EXCEPTIONAL RIBS. 



153 



Fig. 177. 



Tenth rib. 



Single 

 facet on 

 head 



Eleventh rib. 



Single 

 facet 



Twelfth rib. 



The tenth rib has usually only a single articular facet on the head ; it may or 

 may not have a facet on the tubercle. 



The eleventh rib has a single articular facet on the head ; the tubercle is rudi- 

 mentary and non-articular ; the angle and the subcostal groove are slightly marked. 



The twelfth rib has also a single articular facet on the head ; the tubercle is at 

 most a faint roughness ; the angle and the subcostal groove are wanting. 



Development. — The first centre for the shaft appears in the ninth week of 

 fcEtal life, and spreads so rapidly that by the end of the fourth month the perma- 

 nent proportion of bone has been formed. At an uncertain period, probably before 

 puberty, a centre appears for the head and another, except in the last two or three 

 ribs, for the tubercle ; these unite presumably by the twentieth year. 



Variations. — The num- 

 ber of ribs is often increased or 

 diminished by one, generally by 

 a change at the end of a re- 

 gion, as explained in varia- 

 tions of the spine (page 131 ).' 

 Cervical ribs occur by the cos- 

 tal element of the seventh cer- 

 vical becoming free. In the 

 lowest and most common grade 

 it consists of a head, a neck, a 

 tubercle, and a rudimentary 

 shaft one or two centimetres 

 long, ending free. In the next 

 grade it is longer, arid its end, 

 perhaps continued in cartilage, 

 rests on the first rib. Some- 

 times it fuses with the first rib, 

 which then becomes bicipital, 

 as is normal in certain whales. 

 In the third grade, which is 

 very uncommon, it resembles a 

 small first rib, reaching the ster- 

 num. A cervical rib has-been 

 seen more than once with the 

 transverse foramen persisting. 



The explanation of this condition is given under ossification of the vertebrae. When 

 a cervical rib reaches the sternum, the next rib is usually attached to the side of the 

 manubrium by a broad cartilage, fusing with that of the cervical rib. The rib of the 

 eighth vertebra has been seen to end like an ordinary second rib. It is also very 

 rare to have only twelve pairs of ribs, of which the first is cervical. There may be 

 thirteen ribs by the addition of the costal element of the first lumbar. This may 

 be so small as to present no rib-like feature, oPit may resemble an ordinary twelfth 

 rib. In cases of an extra rib from this source the twelfth rib is usually uncom- 

 monly long. Very rarely the first true thoracic rib is imperfect, being continued 

 in ligament to the sternum, joining the shaft of the second rib, or even ending free. 

 A bicipital rib may occur also by the fusion of the first thoracic with the second be- 

 yond the tubercles. The resulting plate later subdivides, to be continued by two 

 normal costal cartilages. Ribs sometimes divide, generally near the front. The 

 parts formed by such cleavage are continued by costal cartilages which usually re- 

 unite, so that a foramen is formed which is bounded laterally or externally by bone, 

 mesially by cartilage. This occurs most commonly in the third and fourth ribs, espe- 

 cially in the latter. 



THE COSTAL CARTILAGES. 

 The costal cartilages * continue the ribs, the first seven going directly to the ster- 

 num, the next three each to the one above it, and the last two ending free. They 

 grow longer from the first to the seventh, sometimes to the eighth. The last two 



' Cartilagines costales. 



Single 

 facet 



Vertebral ends of tenth, eleventh, and twelfth ribs of right side from 



below. 



