THE COSTAL CARTILAGES. 97 
usually large in proportion to their shafts, support a single facet for articulation 
with the eleventh and twelfth dorsal vertebree respectively. The tubercles are ill- 
developed, and have no articular facets. The angle is faintly marked on the 
eleventh, scarcely perceptible on the twelfth. Their anterior extremities are narrow 
and pointed, and tipped with cartilage. The subcostal groove is absent in the 
twelfth, and but slightly seen in the eleventh. The twelfth is considerably shorter 
than the eleventh rib. 
Architecture.—Each rib consists of a curved and flattened bar of bone, the interior of which 
is loose and cancellous, whilst the investing envelope is compact. The inner table is much the 
stronger, attaining its maximum thickness opposite the angle—in front and behind which it 
becomes gradually reduced. The outer table, much thinner, is stoutest opposite the angle; on 
the posterior surface of the tubercle and neck it forms but a thin layer. The compact layers 
forming the upper and lower borders are not so thick as those forming the inner and outer 
surfaces. The cancellous tissue, loose and open in the shaft, is most compact in the region of the 
head and towards the anterior extremity. 
Variations.—The number of ribs may be increased or diminished. Increase may oceur by 
the addition of a cervical rib due to the independent development of the costal element in the 
transverse process of the seventh cervical vertebra. This may happen on one or both sides. The 
range of development of these cervical ribs varies ; they may unite anteriorly with the sternum, 
or they may be fused anteriorly with the cartilage of the first rib, or the cervical rib may be 
free. It may in some instances be represented mainly by a ligamentous band, or its vertebral 
and sternal ends may be alone developed, the intermediate part being fibrous. At times the 
vertebral end only may be formed and may be fused with the first rib, thus leading to the 
formation of a bicipital rib such as occurs in many cetaceans. Increase in the number of ribs 
may also be due to the ossification of the costal element which is normally present in the embryo 
in connexion with the first lumbar vertebra (Rosenberg, Morph. Jahrb. i.). Reduction in 
the number of ribs is less common. The twelfth rib rarely aborts; im some cases the first rib is 
rudimentary. Cases of congenital absence of some of the ribs have been recorded by Hutchinson, 
Murray, and Ludeke. 
Fusion of adjacent ribs may occur. (Lane, Guy’s Hosp. Reports, 1883.) 
Variations in form may be in great part due to the occupation of the individual and the con- 
stricting influence of corsets. Independently of these influences, the fore part of the shaft is 
sometimes cleft so as to appear double ; at other times the cleft may be incomplete so as to form 
a perforation. Occasionally adjacent ribs are united towards their posterior part by processes 
having an intermediate ossicle between (Meckel), thus recalling the condition normally met with 
in birds; more usually, however, the bony projections are not in contact. 
The number of true or vertebro-sternal ribs may be reduced to six, or increased to eight (see 
ante, p. 98). 
Ossification.—Ossification begins in the cartilaginous ribs about the sixth week, and 
rapidly extends along the shaft, so that by the end of the third month it has reached the 
permanent costal cartilage. The sixth and seventh ribs are the earliest to ossify. The 
first rib being the last (Lambertz). At puberty, or before, secondary centres appear ; 
of these there are three—an epiphysis for the articular surface of the tubercle, one for the 
non-articular part of the same process, and one for the head. By the twenty-fifth year 
fusion between these and the shaft is complete. 
‘ THE COSTAL CARTILAGES. 
The costal cartilages, of which there are twelve pairs, are bars of hyaline 
cartilage united to the anterior extremities of the ribs, into which they are recessed 
and held in position by the periosteum. Through these cartilages the first 
seven ribs are connected directly with the sternum by means of synovial joints 
corresponding to the notches along the margins of the breast bone. To this there 
is an exception in the case of the first rib, the cartilage of which is directly blended 
with the manubrium sterni. The eighth, ninth, and tenth are connected indirectly 
with the sternum by their union with each other, and their articulation, through the 
medium of the eighth with the seventh rib cartilage; whilst the eleventh and 
twelfth cartilages tip the ribs to which they belong, and lie free in the muscles of 
the flank. The costal cartilages increase in length from the first to the seventh, 
below which they become shorter. The first inclines obliquely downwards and 
inwards to unite with the upper angles of the manubrium. The second les more 
or less horizontally. The third to the seventh gradually become more and more 
curved, inclining downward from the extremities of their respective ribs, and then 
turning upwards to reach the sternum. The tenth cartilage articulates by means 
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