The contrahentes are muscles widely present in the hands of mammals, including monkeys and gorillas. They are on the palmar/plantar side. There is one each for digits I II IV V but not III. They pull the fingers/toes down and together.

Human anatomy

In humans, the adductor pollicis muscle

Adductor pollicis muscle

In human anatomy, the adductor pollicis muscle is a muscle in the hand that functions to adduct the thumb. It has two heads: transverse and oblique....

 (and the adductor hallucis in the foot) is a well-developed remnant of the first contrahens though it has lost the insertion on the distal phalanx of the thumb

Thumb

The thumb is the first digit of the hand. When a person is standing in the medical anatomical position , the thumb is the lateral-most digit...

.

The other contrahentes only appear as rare atavistic

Atavism

Atavism is the tendency to revert to ancestral type. In biology, an atavism is an evolutionary throwback, such as traits reappearing which had disappeared generations before. Atavisms can occur in several ways...

 abnormalities.

In other mammals, the contrahentes may have their origin either on the carpus or the metacarpus, which suggests that the palmar interossei muscles

Palmar interossei muscles

The palmar interossei are small muscles in the hand that lie on the anterior aspect of the metacarpals. They are smaller than the dorsal interossei of the hand, which lie between the metacarpals.-Structure:...

 also contain elements of the contrahentes.

They appear in the human fetus as a layer of flesh which mostly disappears.

In other animals

The contrahentes of the fourth digit is absent in dogs but present in cats and rabbits.
In primates, the contrahentes vary in number between zero and four. By their insertion onto the proximal phalanges they facilitate convergence of the digits.
In tarsier

Tarsier

Tarsiers are haplorrhine primates of the genus Tarsius, a genus in the family Tarsiidae, which is itself the lone extant family within the infraorder Tarsiiformes...

s, they facilitate the grip by increasing the pressure between the large distal pads and the gripped surface by simultaneously flexing the metacarpophalangeal joint

Metacarpophalangeal joint

The metacarpophalangeal joints are of the condyloid kind, formed by the reception of the rounded heads of the metacarpal bones into shallow cavities on the proximal ends of the first phalanges, with the exception of that of the thumb, which presents more of the characters of a ginglymoid joint...

s and the proximal interphalangeal joint

Interphalangeal joint

Interphalangeal joint may refer to:*Interphalangeal articulations of hand*Interphalangeal articulations of foot...

s and extending the distal interphalangeal joints.