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

Shoulder of the Cat (Felis catus)

Text and Photos by Adam Hartstone-Rose

Introduction:

The cat is a quadrupedal predator.  It is digitigrade (walks on its digits, as opposed to its whole foot/hand or claw).  It has basic adaptations for cursorial (running) locomotion, but, unlike ungulates, has rather flexible shoulders, which allow it to climb and catch prey with remarkable agility.           

The main specializations toward this way of life evidenced by the shoulder region are the vertical position of the scapulae and greatly reduced, proximally free, clavicles which allow for cursorial locomotion.  Also, the flexibility of the limb girdle is marked by highly movable joints with greater stabilization provided by muscular support rather than bony support.

Dissection Technique:           

It is best to remove only the skin overlying the anatomical features under immediate investigation, allowing the anatomy meant for later observation to remain moist and intact.  For the dissection of the shoulder region, remove the section of skin from about halfway down the abdomen (caudally) to the base of the skull (cranially), and just distal to the radiohumeral joint (elbow).  Though this reveals slightly more of the underlying tissue than is necessary for the investigation of the shoulder, some of the muscles of the cat shoulder girdle (e.g., spinotrapezius and latissimus dorsi) originate extremely caudally, and the pectoantebrachialis muscle inserts onto the proximal antebrachium.           

To remove skin use the forceps to lift a small section at the spine about halfway down the lumbar region (halfway down the abdomen, but on the dorsum of the animal).  Nick this fold either with a scalpel or scissors.  Insert a blunt probe into the opening and run it under the skin to break the loose fascia between the skin and superficial muscle layer.  The panniculus carnosus, the most superficial muscle just deep to the skin, is very thin and too closely attached to the skin to warrant an attempt at removing the skin while leaving the panniculus carnosus attached to the cadaver.  Also, it is superficial to the loose connective tissue and comes off easily with the skin.  It can be viewed on the deep surface of the removed skin.  Once the skin is separated from the underlying muscles for a few centimeters, place the blunt probe deep to the skin, parallel to the vertebral column and cut the skin cranially.  Continue loosening the skin and cutting it all the way to the base of the skull in this fashion.

Next loosen and cut the skin in a line circling the neck and another line circling the torso of the animal at the same transverse level at which the dorsal cut was begun. Loosen the skin all the way around the animal until the skin is attached only by the brachia.  At this point, it may be convenient to create a cut parallel to the dorsal cut on the ventral midline, thus separating the skin into two sides.  Continue loosening the skin along the brachia and onto the proximal ends of the antebrachia.  Cut the skin in a circular line around the proximal end of the antebrachia taking care not to disturb the underlying muscle.  The skin can now be removed over the forelimbs, or can be cut away.           

The specimen should be cleaned of remaining loose fascia where possible.  The borders of superficial muscles should be defined using a blunt probe where possible and a sharp probe or scalpel where necessary.

Cutaneous Muscles:

As noted above, the panniculus carnosus muscle was removed with the skin and is visible on the skin’s deep surface.  It inserts into the neck region of the cat and is most likely what accounts for the slightly increased adhesion of the skin to the underlying muscles cranially.Superficial Musculature:The superficial musculature is now revealed (Tab. 1)

Table 1.  Superficial Musculature.  Adapted from [1].

Now detach the trapezius, latissimus dorsi and deltoideus muscles from their origins (mainly along the spine and scapula) and reflect them toward their insertions.  The deep muscles are now visible (Tab. 2).

Table 2.  Deep Musculature.  Adapted from [1].

Discussion:

One interesting feature of the shoulder girdle musculature of the cat is the dorsoventral mobility of the scapula compared to that of humans [2].  This is accomplished mainly by the difference in the shape of the thoracic cavity.  By having a mediolaterally compressed thorax, the cat’s scapulae are rotated into a laterally position in the sagittal plane, whereas the dorsoventrally compressed thorax of humans rotates the scapulae into a dorsal position parallel to the coronal plane.  Most of the shoulder girdle muscles of the cat are parallel to the scapulae and are ideally suited for cursorial activity because the scapular mobility increases stride length.  This arrangement is also well suited for pouncing, which puts especially high forces on the shoulders in the dorsoventral plane.  The highly reduced clavicle (almost non-existent) with no bony connection to the thorax (as is found in humans) allows the scapula to move freely to absorb this force perpendicular to the ground.  An animal with clavicles would not be able to do this as efficiently and would run the risk of breaking their clavicle with too much force.  In comparison, the human shoulder is more stable to allow for greater range of stable movement in brachiation.  We have inherited this trait from our arboreal ancestors.

References:

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

2. Walker, W. F. 1970. Vertebrate Dissection.  Philadelphia: W. B. Saunders Co.