* 



chap, in.] VERTEBRA AND RIBS : MAN. ] 1 1 



the present place only the most summary notice of the various cases 

 can be given. 



Amongst them can be recognized two groups, the first in 

 which the Homceosis is from before backwards, the second in 

 which it is from behind forwards. 



A few words in explanation of the use of these terms are perhaps 

 needed. 



In describing cases of such transformation in the series, it is 

 usual to speak of structures, the pelvis for example, as " travelling 

 forwards" or "travelling backwards." These modes of expression 

 are to be avoided as introducing a false and confusing metaphor 

 into the subject, for there is of course no movement of parts in 

 either direction, and the natural process takes place by a develop- 

 ment of certain segments in the likeness of structures which in 

 the type occupy a different ordinal position in the series. In 

 using the expression, Homceosis, we may in part avoid this con- 

 fusion, and we may speak of the variation as occurring from before 

 backwards or from behind forwards, according as the segment to 

 whose form an approach is made stands in the normal series 

 behind or in front of the segment whose variation is being con- 

 sidered. The formation of a cervical rib on the 7 th vertebra is 

 thus a backward Homceosis, for the 7th vertebra thus makes an 

 approach to the characters of the 8th. On the other hand de- 

 velopment of ribs on the 20th vertebra (1st lumbar), is a forward 

 Homceosis, for the 20th vertebra then forms itself after the pattern 

 of the normal 19th 1 . 



A. Backward Homososis. 



If each segment in the series of vertebras were to be developed 

 in the likeness of that which in the normal stands in the position 

 next posterior to its own, we should expect the whole series to be 

 one less than the normal. The following case makes an approach 

 to this condition. 

 18. Skeleton of old woman. C 7, D 11, L 5, S 5, C 4 (5th and 6th 

 cervicals partially ankylosed). The 7th cervical bore a pair of cervical 

 ribs [of Gruber's class 2, see p. 108], that on the left being ankylosed 

 to the 7th cervical. There were only 11 pairs of thoracic ribs. 

 The 1st lumbar was a true lumbar. Gruber, Wexzel, Mem. Ac. 

 Set. Pet, 1869, Ser. vil, xiil, No. 2, p. 23. Here the 7th vertebra 

 resembles a dorsal in having ribs, the 19th, which in the type is 

 the last dorsal, resembles a lumbar in all respects, the 24th is the 

 1st sacral, and there is no 33rd vertebra. 



1 The same terminology may conveniently be adopted in the case of the parts of 

 flowers. Development of petals in the form of sepals being an outward Homceosis, 

 while the formation of sepaloid petals would be thus called an inward Homoeosis, 

 and so forth. 



