PLATE XXXVII. 



THE FOURTH LAYER OF MUSCLES IN THE SOLE, DEEP 



PLANTAR ARCH, &c. 



In this plate the origin of the adductor hallucis from the heads of the second, 

 third, and fourth metatarsal bones has been removed, and its origin from the long 

 plantar ligament severed. The flexor brevis hallucis and transversus pedis have also 

 been removed from their origin. 



The fourth layer of muscles comprises 



1. Three plantar interossei. 8. Tendon of the peroneus longus. 



2. Four dorsal interossei. 4. Tendon of the tibialis posticus. 

 The two last are sometimes included in a distinct fifth layer. 



The arrangement of the intrinsic muscles of the human hand and foot is merely a 

 modification of a simple type such as is well seen in marsupials. (By ' intrinsic ' is 

 meant those muscles which do not belong to the long flexor system which includes flexor 

 brevis digitorum, accessorius, and lumbricales.) This is 



1. A plantar layer of adductors to each toe. 



2. An intermediate layer of paired flexores to each toe. 

 8. A dorsal layer of adductors to each toe. 



In all cases the marginal muscles, abductor hallucis and minimi digiti, tend to 

 reach backwards and to take an origin from the os calcis. Should the hallux or little 

 toe be absent, the dorsal interossei tend to do the same, provided there is no rudimentary 

 metatarsal bone to confine them. 



Starting from the monotremes (Echidna), where there are a full number (five) of 

 adductors, the tendency is for the centre ones to get suppressed until only the adductor 

 hallucis and minimi digiti remain, and then the latter disappears, leaving the adductor 

 hallucis remaining alone. It may also be taken as a rule that the adductors lie above, 

 and the flexores below the external plantar nerve, and that in most mammals the ad- 

 ductors tend to place themselves obliquely or transversely in order to get a greater pull. 



Now, in the human foot or hand the arrangement of the muscles may be taken in a 

 similar manner, viz. 



1. A plantar or palmar set of adductors, of which the adductor hallucis and trans- 

 versus pedis alone remain in the foot, and the adductor pollicis in the hand. 



2. An intermediate set of flexores breves represented by the flexor brevis hallucis 

 for pollicis), the flexor brevis minimi digiti, and three plantar interossei. 



8. A dorsal set, abductor hallucis (or pollicis) and minimi digiti, and the four dorsal 

 interossei ; the abductors reaching back to the os calcis in the foot or carpus in the hand. 



The flexor brevis hallucis is a paired flexor, the flexor brevis minimi digiti and third 

 plantar interosseus forming a second pair, the third and fourth toes in the foot and 

 second and fourth fingers in the hand only getting one, and the second in the foot and 

 third in the hand losing its flexor entirely. 



In the human foetal foot and hand the whole of the muscles are plantar or palmar, but 

 as development proceeds the metatarsal or -carpal bones widen out, and the dorsal inter- 

 ossei get pressed between them. In most animals the adult stage is similar to that of 

 the human embryo, the muscles being entirely plantar, the bones being too closely 

 compressed to allow room for the dorsal position ; in others, where the bones are close 

 behind but open in front, the edge of the dorsal muscles gets squeezed up between them. 

 In the anthropoid apes they are getting more dorsal, and in man they are dorsal by 

 developmental changes. 



In most animals, as in the human embryo, the dorsal muscles are single-headed 

 but in adult man they have become double-headed, and taken an attachment to the next 

 metacarpal or metatarsal bone. 



In speaking of the adductors it was noticed that they tend to disappear from the 

 centre, it being most common to find an adductor hallucis, next the adductor minimi 

 digiti, then come the adductor indicis and annularis ; it is very rare to find the add. 

 medius. The line of centre to and from which the muscles act is drawn through the 

 third toe in most animals as in the hand, so that an adductor medius is not wanted ; but 

 in man, the gorilla, and tamandus, the line is drawn through the second toe. 1 



1 I am indebted to Professor Cuningham's ' Eeport on the Manus and Pea, Challenjjer Expedition, 

 Zoology, vol. v., for the above information. 



