THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 179 



case on record where cervical and lumbar ribs are present in the same individual. 

 The cervical rib may vary between a small piece of bone connected with the 

 transverse process of the vertebra and a well developed structure long enough to 

 reach the sternum. There are also great variations in the size of the lumbar rib. 

 In case the number of ribs is normal, the last (twelfth) may be rudimentary. 



The eighth costal cartilage not infrequently unites with the sternum. Oc- 

 casionally the seventh costal cartilage fails to fuse with the sternum, owing to 

 the shortening of the latter, but meets and fuses with its fellow of the opposite 

 side in the midventral line. 



The above mentioned anomalies can be referred back to aberrant develop- 

 ment. Primarily costal processes appear in connection with the cervical, lum- 

 bar and sacral vertebrae. Normally these processes fuse with and finally form 

 parts of the vertebrae (p. 153). In some cases, however, the seventh cervical or 

 the first lumbar processes develop more fully and form more or less distinct ribs. 



As an explanation of these variations in the number of ribs, it has been sug- 

 gested that there is a tendency toward reduction in the total number of ribs, and 

 that supernumerary ribs represent the result of a failure to carry the reduction as 

 far as the normal number. In case the twelfth rib is rudimentary, the reduction 

 has been carried beyond the normal limit. This hypothesis is a corollary to the 

 hypothesis regarding the variations in the number of vertebrae. (See under 

 "The Vertebrae.") 



THE STERNUM. Certain anomalous conditions of the sternum can also be 

 explained by reference to development. The condition known as cleft sternum, 

 in which the sternum is partially or wholly divided into two longitudinal bars 

 by a medial fissure, represents the result of a failure of the two bars to unite in 

 the midventral line (p. 153, see also Fig. 130). This is sometimes associated 

 with ectopia cordis (p. 255). The xyphoid process may also be bifurcated or 

 perforated, according to the degree of fusion between the two primary bars 



(P- I54)- 



Suprasternal bones may be present. They represent the ossified episternal 

 cartilages which have failed to unite with the manubrium (p. 154). Morpho- 

 logically the suprasternal bones possibly represent the omosternum, a bone 

 situated cranially to the manubrium in some of the lower Mammals. 



THE HEAD SKELETON. The skull is sometimes decidedly asymmetrical. 

 Probably no skull is perfectly symmetrical. The condition which most fre- 

 quently accompanies the irregular forms of skulls is premature synosteosis or 

 premature closure of certain sutures. The cranial bones increase in size prin- 

 cipally at their margins, and when a suture is prematurely closed the growth of 

 the skull in a direction at right angles to the line of suture is interfered with. 

 Consequently compensatory growth must take place in other directions. Thus 

 if the sagittal suture is prematurely closed and transverse growth prevented, 



