726 THE POPULAR SCIENCE MONTHLY, 



fibrous tissue. In many of the lower animals it is the normal condi- 

 tion. It is seen in nearly all the carnivora, except the plantigrades 

 (though it has been found in the cave-bear) ; it is also seen in monk- 

 eys, lemurs, and sloths. In these it is generally completed by bone, 

 though in some by bone and ligament as in man. In the animals above 

 mentioned it serves the purpose of protecting the great nerve and ves- 

 sel of the fore-limb from pressure during flexion, and it also affords a 

 more direct course by which these structures can supply the parts be- 

 low (see Fig. 4). In man when this arrangement occurs, owing to 

 the altered position of the limb, the nerve and blood-vessel are actu- 

 ally dragged out of their course to pass through this opening ; so in 

 him it serves no useful purpose. This variation is, as was first pointed 

 out by Professor Struthers, well known to affect certain families. The 

 only reasonable explanation of the occurrence of this structure ap- 

 pears to be that of reversion to the type of some mammalian ancestor 

 in which this part was functional, or in other words served a definite 

 purpose (Struthers). 



Third Tkochai^tee. — The third trochanter of the thigh-bone 

 occurs about as frequently as the supra-condyloid process. On the 

 Tipper part of the thigh-bone there are two prominences called the 

 greater and less trochanter ; a third prominence (trochanter tertius) 

 sometimes occurs ; it is situated a little below the great prominence, and 

 gives attachment to the large muscle of the buttock {glutceus maxi- 

 mus). According to Fiirst, in forty skeletons of Swedes examined by 

 him in the Caroline Institute in Stockholm, fifteen possessed this pro- 

 cess, and, in six skeletons of Laplanders, four had a third trochanter. I 

 have seen it in only about one per cent of the skeletons I have exam- 

 ined. In many of the lower animals this process is enormously devel- 

 oped ; it is very prominent in the horse and rhinoceros, and in many 

 others it exists in a slighter degree. 



One more example from the osseous system, and I shall pass to the 

 softer structures. In the human wrist are eight small bones called car- 

 pals, and arranged in two rows ; occasionally between the two rows we 

 have a ninth bone called the os centrale. This os centrale is always 

 present in the higher apes and some of the rodents. We also find 

 that in every human foetus at an early period a rudiment of this bone 

 exists, but it has entirely disappeared by the fourth month of fetal life. 



Circulatory System. — Every naturalist now admits that the vari- 

 ous stages of development of an animal, as well as its specialized parts, 

 are often found to correspond with permanent conditions of animals 

 lower in the scale. A good illustration of this is seen in the develop- 

 ment of the human heart and blood-vessels. In the early stages of 

 development we have a heart with a single cavity, connected with a 

 vessel at each end as in ascidians ; later on the blood-vessels consist of 

 a series of arches which go to the gills or branchial clefts as in fishes 

 and amphibia, while the heart consists of two chambers separated by 



