Below the cleft of the chin, the chin itself protrudes. Its breadth at the base is marked by two lines which, prolonged, would meet at the septum of the nose, making a triangle that wedges upward into the base of the lower lip. It is bordered on each side by two planes which reach to the angle of the jaw.
Variations in chins present the following comparisons: high or low; pointed or ball; flat, furrowed or dimpled; elongated, double, etc.
The Mouth. Details of Mouth and Lips.
The upper part of the hotly is built around a bony cage called the thorax, conical in shape, and flattened in front. The walls of this cage are the ribs, twelve on each side, fastening to the spine behind and to the sternum or breast bone in front. The upper ribs are quite short and make a small circle; they grow longer until the seventh, which is the longest and the last to fasten to the breast bone. The next three grow shorter and shorter, and reach the sternum only through a long costal cartilage, which with the projecting end of the sternum (ensi-form cartilage) form the abdominal arch. The last two ribs are quite short and are free at their front ends. The first seven are called true ribs, the next three false, and the last two floating ribs.
To the breast bone at the top of this cone the collar bones are attached, lifting the whole mass away from the cone and making it a flat surface, wedging downward. The inner ends of the S-shaped bone are curved around the apex of the cone, but the outer ends move forward again, bringing the mass of the shoulders with them to form the flat front surface.
Thus, without the shoulders, the cage of the chest is a cone with the apex upward. Under the muscles this form may easily be seen.
With the shoulders, it is a wedge with the apex downward. The profile of the sides forms a wide wedge, buttressed by a mass of lateral muscles over the iliac crest.
The front surface, formed mainly by the pectoral and rectus abdominis muscles, forms a much more slender wedge. Its upper third bevels more sharply in, as far as the lower border of the breast muscle, paralleling the edge of that muscle, bounded by a line across the bottom of the breast muscle, just below the nipple, through the epigastric pit. Its lower two-thirds bevels more sharply down, following the edge of the rectus muscle; its lines almost meet at the symphysis pubis below.
A vertical central groove divides the symmetrical halves of the front, running its full length. It begins in the pit of the neck, between the collar bones. Over the breast it marks the breast bone, and is deepened by the bulge of the pectoral muscles on either side. At the end of this, its upper third, is a pit (epigastric pit) marking the divergence of the ribs.
At the end of its middle third is another pit. the umbilicus or navel. At the end of its lower third is the mound of the symphysis. This line is useful for placing the masses of the chest, the epigastrium (over the stomach) and the abdomen.
A straight line marking the collar bones defines the top of the first mass; and paralleling it, a line through the base of the breast muscles and pit of the epigastrium forms its base.
Although the shoulders are freely movable, changing the lines of the first mass, and bulging the pectoral muscles, yet the mass itself changes little except the slight change in respiration. Even in respiration the upper portion, as far as the level of the epigastric pit, changes little; the lower ribs perform most of the respiratory movement.
Centering on this pit is the abdominal arch, made of the cartilages of the false ribs. At its centre, the end of the breast bone (ensiform cartilage) hangs pendent; on either side the arch descends diagonally, variously curved, separating thorax from abdomen.
Below this arch is the abdomen, the most movable part of the mobile portion. It is bounded below by a line passing approximately through the anterior points of the iliac crests. Its profile shows the lines of the cone of the thorax diverging downward, the lines of the wedge of the chest and shoulders converging downward, and the buttressing of the lateral muscles.
In the bending or turning of the body the central line of this portion bends always to the convex side, always paralleled by the borders of the rectus muscle.
By this movement the straight wedge of the front is broken. It becomes not a bent wedge, but two wedges; one the upper half of the original wedge, prolonged but not completed downward; the other the lower half, prolonged upward to meet the one above.