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



Chimpanzee, Pithecia hirsuta, and the Hapale pencillata both are present, but they are 

 coalesced so as to form a single muscular mass. In the Lemur the two muscles are largely 

 developed, and although they lie side by side are yet easily separated from each other. 



The foregoing facts have an important bearing upon the development of these muscles 

 in the human foot. Dr. Ruge ' has conclusively proved that the transversalis pedis or 

 adductor transversus at an early stage in the development of the human embryo Lies in 

 apposition to the adductor oblicpius, and that its transverse position in the foot of the 

 adult is due to its travelling forwards towards the heads of the metatarsal bones. The 

 occasional absence of the adductor transversus in Man, together with the fact that, in 

 the young subject, it is always better marked than in the adult (Ruge) would seem to 

 indicate that this muscle is undergoing regressive changes (probably from the confinement 

 of the foot in a boot, and the consecpient limitation of the range of movement of the hallux) 

 and that in the course of time it will disappear altogether, or only appear occasionally as 

 an interesting abnormality. 



A study of the other members of the plantar layer in the Quadrumana yields results of 

 an equally interesting nature. As a general rule all three (viz., the adductor indicis, adductor 

 annularis, and adductor minimi digiti) are present, so that the plantar group of muscles is 

 represented by its typical number of elements. These are inserted so as to draw the 

 digits towards a line drawn through the medius. The following abstract from Bischoff's 

 memoir upon the Hylobates leuciscus will show the arrangement of the adductors : — 



~ , 7 . , / 1. Adductor hallucis (the two 



Cynocephatus maimon. 



Macacus cynomolgus. 



Cercopithecus sabaeus. 

 Pithecia hirsuta. 

 Ateles. 



Plantar layer 



heads of which may be sepa- 

 rated or united), 

 consists of i 2. Adductor indicis. 



3. Adductor annularis. 



4. Adductor minimi digiti. 



To this list we may add the Cebus apella (Ruge) and the Lemur, 2 in both of which 

 the same factors of this group of muscles are found. 



{1. Adductor hallucis (the two heads 

 united). 

 2. Adductor indicis. 

 3. Adductor minimi digiti. 



1 Processes in the Development of the Muscles of the Human Foot, Morphologisches Jahrbuch, 1878, p. 132. 



2 Murie and Mivart in their memoir upon the Lemuroidea figure and describe only two adductors (in addition to the 

 adductor hallucis), one " going to the peroneal side of the flexor of the index " and the other " going to the tibial side of 

 the fifth digit." In the specimen I dissected there was also an adductor annularis, but it was feebly developed and 

 partially fused with the subjacent muscles. The deep division of the external plantar nerve passed under it, and on 

 detaching the other two adductors from their origin and throwing them forward it also was raised. Its feeble 

 development may be due to abduction taking place, with reference to the annularis. Ruge, on the other hand, in his 

 article upon the deep muscles of the sole of the foot (p. 649), says : — " In the Lemurs, in Loris gracilis, I could find 

 contrahentes for all and even the third toe. The first contrahens (abductor hallucis) consists of two quite distinct heads." 



