Rib Cage

Rib Cage, front. Gray, Henry.

More from, Human Anatomy For Artists: The Elements of Form, by Eliot Goldfinger.

The rib cage is made of twelve pairs of ribs, their costal cartilages, the sternum, and the thoracic vertebrae. It forms a semi-rigid enclosure, protecting organs, and allows negative pressure to develop within for breathing, so says Goldfinger.

Egg shaped, with a vertical axis slightly tipped backwards, and an opening, relatively, small compared to lower.

The costal cartilages of teh seventh ribs attach to bottom of sternum, called, costal arch, or skeletal arch etc., at an angle of roughly ninty-degrees, in male, and sixty in female. (during inhalation angle increases.

Bell shaped; lower half covered only by a thin layer of muscles: therefore directly determine’s trunk shape in life. Upper portion covered by shoulder girdle, pectoral muscles, and reveals its basic shape when shoulder apparatus moves (because it is set atop the ribs).

Male rib cage: ranges from slightly wider to narrower than the width of the iliac crest of the pelvis. In female: always narrower.

Each rib attaches to a costal cartilage of vertebra. Cartilages of first seven ribs connect direct to sternum (true ribs). Cartilages of eight and ninth ribs attach to costal cartilage immediately above, while the tenth, only occassionaly reaches cartilage of the one above (these are false ribs)(sometimes eight reaches bottom of sternum). Eleventh and twelfth are not connected to any rib cartilages (floating ribs).

Outer edge of ribs, because they are flat, contribute to overall form of rib cage.

Rib Cage, backside. Gray, Henry.

On the back of the thorax, a short distance from the vertebrae, each rib abrubtly curves, formingwhat is called, the angle of the rib. This can be seen in life as a line, because erector spinae muscle inserts up to, but not past, the angles of the ribs.

The side planes of rib cage converge slightly anteriorly.

Ribs seen from the side, angle downward and forward (slightly less than 45 degrees): they have a slight S curve. Inhalation: more horizontal, as front ends rise, pivoting at the vertebrae.

Lower ribs most visible during inhalation and when trunk is flexed forward along with shoulders.

Posterior surface projects backward almost as far as spinous processes of vertebrae. Superiorly, there is increased posterior projection of spinous processes.

The tip of twelfth rib is the lowest point of the rib cage; in the male it can be as close as one inch from the iliac crest, much closer than is usually indicated in illustrations and in articulated skeletons. Says, Goldfinger. The distance is greater in females.

Costal cartilages, are extensions at anterior ends of ribs, from ribs five through eleven cartilage curves upward and medially; cartilage of three and four, generally, straight and directed upward and medially; cartilage of two horizontal; cartilage of one downward and medially. Except for first two ribs, cartilages taper medially, especially eight through twelve, which are pointed. Cartilage of six through eight, adjacent cartilages come into contact with each other, usually swelling.

Cartilage of seventh ribs attach to body of sternum and adjacent xiphoid process. Costal cartilages, here, meet at a sharp angle, articulating with the sternum: creating the top of the costal or skeletal arch. (Ligaments attached to the tips of cartilage, of the xiphoid process, may soften the sharp angle). Skeletal arch proceeds downward through ribs seven to ten.

Side view: anterior costal cartilages seven through ten may form a rounded or straight profile.

Costochondral articulation: connection of the rib to its costal cartilage. This articulation at the fifth ribs lies at the bottom of the sternum. Costal cartilage attached to sternum is called sternocostal articulation. And except for the first rib, these articulations are true synovial joints. (Synovial: relating to or denoting a type of joint that is surrounded by a thick flexible membrane forming a sac into which is secreted a viscous fluid that lubricates the joint.)

Rib Cage, sideview. Gray, Henry.

Sternocostal articulation gets tighter the lower the rib (less space between ribs).

In thin individuals sterno and costo chondral articulation may be seen in life, especially, under upper half of pectoral muscles. Or lower in inhalation.

Semilunar line, between rectus abdominis and external oblique, crosses the tip of the tenth costal cartilage, indicating the bottom of skeletal and abdominal arches.

The sternum, or breastbone, is made of the manubrium, the body of the sternum, and xiphoid process. In female, sternum is shorter and arched forward.

Manubrium, shield-like shape (short and wide), meets sternum at an angle, the “sternal angle,” usually visible on the surface. And directed more upward than plane of the body of sternum.

Sternal angle is also due to swelling of lower edge of manubrium and upper edge of the body of the sternum: into thickened lips or ridges where they meet.

“The costal cartilage of the second rib attaches to the sternum at the sternal angle, making it a good point to begin counting the ribs in life.”

And

“The medial ends of the clavicles articulate with the upper outer corners of the manubrium, and, along with the upper edge of the manubrium, form the suprasternal (jugular notch), also called the pit of the neck.” And since the pit of the neck is lower than the spinous process of the seventh cervical vertebrae in back, the neck descends lower in front.

The body of sternum is elongated and teardrop-shaped. Gentle anterior curve.

Three evenly spaced transverse ridges on front of sternum, may be seen in life.

The uppermost ridge, where the third costal cartilage articulates with sternum, may be prominent, even more than sternal angle. The bone at sternum swell slightly at sternocostal articulation, which may be seen on the chest in life.

A narrow subcutaneous strip of the sternum, between pectoral muscles, getting deeper and wider inferiorly, into a flat triangular area, exposing front surface and lower end of sternum body.

Lower edge of sternum is located below nipple level and above lower border of pectoralis major.

There is a depression immediately below the sternum, called, the infrasternal notch or episgrastric depression. Upper ridge is created by the seventh costal cartilage attaching to the sternum.

The xiphoid process, or ensiform cartilage, is a thin, flattened, cartilage attached to the bottom of the sternum, projecting downward and slightly forward. Superiorly it is deeper, helping create infrasternal notch, and varies in size according to the size of the distal phalanx of the thumb, and shape.