98 OSTEOLOGY. 
of a synovial joint with the ninth, the ninth with the eighth, and the eighth with 
the seventh. There are also surfaces for the articulation of the seventh with the 
sixth, and sometimes for the sixth with the fifth. 
Variations.— Occasionally a costal cartilage is unduly broad, and may be pierced by 
a foramen. The number of costal cartilages connected with the sternum may be reduced 
to six or increased to eight (see p. 93). In advanced life there is a tendency towards 
ossification in the layers underlying the perichondrium, more particularly in the case of 
the first rib cartilage, in which it may be regarded as a more or less normal occurrence. 
THE THORAX AS A WHOLE. 
The bony and cartilaginous thorax is barrel-shaped, being narrower above than 
below, and compressed from before backwards. Its posterior wall is longer than its 
anterior, aud its trans- 
verse width, which 
reaches its maximum 
opposite the eighth or 
ninth rib, is much in 
excess of its sagittal 
diameter. This is 
largely owing to the 
forward projection of 
the thoracic part of the 
vertebral column into 
the thoracic cavity. 
The anterior wall 
is formed by the ribs 
and rib cartilages, to- 
vether with thesternum. 
The posterior wall 
comprises the thoracic 
part of the vertebral 
column and the ribs as 
far as their angles. 
Owing to the backward 
curve of the ribs, and 
the projection forwards 
of the vertebral bodies, 
the antero - posterior 
ciameter of the thoracie 
cavity 1s considerably 
greater on either side 
of the middle line than 
in the mesial plane, thus 
allowing for the lodg- 
ment of the rounded. 
posterior borders of the lungs. For the same reason the furrow on either side of 
the spinous processes of the thoracic vertebre is converted into a broad groove 
(vertebral groove), the floor of which is in part formed by the ribs as far as their 
angles. The grooves so formed are each occupied by the fleshy mass of the erector 
spine muscle. 
The lateral walls are formed by the costal arches. The ribs which run 
obliquely from above downwards and forwards do not lie parallel to each other, but 
spread somewhat, so that the intervals between them (intercostal spaces) are wider 
in front than behind. 
The superior aperture or inlet formed by the body of the first thoracic 
vertebra behind, the arches of the first rib on either side, and the upper border of 
the manubrium sterni in front, is contracted and of reniform shape. The plane of 
the inlet is oblique from behind downwards and forwards, so that in expiration the 
Fic. 75.—lHE THORAX AS SEEN FROM THE FRONT. 
