THE RIBS. 95 
The tubercle consists of an articular and a non-articular part; the former is 
internal to and below the latter. Its articular surface, of rounded or oval shape, 
is directed downwards, back- 
3 : : Facets on head 
wards, and a lttle inwards, AC Neck 
and rests upon a facet on i \ 
the transverse process of the 
vertebra in numerical cor- 
respondence with the rib. 
The non-articular part, most 
prominent in the upper ribs, 
has the fibres of the posterior 
costo - transverse lgament 
attached to it. It is usually 
Non-articular 
-part of tubercle 
Articular part of tubercle 
: m for transverse process of 
separated from the upper vertebra 
border of the neck and shaft Fic. 73.—FirtH RicHT RIB AS SEEN FROM BEHIND. 
by a groove, in which les 
the external branch of the posterior division of the thoracic nerve. 
The shaft (corpus cost) is thin, flattened, and band-like. Its length varies 
much; the seventh and eighth, which are usually the longest, are from two and a 
half to three times the length of the first and twelfth ribs respectively. The shafts 
are curved so as to adapt them to the form of the thoracic wall. More acute in 
the upper members of the series, where the shafts are shorter, the curve opens out 
in the middle and lower parts of the thorax, where the diameters of that cavity 
are greater. The curve, however, is not uniform. Including the whole length of 
the bone, it will be seen to be most accentuated towards the hinder part, where, in 
correspondence with the point at which the bend is most pronounced, there is a 
rough ridge placed obliquely across the outer surface of the shaft; this is the angle 
(angulus costs). The distance between the angle and the tubercle is greatest on 
the eighth rib; above that, the width between these two points gradually decreases 
until, in the case of the first rib, the two coincide. Below the level of the eighth 
rib the distance slightly diminishes in conformity with the general narrowing of 
the thorax below that level. 
Combined with this curve, there is in many of the ribs a twist. This may best 
be understood if the student will take a strip of stiff paper and bend it in the form 
of the curve of the rib. If, after he has done this, he pulls down the fore end and 
turns up the hind end of the strip, he will have imparted to the strip of paper a 
twist similar to that met with in the rib. This appearance is best seen in the 
middle members of the series, notably in the seventh and eighth ribs, above and 
below which it gradually becomes less marked. It is the occurrence of this twist 
which prevents the extremities of the ribs, together with the shaft, from resting on 
the same plane surface. To this rule there are certain notable exceptions, viz. the 
first and second, the twelfth, and not infrequently the eleventh. 
The shaft has two surfaces, an internal and an external, and two borders, a 
superior and an inferior. The external surface, which is smooth, conforms to the 
general vertical convexity of the thorax, being directed upwards in the first rib, 
upwards and outwards in the higher ribs, outwards in the middle series, and out- 
wards and slightly downwards in the tenth, eleventh, and twelfth. The internal 
surfaces are arranged conversely and are covered by the parietal pleura. Towards 
the sternal end of the middle ribs, where the downward twist is most marked, there 
is often an oblique line across the outer surface. This is sometimes referred to as the 
anterior angle. The upper border of the shaft is thick and rounded behind, thinner 
and sharper in front; to it are attached the fibres of the internal and external inter- 
costal muscles. The lower border is grooved behind at the expense of the inner 
surface, and is overhung externally by a sharp margin. Anteriorly this subcostal 
groove (sulcus costalis) fades away, and its lips coalesce to form a rounded edge. 
The intercostal vessels and nerve are lodged in this groove, whilst its lips afford 
attachment to the external and internal intercostal muscles respectively. On the 
floor of the groove may also be seen the openings of the canals for the transmission 
of the nutrient vessels, which are directed towards the vertebral end of the rib. 
