458 THE VOYAGE OF H.M.S. CHALLENGER. 



the same connections, and probably a distinct function to play in the mechanism of the 

 foot. 



The plantar layer constitutes a very constant part of the intrinsic muscle apparatus 

 of the Mammalian foot. In the feet of forty-six different species possessed of three or 

 more toes, it was absent entirely in three cases only. In the monodactylous and didacty- 

 lous feet of Solipeds and Ruminants not a trace of adducting muscles is to be found. 

 The original number of these muscles is five, one for each toe, but they exhibit a distinct 

 tendency to disappear from the centre of the foot towards the margins, and this 

 disappearance takes place in a more marked degree outwards towards the minimus than 

 in an inward direction towards the hallux. The central adductor {i.e., adductor medii) 

 was only found in three specimens. The sudden disappearance of this adductor is 

 probably due to the tendency which these muscles have to arrange themselves so as to act 

 with reference to the middle toe. 



Fig. 172. — Schematic view of a transverse section through the metatarsus showing the intrinsic muscles of the left 



human foot. 

 l.-V. Metatarsals ; /', flexor brevis hallucis ; f 3 t, tibial head of flexor brevis medii (1st plantar interosseous) ; ft, tibial 

 head of flexor brevis annularis (2nd plantar interosseous) ; fH, tibial head of flexor brevis minimi digiti (3rd plantar 

 interosseous); />/, fibular head of flexor brevis minimi digiti ; p 1 , adductor obliquus hallucis ; d l , abductor hallucis ; 

 d*, abductor minimi digiti ; d--d 5 , dorsal interossei ; e.p.u., external plantar nerve. 



The intermediate group of flexores breves is the most constant layer, and is closely. 

 associated with the dorsal layer, which is the least constant and most variable. It is 

 quite possible that the latter (i.e., dorsal interossei, abductor hallucis, and the abductor of 

 the minimus) may have originally been derived from the former. Ruge's ' investigations 

 into the development of the muscles of the human foot favour this view. 



If the human foot be studied in the light of these results, it will be seen that the 

 dorsal layer is the most fully represented. , It consists of (l) the abductor hallucis, (2) 

 the abductor minimi digiti, and (3) the four dorsal interossei. 



The plantar layer is represented by the adductor hallucis and the transversalis pedis, 



1 Zur vergleichenden Anatomie tier tiefen Muskeln in der FoMSohk, Morphol. Jahrh., Bd. iv. pp. 644-660, 1878, 



