290 



SCIENCE. 



[N. S. Vol. V. No. 112. 



longer virile. I venture to call it the post- 

 genetic period. The term senility, in addi- 

 tion to the usual ontogenetic sense, will be 

 employed to express the changes that go on 

 in a given group of animals on the decline 

 from its acme of evolution, and attempts 

 will be made to correlate these states with 

 those occurring in the individual. (A. 

 Hyatt.) 



I propose to submit a number of facts 

 which, for convenience, I have placed under 

 the head of propositions, the number and 

 variety of which give a clue to the intricacy 

 of the subject. 



It will be taken for granted throughout 

 that the subject of variations of structure as 

 usually limited has been carefully considered 

 and set aside, for these do not invalidate 

 any of the propositions. 



I. Reversion to lower types is sometimes wit- 

 nessed in the senile human skull. 



Outside of Primates no mammal exhibits 

 an orbitotemporal septum. In the senile 

 human skull thinning, and often notable ab- 

 sorption of the septum is almost constantly 

 exhibited.* 



II. Complex bones are sometimes analyzed in 

 part by the manner in which absorptive processes 

 occur in them. 



Many examples can be cited to illustrate 

 the fact that bones originally separate tend to 

 localize effects of diseased action even when 

 the lines of separation between primal parts 

 disappear. It is not expected that when 

 the precoracoid bone of the cod is hyperos- 

 tosed — a condition which has been often 

 detected — that other portions of the suspen- 

 sorium should participate. Neither do we 

 find that when one-half of the lower jaw of 

 a kangaroo is diseased that the other sepa- 

 rate half should be involved. In the do- 

 mestic cat the halves of the lower jaw are 

 disposed to unite in old age, but even in 

 this animal effects of inflammatory pro- 



* I described this in Am. Journ. 3fed. Sci., 1870, 

 405. 



cesses may be confined to one-half of the 

 bone. In a specimen of an old cat in the 

 Cornell collection the symphysis was united, 

 the entire mentum thickened ; but the right 

 side of the jaw was without incisor teeth 

 and was much larger and thicker than the 

 left, which was provided with teeth. 



The center for the premaxilla in the so- 

 called superior maxilla of the human sub- 

 ject is sometimes absorbed after the suture 

 lines between the bone and the maxilla 

 proper have been obliterated.* In the 

 malar bone of the human subject the lower 

 half is sometimes separated from the upper 

 by a suture. The bone when thus marked 

 is said to be bipartite. The skull of a 

 syphilitic subject in my possession exhibits 

 absorption of the lower half of the left 

 malar bone. Since the bipartite bone 

 is exceedingly rare, it is probable that 

 the specimen had not possessed the su- 

 ture and that the absorptive process had 

 stopped when the area corresponding to a 

 distinct center of ossification had been 

 covered. Even if the bone had been bi- 

 partite the circumscription of an absorp- 

 tive process to an individual bone would 

 have been none the less striking. In 

 old age the peripheral venous openings on 

 the bones tend to become enlarged. The 

 arrangement of these openings sometimes 

 defines the regions occupied by epiphyses 

 which have long since disappeared. In the 

 femur of the senile dog, for example, the 

 line of separation of the distal epiphysis 

 from the shaft is indicated by the venous 

 foramina being enlarged on the periphery. 

 I have never observed such disposition to- 

 manifest itself in vigorous adults. 



III. Bone processes are sometimes increased 

 in size in senility, or appear in places where they 

 are not seen in antecedent stages, though occur- 

 ring normally in the species of related groups. 



In the aged human subject the styloid 

 process is sometimes longer than in younger 



* Ibid, p. 403. 



