50 



THE VOYAGE OF H.M.S. CHALLENGER. 



dorsum of the foot. 1 Euge accounts for this peculiarity by looking upon the second and 

 first interosseous muscles in these cases as being compound muscles, — receiving accessory 

 heads from the extensor brevis digitorum. The nerve filaments which go to them, from 

 the anterior tibial, are therefore for the supply of these extraneous fibres. 2 



Conclusions Kegarding the Dispositions of the Intrinsic Muscles. 



I. That the typical arrangement of the intrinsic muscles of the pes is the same as in 

 the hand, and that this arrangement is seen to best advantage in the feet of certain of the 

 Marsupialia. In these animals the muscles are disposed in three layers : — 



1 . A plantar layer of adductores. 



2. An intermediate layer of flexores breves. 



3. A dorsal layer of abductores. 3 



Fig. 1 . Schematic view of a section through the metatarsus of a typical foot. 



(I. to V.) Metatarsal hones. (p 1 to ;; 5 ) Plantar layer of adductores. (f 1 to/ 5 ) Intermediate layer of 

 flexores breves, (d 1 to rf 6 ) Dorsal layer of abductores. (e.p.n.) External plantar nerve. 



Deviations from this typical trilaminar disposition may take place — (a) by sub- 

 division of certain of the members of one or other of the layers, (b) by fusion of certain of 

 the elements of the different strata, or (c) by suppression or non-development of some of 

 the muscles. 



The first of these deviations is to be found in a few Marsupial animals, in which a 

 tendency is exhibited to the development of a fourth layer of muscles by the splitting 



1 Rudinger Die Gelenknerven des Menschlichen Korpers, Erlangen, 1857. Cunningham, Journal of Anatomy and 

 Physiology, vol. xiii. 



2 hoc. cit., p. 50. 



3 Professor Humphry, in his Memoir upon the Myology of the Orycteropus capensis (Jour, of Anat. and Phys., 

 May 1868), alludes in a footnote to this typical arrangement, and singles out the Rabbit as affording a good example. 

 In this animal, however, the dorsal and intermediate muscles have undergone fusion, and it does not exhibit the 

 trilaminar disposition so well as the marsupial hand or foot. 



