Febeuaey 19, 1897.] 



SGIENGE. 



293 



in whicli cusps are lost in bats in passing 

 from an insect-eating to a fruit-eating habit. 

 Thie fruit-eating bats might be called senile 

 forms, because they lost their cusps, just as 

 reasonably as we may use similar language 

 in describing an old dog. Both forms ex- 

 hibit degeneration effects ; in one it is phyl- 

 logenetic ; in the other it is ontogenetic. 



VII. Entire loss of teeth in the human sub- 

 ject from old age will often he folloiued by hyper- 

 ostosis which presents three kinds nearly answer- 

 ing in position to the three series of teeth, the 

 incisors, the premolars and the molars. 



The same law which created the differen- 

 tiation of the teeth continues to operate 

 after the loss of teeth. This is modified 

 by the characters of chronic rheumatism 

 and syphillis. The best examples in illus- 

 tration of the proposition are met with in 

 the jaws of savages.* 



VIII. The manner of obliteration of sutures 

 in the skull of mammals may be definitive. 



The parieto-squamosal and the sphenoido- 

 squamosal sutures rarely disappear in the 

 human skull, no matter how old the indi- 

 vidual. The suture last named disappears 

 in the dog in the early stages of senility. 

 The two forms are thus distinguished as 

 readily as by the employment of other 

 characters. We do not hesitate to distin- 

 guish by rates of disappearance of cranial 

 suture lines the Ophidia from other reptilian 

 orders. 



IX. Muscles modify the shapes of long bones 

 in proportion to the length of time they have 

 been exercised. 



The power of the flexor muscles of the 

 limbs, on the whole, is greater than that of 

 the extensors. It is the flexors that hold 

 the extensor tendons firmly against the 

 long bones, even, indeed, if they do not 

 create grooves for the accommodation of 

 their tendons. The longer the time the 

 extensors are thus held against the bones 

 the deeper become the grooves. Hence the 



*Proo. Acad. Nat. Sci., 1893, 1]. 



relatively deep grooves on the extensor sur- 

 faces of long bones, as in the patellar notch 

 and the grooves on the distal end of the 

 radius, tibia and fibula in old animals. In 

 an old cat in the collection of Professor 

 Wilder at Cornell University the groove 

 for the Tibialis posticus was converted into 

 a bony canal. 



Proposition IX. is also illustrated in the 

 groove for the Extensor longus pollicis of the 

 common brown bat {Adelonyderis fusca) be- 

 coming deeper in old individuals. At times 

 the sides of the groove become enormously 

 thickened by hyperostosis. 



X. In ontogeny a senile form of an annually 

 recurrent structure may resemble the juvenile 

 form and recall in phyllogeny the primitive form,. 



The horn in an old specimen of Cervus 

 canadensis tends to reassume the form of the 

 spiked horn of young specimens, which at 

 the same time recall the shape of the primi- 

 tive horn in Pecora generally. 



XI. Inflammation of bone modifies the shape 

 in places ivhich exhibit the greatest physiological 

 activities, and these prepare the way to senile 

 changes in the same regions. * 



The deepening of extensor muscle grooves 

 when carried far will, we assume, develop 

 friction and with the friction excess of 

 heat and with the heat inflammation and 

 resulted hyperostosis. But excess of heat 

 may arise from undue exaltation of func- 

 tions of any kind. In the following in- 

 stances it has been productive of grave re- 

 sults from the conjunction of restrained 

 growth force in the halves of the lower jaw 

 and the eruption of the incisor teeth. 



In the lower jaw of the domestic cat 

 whose permanent premolars are just being 

 erupted the symphysal articular surfaces 

 are nearly smooth ; in that of an animal 



* In studying the jaws of the domestic cat I have 

 been struck by the frequency of effects of traumatism, 

 especially of fractures of the teeth. It will be as- 

 sumed that in the above statement all instances of 

 disease following injuries have been excluded. 



