Marsh Collection, Peabody Museum. 175 



arrangement found in one section of lemurs appears to be as 

 advantageous as that of monkeys and man, and upon any 

 hypothesis which involves this blood supply through the ento- 

 carotids, the brain of these forms of lemur should have devel- 

 oped as rapidly as that of the monkeys. 



The explanation of this seeming inconsistency, I apprehend, 

 is to be found in the relative importance of the two sets of 

 arterial feeders of the cerebral hemispheres ; viz., the ento- 

 carotids and vertebrals. The latter, it should be remembered, 

 lie deeply in the neck and pursue a somewhat tortuous course 

 through the foramina of the neck vertebrae, finally uniting on 

 the floor of the posterior chamber of the skull to form the 

 basilar artery. From this trunk, a pair of arteries is given off, 

 which diverge and pass forward along the floor of the middle 

 chamber, to unite with the entocaroticl branches, forming, in 

 conjunction with an anterior connecting branch, the circle of 

 Willis. From the union of the entocarotid with the anterior 

 branch of the basilar springs a large artery which passes 

 upward through the Assure of Sylvius, and gives the principal 

 supply of blood to the cerebrum. 



In monkeys and man, the vertebral branches are relatively 

 small and insignificant, and the entocarotids correspondingly 

 large. In the lemurs, on the other hand, this condition is 

 reversed ; the vertebrals are large and the entocarotids small. 

 In the one case, the entocarotids became the predominant 

 feeders of the cerebral hemispheres, and there was correspond- 

 ing increase in size and a general increase in intelligence ; in 

 the other, this office was performed largely by the vertebrals, 

 and the brain developed more slowly. It is evident that the 

 advantage lay on the side of the more direct current, which 

 must have resulted in giving a greater stimulus to the psychic 

 activities of the monkey, and have been responsible for his 

 evolution in this direction. 



Having now stated the probable relationship which must 

 have existed almost from the very beginning between the 

 development of the cerebral hemispheres, the prehensile 

 extremities, and the carotid circulation, I proceed next to an 

 examination of the skeletal structure of the manus and pes, in 

 which the osseus index of some of these conditions above 

 described is plainly exhibited. Selecting one of the more 

 typical representatives of the order, the chief osteological 

 characters of the hand and foot may be briefly stated as 

 follows : 



The Pes. — When the four outer digits are made to rest 

 upon a plane surface, the tibial facet of the astragalus looks 

 almost directly inward, while the fibular facet looks upward 

 and a little outward.* The head of the astragalus is obliquely 



* These two characters do not apply to the human foot, which has been 

 modified for terrestrial progression in the erect position. 



