SECTION' XI 



VESTIGIAL AXD ABNORMAL 

 STRUCTURES 



By ARTHUR ROBIXSOX. M.D.M.R.C.S. 



LECTURER ON ANATOMY IX TUE MIDDLESEX HOSPITAL MEDICAL SCHOOL; EXAMINER IN ANATOMY FOR THE CON- 

 JOINT BOARD OF ENGLAND 



The vestigial structures met -with in the human body may be classified in 

 two groups: — 



(1) Renniants of organs which phiyed an important part during fietal life. 



(2) Structures which appear regularly in the human body, but which possess little 

 or no function, Ijeing merely vestiges of organs which are much better devel- 

 oped in some of mankind's ancestors. 



The abnormal structures which occasionally appear in the human subject are 

 produced by retardation or excess of ordinary develojunental i)rocesses, 1)V the non- 

 union of parts Avhicli usually fuse together, by the fusion of parts which usually 

 remain separate, or the formation of organs not usually develoi)ed in man. ])Ut 

 which are, however, frecjuently met Avith in some of his more or less remote ances- 

 tors; organs of the latter class are of atavistic nature, tliat is, they are due to a 

 'reappearance of a more primitive organisation, or a reversion to a i)rimary state.' 



THE SKELETON 



THE SKULL 



The epipteric bone. — This term wa.« applie<l by Professor Flower to a small 

 bone occasionally found at the bottom of the temporal fossa, separating the great 

 wing of the s])h('noid from the anterior inferior angle of the parietal Ixme. It is of 

 the nature of a \\'ormian l)one, and is due to the apjiearance of a centre of ossifica- 

 tion in the region usually occupied by a portion of the jiarietal l)one or the great 

 wing of the sjthcnoid. 



The interparietal bone, or os Incae, and the praeinterparietal bone.— The 

 posterior section of the occipital bone, the stjuama occii»italis, is usually deveiope<l 

 from at least three pairs of centres, which are superposed. As a rule, each pair of 

 centres soon fuses into a sinsle nucleus of ossification, and the three segments of 

 bone formed from the three pairs of centres fuse together to complete the squama 

 occipitalis. 



Occasionallv, liowever, the two upper segments may unite, but remain separate 

 from the lower segment, and then, in the adult, a V)one is found lying between the 



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