Bones of Metatarsus and Digit/Digits | Gross Anatomy | Anjani Mishra

  STUDY OF MetaTARSUS & DIGIT/DIGITS

 Bones of the Pes

                                                                                        Written by Anjani Mishra

Pes is the fourth segment of hindlimb/pelvic limb
It consists of three segments;

• Tarsus-consists of carpal bones
• Metatarsus-consists of metacarpal bones
• Digit/digits-consists of phalanges and sesamoid bones 

General plan for metatarsus (in domestic animals and birds)

• A group of metatarsal bones are known as metatarsus.
• Metatarsus is the second segment of pes and is located between tarsus above and digit/digits below.
• Metatarsus in general, resembles, those of corrosponding regions of the fore limb/thoracic limb with some differential features noted in species comparision.
• Metatarsus consists of number of bones ranging from three to five metatarsal bones in domestic animals and birds.
• They are long bones and are arranged numerically from medial to lateral as first, second, third, fourth, and fifth (I, II, III, IV, V).
• Such arrangement mainly occurs in dog, although the first metatarsal is much smaller than the others and the second and fifth are somewhat reduced where the third and fourth metatarsals are well developed that carry the chief digits.
• In the ox, first and fifth metatarsals are absent, the third and fourth are well developed metatarsals that carry the well developed digits; they are fused in the ox, sheep/goat. The second one is quadrilateral disc like located at medioplanter angle of the proximal end of third metatarsal bone.
• In the horse, first and fifth metatarsals are absent, the third one is well developed and carries a single digit, where as the second and fourth are much reduced.
• In the pig, first metatarsal is absent, the third and fourth metatarsals are well developed that carry the chief digits where the second and fifth are somewhat reduced and carry accessory digit.
•  In the fowl, four metatarsal bones are present where first one is very small. The second, third, and fourth are elongated and carry a chief digit. 

METATARSUS(OX)

The metatarsus of ox consists of  three metacarpal bones (II, III, IV). Of which third one is located on the medial side and the fourth one is located on the lateral side during fetal life, but later becomes fused forming a large metatarsal bone. There is a medial small rudimentary second metatarsal bone.

THE LARGE METATARSAL BONE:

This resembles to the large metacarpal but differs from it as;

• The large metatarsal bone (III and IV) is about one-seventh longer than the   corresponding metacarpal.
• Metatarsal is larger and distinctly quadrilateral in form.
• Shaft is compressed transversely, and is distinctly four sided.
• The vascular groove on anterior surface is wider and deeper.
• The foramen at proximal part of this surface doesn't pass through the shaft but runs obliquely through the extrimity and opens on the planter part of its proximal surface.
• The dorsal longitudinal sulcus is deep and wide.
• The planter surface is marked by variable grooves.
• The medial surface is flat and wide above. 
• The distal extremity is slightly narrower than metacarpal bone. 
• Proximal end bears a facet at its medial aspect for small metatarsal bone.

The SMALL METATARSAL BONE:

• It is medially situated, quadrilateral and disc like bone about 2cm in width and height.
• It is rudimentary bone.
• Its dorsal surface bears a facet for articulation with large metatarsal bone.
• It may be a sesamoid bone, and regarded as metatarsal sesamoid bone. 

Comparison with:

A) Metatarsal of Horse:

 
THE LARGE METATARSAL BONE:

 The metatarsus of horse consists of three metacarpal bones (II, III, and IV) and have the same general arrangement as the metacarpal bones, but differs in some aspects as;

• Their direction is slightly oblique, distal and a little dorsal.
• The third or large metatarsal bone is about one-sixth longer than the corresponding metacarpal; in median size animal the difference is about 5cm.
• The body or shaft is more cylindrical and is almost circular on cross section, except in its distal part.
• At the proximal part of its lateral surface there is a groove which is directed obliquely distally and plantarly and is continued by the furrow formed by the fourth metatarsal bone.
• The nutrient foramen is relatively higher than on the metacarpal bone   
• Vascular groove is absent.
• Proximal extremity is wider and much greater than that of the metacarpal bone.
• Its articular surface is slightly concave and is marked by a large, central, non-articular depression, continued laterally by a deep notch.

THE SMALL METATARSAL BONE:

•  The second and fourth  small metararsal bones are a little longer than the corresponding metacarpals.
• The fourth metatarsal bone is relativelt massive, specially on its proximal part.
• The base or  proximal extrimity is large and bears one or two facets proximally for the fourth tarsal and two dorsomedially for articulation with the large metatarsal.
• The second metatarsal bone is much more slender than the fourth, specially towards its base.
•  Proximally it bears two facets for the fused first and second tarsals, sometimes one for the third tarsal and dorsolaterally, two facets for the third metatarsal. 

B) Metatarsal of Dog:

 The metatarsus of dog consists of five metacarpal bones (I, II, III, IV, and V) and have the same general arrangement as the metacarpal bones, but differs in some aspects as;

• First metatarsal is very small and pea like, somewhat compressed laterally.
• It articulates with the first tarsal
• Third and fourth  metatarsals are nearly equal in length being slightly larger than the second and fifth.
• Third and fourth  metatarsals have facets for articulation with two small, rounded sesamoid bones.
• The second is slightly longer or equal to the fifth metatarsal bones. 

C) Metatarsal of Pig:

 The metatarsus of pig consists of four metacarpal bones (II, III, IV, and V) and have the same general arrangement as the metacarpal bones, but differs in some aspects as;

•      The metatarsal bones are somewhat longer.
•      The proximal ends of the third and fourth metatarsals have a conciderable planter projection.
•      The process on the third metatarsal has a facet for articulation with a discoid metatarsal sesamoid bone.
•      The second and fifth are placed more towards the planter aspect of the third and fourth than in the fore limb.

D) Metatarsal of Rabbit:

•  Four metatarsals are present.
• They are narrow rod shaped structures.
• First digit is absent and rest digits have three phalanges.

E) Metatarsal of Fowl:

The metatarsus consists of four metacarpal bones (I, II, III, and IV) and differ widely in the  arrangement compare to metacarpal bones as;

• The first metatarsus is very small
  
• In adult the tarso-metatarsus is represented by a single long bone resulting from the fusion of 3 bones ( 2^nd, 3^rd & 4^th metatarsus).
• The proximal extremity is compounded with tarsus, and is very irregular and has two concavities for articulation with tibia.
• The distal extremity is divided into three articular projections, which supports three principal digits.
• In the male, there is a conical, slightly curved projection at the distal third, which serves as support for horny spur. 

 DIGIT/DIGITS OF HIND LIMB

• Digit/digits are the third segment of pes.
• It consists of phalanges and sesamoid bones.
• digital bones in general, resembles, those of corrosponding regions of the forelimb/thoracic limb with some differential features noted in species comparision.

DIGITS (OX)

• The digits of hindlimb/pelvic limb resembles those of forelimb/thoracic limb very closely except in their dimension and inclination. 
• It is very difficult to make a difference in ruminants. so, the separate description is not necessary.
• Sesamoid bones are similar with that of forelimb.

DIGIT (HORSE)

• The axis of the phalanges of the hind limb forms an angle which is about five degree greater  than the fore limb.
• The major differences in the form and size of the bones are as follows;

First phalanx

• The first phalanx is little shorter, wider proximally and narrower distally.

Second phalanx

• The second phalanx is narrower and slightly longer.

Third phalanx

• The distal phalanx is narrower.
• The planter surface is more concave.
• The planter processes are less prominent and closer together.

Sesamoid bones

•  The proximal sesamoid bones are little smaller, except in thickness.
•  The distal sesamoid bone is narrower and shorter.

DIGITS (DOG)

• The first digit is often absent. If present, its development varies and it may contains either one or two phalanges.
• In other cases, specially in very large dogs, a sixth digit is also present; it does not articulate with the metatarsus, but attached with fibrous tissue.
• The other digits except first resemble those of forelimb.
• Sesamoid bones are similar with that of forelimb.

 DIGITS (PIG)

•  The first and second phalanges of each digit are coparatively longer and narrower than those of the forelimb.
• Sesamoid bones are similar with that of forelimb.

DIGITS (FOWL)

• The digits of fowl varies greatly in form, size and number.
• The digits of fowl are four in number, arranged numerically as (I, II, III, IV)
  
• The first digit(the hallux) is very small which is posteriorly directed toe consists of two phalanges.
• Digits second,third, and fourth are located numerically from medial to lateral to their respective trochleae and consists of three, four, and five phalanges respectively.
• The third phalanx of each digit is a claw which is covered by horny structure exteriorly.

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