THE HUMAN STERNUM 6i 



IV. TERATOLOGY OF THE STERNUM 



I have had no opportunities of making personal observations on this 

 aspect of the subject. Anthony devotes a chapter to the subject in his 

 valuable memoir,' from which the following summary is taken : — 



Absence of the Sternum. 



The metasternum is occasionally wanting in the human subject, and 

 in rare cases the mesosternum and metasternum are both absent together, 

 the presternum alone being present. It is doubtful if the presternum alone 

 is ever absent : such conditions as have been described being possibly cases 

 of fissure. One case is recorded of absence of the presternum and mesosternum 

 together, the metasternum alone being present. In this case the thorax was 

 enclosed by a fibrous membrane. 



Defective or Excessive Attachment of the Sternal Ribs. 



The first or the seventh pair of ribs may fail to reach the sternum. 

 The former is the rarer condition, and is found most commonly in 

 anencephalic monsters. It may or may not be associated With, fissura sterni. 



On the other hand, the last cervical vertebra may carry sternal ribs 

 (Turner^"' '*). This is a very rare occurrence. It is much more common to 

 find the eighth pair of ribs associated with the sternum (Tredgold,*' 

 Cunningham"). It is commoner in the male than in the female, and on the 

 right side than on the left. 



Perforations in the metasternum are regarded by Anthony as indications 

 of the primitive lateral separation of the parts. Perforations never occur In 

 the presternum. 



Fissures may be partial or complete ; the former occurring in the pre- 

 sternum or metasternum, separately or together ; the latter occurring so as to 

 produce two lateral plates, each associated with its corresponding ribs. Many 

 cases occur in anencephalic foetuses. The two hemi-sterna are in such cases 

 (Turner") united across the middle line by a fibrous membrane. 



