Mr Appleton, The Gluteal Region of Tarsius spectrum 471 

 Table III. 



How then shall we account for the great difference in Tarsius'^. 



The length of the femur relative to other parts of the body 

 appears to he a matter of great importance. Cursorial animals and 

 jumpers require the action of powerful knee extensors at the 

 moment of springing from a foot, whether in galloping or leaping; 

 for the line of thrust from the foot is placed approximately at a 

 right angle to the long axis of the femur. The quadriceps itself is 

 situated disadvantageously for powerful action when the knee is 

 fully flexed. And the longer the femur, the greater the moment of 

 the animal's weight about the knee-joint. It is as a mitigation of 

 the strain so placed upon the knee extensors that I regard the 

 shortening § of the femur in cursorial animals; and these muscles 

 are not of disproportionate development in them. In the Macro- 

 podidae also, shortening of the femur occurs with a saving of 

 knee-extensor musculature. In the small jumpers, however, such 

 as Pedetes, Macroscelides, Dipus and Tarsius, this compensatory 

 shortening is not a feature, and a powerful quadriceps is essential. 

 This muscle-group is here shown to be of extraordinary size in 

 Tarsius: it w^ould prove of interest to ascertain the muscle- weights 

 in the other saltatory mammals: I am aware of no records. 



It appears that in the production of such a form as Tarsius, 

 the rectus femoris took no part in the great development of the 

 quadriceps extensor; the remaining muscles (forming the "triceps 

 extensor") alone have been concerned, — most of all the vastus 

 externus. 



* Haughton, assuming his identification of muscles to be the same. 



t Haughton, P. R. Ir. Acad, ix, fails to distinguish a caudofemoralis. It is, how- 

 ever, in Marsupialia a distinct muscle from the quadra tus femoris as Carlsson 

 (K. Sv. Vet. Akad. Handl. 191.5) {Hypsiprymnodon), Parsons (Petrogale), McCormick 

 (Dasyurus) have shown. Cunningham, lor. cit. 1882, saw both muscles in Thylacmus 

 and Phalangista. 



X Inclusion of tensor fasciae latae as a knee extensor would lower this ratio 

 slightly, while admission of semimembranosus as a hip extensor raises this ratio 

 considerably. 



§ Appleton, Proc. Camb. Phil. Soc. 1921, p. 386. 



