1210 VESTIGIAL AND ABNORMAL STRUCTURES 



parietal bones and aV)ove the occipital. This bone is known as the interparietal 

 bone, or, because it appears more frequently in ancient Peruvian skulls (tive to six 

 j)er cent.) than in European skulls (one to two per cent.), it is also called the os 

 Inc^. 



The interparietal bone may consist of lateral halves, a condition produced by 

 the centres on each side failing to unite across the middle line. 



In a relatively small niunber of cases the upper segment alone may remain 

 separate in adult life as a pne-interparietal bone, which corresponds in position with 

 the apex of the occipital bone; this also may consist of two lateral segments. In a 

 still smaller numlier of cases both interparietal and i)ra?-interparietal bones may be 

 present in the same skull. 



The interparietal bone appears first among mammals, and is best developed in 

 some of the lower forms. The prae-interparietal bones are only constantly. present 

 in the horse. 



Fontanelle bones. — Accessory centres of ossification occasionally appear in 

 the membranous wall of the skull in the regions of the anterior and posterior 

 fontanelles, where they give rise to small segments of bone, which remain separate 

 throughout life. The anterior fontanelle bone lies in the region of the bregma, 

 l)etween the parietals and the frontal, and the posterior fontanelle lione is situated 

 at the lambda, between the parietals and the occipital. Both are of the nature of 

 Wormian ])ones, and the posterior fontanelle bone must not be confounded with the 

 pne-interparietal l:)one. 



Os Japonicum. — This term has been apiDlied to a malar bone, which consists 

 of two separate fragments, under the idea that a bifid condition of that bone is more 

 common in the Japanese than in other races; but a double malar bone is not unfre- 

 quently met with in other races, and its significance is not at present thoroughly 

 understood. 



THE STERNUM 



Cleft sternum. — Occasionally the sternum, instead of being a single median 

 bone, consists of two lateral parts, each part being connected Avith the cartilages of 

 the true ribs of its own side. This condition is due to the persistence of one of the 

 embryological phases through which the sternum normally passes, for in the early 

 periods of development each half of the sternum is formed by the fusion of the 

 cartilaginous extremities of the upper nine ribs, and thereafter the two cartilaginous 

 bars thus produced fuse together to form the single median sternum with which the 

 eighth and ninth ribs subsequently lose their connection. If the fusion of the two 

 halves of the cartilaginous sternum does not occur, ossification proceeds in each half, 

 and the condition of cleft sternum is j^roduced. 



Perforated sternum. — Xot unfrequently an aperture, of larger or smaller size, 

 is found in the lower part of the body of the sternum. It is filled, in the recent 

 condition, by fibrous tissue or by cartilage, and it is due to the non-union of the 

 lateral centres of ossification from which the loAver part of the sternum is ossified. 



Ossa suprasternalia. — The two small bones or round nodules of cartilage 

 which are known as the ossa suprasternalia, or Brecht's cartilages, are found, one 

 on each side, above the sternum and immediately internal to the sterno-clavicular 

 joint. They are to be looked upon as rudiments of the episternal bone, which is 

 met with in so well-develoi»cd a form in the ornithorhynchus. 



THE RIBS 



Additional ribs. — There are usually twelve pairs of ribs in the human subject, 

 but there is embryological evidence to show that man has inherited a larger 

 number from liis ])redecessors. Therefore the occurrence of additional ribs is an 

 atavistic phenomenon. In the embryo rudiments of additional ribs are found both 

 at the upper and lower ends of the ordinary series, — at the up]ier end in connection 

 with the last two cervical vertel^nc, and at the lower end in connection with all the 

 hniibar and sacral vertebra\ In the adult, however, additional rilis are only 



