Table 1).

Table 1.  Antebrachial musculature.  Adapted from [2].

Paw musculature:

Because of their varied use of their paws and their retractable claws, cats have many more muscles in their paws than most other quadrupeds (Table 2).

Table 2.  Manual musculature (not marked in figures).  Adapted from [2].

Discussion:

The dominant trend in cursorial mammals is the reduction of weight at the distal end of the limb, often accompanied by a loss of digits and a lengthening of the distal elements (e.g., long metapodials and short femora).  This lengthens stride.  However, there is a trade-off in this adaptation: the maximization of gait efficiency generally comes at a loss of strength and dexterity of movement [3].  The cat has found a balance with a cursorial-type shoulder girdle and brachium and a dexterous manus with accompanying antebrachial musculature.  Cats can run and jump somewhat efficiently, but can also climb and manipulate objects with their paws.

Different felids strike different balances on this continuum, with cheetahs (Acinonyx jubatus) maximizing stride length and sacrificing manual dexterity, and lions (Panthera leo) maximizing forearm and paw strength at the expense of speed.  These differences reflect hunting techniques and prey selection:  The cheetah hunts fast animals, smaller than its own body size, that it chases down with speed and strangles with a bite.  Lions cannot sustain a fast chase and attack strategically in groups using stealth and teamwork to get close to their large prey, which can be several times their own body weight.  They then use their strong forelimbs to grab onto and hold the target animal to either tire it, or slow it long enough for another animal to suffocate it by covering the nose and mouth while holding tightly to the head with its paws. [3, 4]

Thus, felids in general are neither perfectly suited for either speed or agility (though the cheetah certainly approaches the former) but are able to do a mixture of both.

References:

1. Russel, A.P. & Bryant, H.N. 2001. Claw retraction and protraction in Carnivora: the cheetah (Acinonyx jubatus) as an atypical felid.  J. Zool. 254:67-76.

2. Reighard, J. & Jennings H.S. 1935.  Anatomy of the Cat.  New York: Henry Holt and Co.

3. Turner A. 1997.  The Big Cats and Their Fossil Relatives.  New York: Columbia Univ.

4. Kunzel, W. & Probst, A. 1999. Anatomical features of the carpal joint of the cheetah (Acinonyx jubatus) compared to the domestic cat (Felis catus). Anat. Hist. Emryol. J. 28:177-182.