606 report— 1878. 



The specimens illustrated in an interesting way the different degrees in which 

 the transition between the sacral and lumbar vertebral form might occur ; and, in 

 connection with the examples of symmetrical form, led the author to remark upon 

 the vai'iations in the number of sacral and lumbar vertebrae which occur not un- 

 frequently both in man and animals, and, according to some observations which he 

 has made, very commonly among the anthropoid apes. 



With reference to this subject, the author held that the most common cause of 

 the occurrence of six component pieces in the human sacrum (more frequent in the 

 male than in the female) is the increased development and union of the first piece 

 of the coccyx with the fifth sacral vertebra, without there being any actual 

 increase in the whole number of vertebrae in these two portions of the verte- 

 bral] column. But he further held it to be pi'obable that in some instances, 

 though very rare, an addition may take place to the number of sacral vertebrae 

 by the conversion of the lowest lumbar vertebra into the sacral form ; thus 

 giving rise, in an instance in which there is the usual number of ribs and dorsal 

 vertebrae, to the existence of only four lumbar vertebrae. 



It is more certain that in instances of six lumbar vertebrae (with twelve 

 developed ribs and dorsal vertebrae), the sixth lumbar vertebra may be regarded 

 as a transformed sacral vertebra ; as in an example of this kind observed by the 

 author, the sixth lumbar vertebra showed a slight approach to the sacral characters, 

 while the normal number of five sacral and four coccygeal vertebral pieces was 

 still present, thus indicating distinctly the interpellation of an additional vertebra, 

 and the transference, as it were, of one from the coccygeal to the sacral series, 

 and of one from the sacral to the lumbar series. 



Dr. Thomson showed also an example of a sacrum of four pieces, and pointed 

 out that this might arise either from the conversion of the upper piece into the 

 lumbar form or from the detachment and retrogression of the lower vertebra into 

 the coccygeal form. 



At the suggestion of Professor Macalister, Dr. Thomson exhibited from the 

 Museum of Trinity College a skeleton of the wombat {Phascolomys) presenting 

 the same unilateral retrogression of the upper sacral vertebra into the lumbar 

 form which is the cause of the oblique pelvis in man, thus showing that the 

 abnormal form of development which had been described is not confined to the 

 human species. 



4. Note on the Occurrence of a Sacral Dimple and its possible Significance. 



By Lawson Tait, F.B.C.S. 



The author had noticed casually amongst his hospital patients a curious pit-like 

 depression or dimple in the skin over the lowest bone of the sacrum, and his atten- 

 tion had been drawn to a case in which it was present in a marked degree in all 

 the children of a woman who had it. In one of these the pit was a centimetre in 

 depth, its apex adherent to the aponeurotic structures, and it expanded outwards 

 so that its mouth had a diameter of about 13 millimetres. Of the patients sub- 

 jected to examination (for other and special reasons) he found no trace of it in 55 

 per cent. ; in 22 per cent, it was faintly marked ; and in 23 per cent, it was well 

 marked. A double pit was noticed in about 6 per cent, of the cases. The average 

 age of its occurrence was slightly over thirty-two years, and the average age of 

 those in whom it was entirely absent was nearly forty-five years. In children aver- 

 aging about five years it was found entirely absent in only 26 per cent., and it was 

 well marked in 40-7 per cent, of the cases ; so that there is a manifest tendency for 

 it to disappear with age. 



The author further described the appearances of a kitten which he had dissected, 

 and in which there had been a spina bifida in the lower sacral vertebrae ; all, the 

 vertebral elements below the seat of the malformation had been arrested in deve- 

 lopment exactly as is found in the coccyges of those animals in which a tail has 

 been lost — man, the Manx cat, the guinea-pig, &c. The nervous nutrition of the 

 posterior limbs had not been interfered with in any way. 



