Components of the skeleton

Components of the skeleton

(i) Vertebral Column (See Figs. 1-N and 1-O)

The vertebral column consists of a series of unpaired, median irregular bones, the

vertebrae, extending from the skull to the tail. Some vertebrae may become

fused (e.g. in the sacral region). The number of vertebrae in each body region

(cervical, thoracic, lumbar sacral, caudal) is fairly constant so a vertebral

formula can be expressed (e.g. C7T18L6S5Cd15-21 for the horse, C7T13L7S3Cd20-23 for

the dog). Note the differences from the human vertebral formula, particularly in

the number of thoracic vertebrae (and hence ribs). Note also the differences

between the two species, again especially in the thoracic region.

A typical vertebra consists of:

(1) BODY - cylindrical mass articulating cranially (convex) and caudally

(concave) with other vertebrae.

(2) ARCH - two lateral halves which, together with the body, form the vertebral

foramen containing the spinal cord and its vessels. The base of the

arch (pedicel) has vertebral notches for the passage of spinal nerves and

vessels.

(3) PROCESSES

articular processes, two anterior and two posterior articulate with adjacent

vertebrae.

spinous process projects from the middle of the arch for the attachment of

muscles and ligaments

transverse process, two project laterally from the arch.

mammillary process, in most mammals on the posterior thoracic and

anterior lumbar vertebrae, between the transverse and anterior articular

process.

Vertebrae from different regions are distinguished by characteristic features:

Cervical – transverse foramina (except C7), atlas and axis unique

Throacic – long, caudally pointing spinous processes until T11 (anticlinal);

Costal fovae for ribs

Lumbar – long cranially pointing transverse processes

Sacral – fusion, processes become crests, notches for nerves become foramina

Caudal – reduction in processes

Figure 1-N: Cervical vertebrae of the dog

Figure 1-O: Vertebrae of the dog (Thoracic, lumbar, sacral and caudal

vertebrae)

(ii) Ribs and Sternum (See Fig. 1-P)

Ribs are paired curves bones which form the skeleton of the thoracic wall. The

shaft of the rib varies in length, width and curvature. The costal groove on the

medial surface contains the intercostal vein. The head is at the end of the rib and

articulates with two adjacent vertebrae. It is connected by a neck to the shaft and

the tubercle which is directed caudally, articulating with the transverse process of

the vertebra.

The sternal (ventral) end of the ribs is usually slightly enlarged and roughened

where it joins the costal cartilage.

Three types of ribs can be distinguished:

(1) STERNAL RIBS - articulate with the sternum by the costal cartilage.

(2) ASTERNAL RIBS - do not articulate with the sternum, but may have

overlapping costal cartilages, united by elastic tissue to form a costal arch.

(3) FLOATING RIBS - ventral ends free and not attached to adjacent ribs.

The sternum is composed of 6-8 unpaired median bony segments (sternebrae).

Its shape varies considerably. The cranial end (manubrium) is relatively small

and terminates in cartilage (cariniform) providing attachment for breast and neck

muscles. The body of the sternum articulates with the costal cartilages of sternal

ribs and gives attachment to pectoral muscles. The ventral border of the sternum

may have a prominent crest. The sternum terminates caudally in a thin plate of

xiphoid cartilage which provides attachment for the diaphragm and abdominal

muscles.

(iii) Thoracic Limb (See Fig. 1-Q)

The shoulder (pectoral) girdle typically consists of three parts, (scapula,

coracoid and clavicle):

• The scapula is the only well developed component in domesticated mammals.

It is a flat bone, with its lateral surface divided by a spine. This

terminates in the acromion in the dog. The glenoid cavity articulates with

the head of the humerus and the supraglenoid tubercle provides attachment

for biceps brachii muscles.

• The coracoid process has fused with the scapula

• The clavicle is either absent (horse) or rudimentary in the

brachiocephalicus muscle (dog). Thus, the shoulder has no articulation

with the axial skeleton, but is supported by a syndesmosis (see chapter 3).

Humerus:

A long bone, with a cylindrical, slightly twisted shaft. The anterior surface of the

shaft has a deltoid tuberosity. The proximal end has a head (which articulates

with the glenoid cavity) a neck and greater and lesser tubercles. The distal

extremity consists of an oblique trochlear surface for articulation with the radius

and ulna, and medial and lateral epicondyles separated caudally by the olecranon

fossa and cranial by the radial (coronoid) fossa. The radial and olecranon

fossae are often joined by a large supratrochlear foramen.

Bone composition

Bone composition

Dried bone consists of roughly 1/3 organic matter and 2/3 inorganic salts (CaPO4

57%, CaCO3 4%, MgPO4 2%, NaCl and Na2CO3 3%). Most of the inorganic salt is a

form of calcium phosphate called calcium hydroxyapatite.

Bone derives its hardness from the deposition of mineral salts within the soft organic

matrix. It has a compressive strength of 20,000 lbs/in2 and a tensile strength of 15,000

lbs/in2. In behaviour, bone is similar to reinforced concrete: the organic matter

(collagen fibres, equivalent to steel girders) resist tension and the inorganic matter

(mineral salts equivalent to concrete) resist compression.

Mineralization of bones is a compromise between increasing strength and increasing

stiffness (making them more brittle). The combination of lightness and compressional

strength is achieved by internal sculpturing with trabeculae (from the Latin: little

beams) orientated parallel to compressional cortices.

Bone is ALIVE: it requires oxygen and nutrients, it can grow, change shape, erode,

become infected and die.

Bone development

Bones develop either in fibrous tissue (membranous or dermal bones – flat bones;

mainly cranial or facial) or in cartilage (cartilage bones – long bones). Some bones eg

the ethmoid and temporal bones of the skull, have components of both.

Membranous ossification

The loose mesenchyme in the region of the future bone is invaded by osteoprogenitor

cells, which develop into osteoblasts. These lay down calcium salts on a randomly

arranged framework of collagenous fibres to form woven bone.

Osteoclastic erosion of and osteoblastic remodelling coverts this weaker woven bone

into lamellar bone. This takes the form either of a continuous latticework of trabeculae

i.e. cancellous (spongy) bone, or compact bone with Haversian systems, according

to the stresses each region of the bone is required to endure.

Osteoblasts in the periosteum lay down trabeculae in dense parallel sheets to form

circumferential lamellae of compact bone.

Blood vessels carry stem cells which colonize the marrow forming haemopoietic tissue.

Figure 1-M: Ossification in a long bone

Endochondral ossification (see Fig. 1-M)

This occurs by replacement of a pre-existing cartilage model with bone. Ossification

begins in the mid-diaphyseal region (primary ossification centre), when osteoprogenitor

cells of the perichondrium differentiate into osteoblasts, forming a periosteal collar of

woven bone. This is remodelling into lamellar bone, as described above. Trabeculae of

cancellous bone and compact bone Haversian systems are formed, and haemopoietic

cells colonise the marrow.

Increase in length of the bones takes place by the continued growth of either end of the

cartilage model. Eventually secondary ossification centres begin to form in each

epiphysis, but a collar of epiphyseal cartilage (the growth plate) is retained and continues

to grow between the primary and secondary ossification centres. It is steadily

replaced by endochondral ossification on the diaphyseal side of the growth plate

The chondrocytes of the epiphyseal cartilage multiply (zone of hyperplasia) to form

columns, and then increase in size (zone of hypertrophy). The hyaline matrix thins out

and mineral crystals appear in it (zone of calcification), until the nutrient supply of the

chondrocytes is cut off and they die (zone of regression). Osteoprogenitor cells migrate

along the scaffold of the calcified matrix and differentiate into osteoblasts which secrete

collagen fibres and the matrix material of bone (osteoid) which becomes mineralised

(zone of ossification). Finally, remodelling by osteoclasts and osteoblasts forms compact

bone.

Increase in diameter occurs by the periosteal deposition of compact bone around the

spongy core. Finally,there is a breakdown of spongy bone to leave a hollow interior

filled with marrow.

Blood supply to bone (see Fig. 1-K)

Blood supply to bone (see Fig. 1-K)

The blood supply to bone is of critical importance in maintaining the health of normal

bone and in assisting the repair process after damage, fractures, etc. There are 3 sources

of afferent blood to a typical long bone:

a) Nutrient Artery (usually single)

This passes to the medulla via the nutrient foramen where it branches to proximal

and distal medullary arteries through the marrow. These further divide to provide

the major blood supply to the diaphysis and may anastomose with epiphyseal and

metaphyseal arteries at each end of the bone.

b) Metaphyseal Arteries

Numbers of these enter the proximal and distal metaphyses at all sides. Their

final branches anastomose with the medullary arteries. Normally this anastomosis

is at a capillary level so the metaphyseal arteries make little contribution to the

medullary blood supply but if the nutrient artery is blocked they can enlarge to

take over the medullary supply.

c) Periosteal arterioles

Pass to the diaphyseal cortex only at areas of strong fascial attachment. They

supply the outer third of the cortex where they anastomose with branches of the

medullary artery. Their extent and significance is questioned, but they may be

important in bone repair following fracture.

Figure 1-K: Blood supply of bone

In a young animal arteries do not usually penetrate the epiphyseal cartilage.

Most drainage of blood from the bone is by veins following the path of the arteries -

nutrient and medullary. The cortex is drained by venous channels and capillaries to the

periosteum, always centrifugal (medulla to periosteum).

Nerves are largely distributed to the blood vessels. The periosteum is richly endowed

with sensory nerve endings, and is second only to skin in sensitivity. This explains why

fractures, infections and tumours involving bone are so exquisitely painful, and always

require early attention to pain relief.

Young growing bones have a separate circulation to the epiphysis since arteries do not

generally penetrate the epiphyseal cartilage.

Bone structure (see Fig. 1-L)

Bone structure (see Fig. 1-L)

Bone consists of an external layer of dense compact bone which varies in thickness

according to the forces applied to it. This encloses the cancellous (spongy) bone of

more loosely arranged bony plates and spicules which are aligned according to the

mechanical stresses experienced. The cavities between plates are occupied by marrow

(marrow spaces). Cancellous bone forms the bulk of short bones and the extremities of

long bones. The hollow shaft of long bones is termed the medullary cavity and contains

marrow. Much of the bone marrow in the body is red marrow in young animals

(haemopoietic; blood forming), but this is largely replaced by adipose tissue to form

yellow marrow in older animals.

Some regions, such as the sternum, vertebrae, ribs, skull, pelvis and proximal epiphyses

of the femur and humerus, contain red marrow throughout life. The wing of the ileum

and the sternum provide a convenient location for its collection.

The periosteum envelops the outer surface of the bone and consists of two parts - an

outer protective layer and an inner osteogenic layer. It is critically important in repair of

bone following injury, and excessive stripping of the periosteum during surgery or

fracture is detrimental to new bone formation. The thin fibrous endosteum lines the

medullary cavity, and is important in early callus formation during fracture repair.

Figure 1-L: Sagittal section of a long bone

The skeleton

The skeleton

These notes provide basic information on the skeleton which is intended to help you

understand the general osteology of the horse and dog. Much of this is also revision,

and more detailed information is provided where relevant, in subsequent chapters.

Introduction

The skeleton consists of the framework of hard structures which protect and support

its soft tissues. These include bones, cartilages and ligaments.

The skeleton can be divided into cranial (head) and post-cranial parts. The latter can

be divided into three parts:

(1) AXIAL SKELETON - the vertebral column, ribs and sternum.

(2) APPENDICULAR SKELETON - the bones of the limbs including the pelvic and

pectoral girdles

(3) SPLANCHNIC or VISCERAL SKELETON - bones developed within some soft

organs; e.g. os penis of the dog.

The number of bones in the skeleton varies with age (fusion in older animals) and

between individuals (e.g. caudal vertebrae and, in the horse, tarsus 6 or 7, carpus 7 or

8).

The bones can be roughly divided into four classes, depending on shape:

(1) LONG BONES - act as columns and levers in the limbs and consist of cylindrical

shafts (diaphyses) containing marrow and enlarged extremities (epiphyses). They

develop from at least three centres of ossification, one for the shaft (diaphysis)

and one for each extremity (epiphysis).

(2) FLAT BONES - provide protection for underlying organs (e.g. brain) and a large

area for muscle attachment (e.g. scapula).

(3) SHORT BONES - diffuse concussion (e.g. in the carpus and tarsus) or reduce

friction and modify the action of muscles and tendons (e.g. sesamoid bones

developed in some joints and tendons). They generally have only one centre of

ossification.

(4) IRREGULAR BONES - varied functions (include the median, unpaired bones of

the vertebrae and base of skull).

Outer body form – shape and general appearance of horse and dog

Outer body form – shape and general appearance

of horse and dog

The following drawings (Figs. 1-G and 1-I) illustrate the outward appearance of

the horse and the dog. Opposite each drawing is another drawing which shows

the skeleton of the species in relation to the outer shape of its body (Figs. 1-H

and 1-J).

The purpose of these drawings is to illustrate:

(a) the general organization of the skeleton as it relates to each species; and

(b) how the skeleton contributes as a protective and supportive element to the

overall shape of each animal.

Study each of these pairs of pictures, therefore, so that when you see an animal,

you will be able to envisage the arrangement of its bony framework. Take particular

notice of those areas that appear to have bone as protection and which

appear to be supported by the skeleton.

There are also a few lay terms with which human anatomists may not be familiar,

but which owners of these animals will inevitably use. These are explained in

this section illustrating general appearance of the horse and dog. Take particular

note of the following:

a) The carpus of the horse is generally referred to as the “knee”; in the dog,

some will use the term “knee”, others “wrist”.

b) The tarsus (human ankle joint) is referred to in all animals as the “hock”.

c) The femorotibial and femoropatellar joints (which form the true knee joint

in humans) are referred to together in all animals as the “stifle”.

d) The part of the forelimb of the horse below the carpus and tarsus (which is

in fact the metacarpus and metatarsus respectively), is referred to as the

cannon (or shin, in the forelimb). An old term for this region of the hind

limb is “shannon”, but this is usually also referred to as cannon.

e) The metacarpophalangeal and metatarsophalangeal joints in the horse are

referred to as the “fetlock” joints.

f) The proximal interphalangeal joints in the horse are referred to as the

“pastern” joints, and the region between the fetlock and the hoof in which

this joint is situated is referred to generally as the “pastern”.

g) The distal interphalangeal joints in the horse are referred to as the “coffin”

joint.

Figure 1-G: Points of a dog (outward appearance)

Figure 1-H: Skeleton of the do(Rg e-drawn from Popesko)

Figure 1-I: Points of the horse (outward appearance)

Figure 1-J: Skeleton of the hor(sRee -drawn from Popesko)

Comparative Animal Anatomy

Topographical terms

(A) TERMS USED TO INDICATE THE PRECISE POSITION AND DIRECTION

OF PARTS OF THE BODY

N.B. It is to be assumed that the terms listed below apply to a quadruped

(four-legged) animal in an ordinary standing position.

PLANE: A flat surface, real or imaginary, passing through the animal, or part of it.

TYPES OF PLANES: (See Figs. 1-A and 1-B)

1. MEDIAN OR LONGITUDINAL: (divides the body into similar halves)

divides the head, body of the limb longitudinally into equal right and left

halves.

2. SAGITTAL: passes through the head, body or limb parallel to the median

plane.

3. TRANSVERSE OR SEGMENTAL: cuts perpendicular to the median

plane, or at right angles to its long axis or an organ or limb.

4. FRONTAL (OR CORONAL): perpendicular to the median and transverse

planes.

SURFACES: The outer or external aspects of an object or body.

TYPES OF SURFACES: (see Figs. 1-C and 1-D)

1. VENTRAL: the surface directed towards the ground. Towards or relatively

near to the underside of the head or body.

2. DORSAL: the opposite surface to the preceding (i.e. towards or relatively

near to the top of the head, back of the neck, trunk or tail). On the limbs, it

applies to the upper or front surfaces of the carpus (knee),, tarsus (hock),

metapodium (homologous to the hand and foot), and digits.

3. MEDIAL OR INTERNAL: a surface or structure which is nearer than

another to the median plane (i.e. towards or relatively near to the median

plane).

4. LATERAL OR EXTERNAL: a surface which is further than another

from the median plane (i.e. away from or relatively further from the median

plane).

5. CRANIAL: is the head-end of the body. A surface towards or relatively

near to the head. On the limbs, it only applies to structures above the

carpus and tarsus. You may also encounter the term CEPHALIC which

means the same thing.

Figure 1-A: Terms of planes and of decirtion( Redrawn from Sisson and

Grossman)

6. CAUDAL: is the tail-end of the body. A surface towards or relatively near

the tail. On the limbs, again it applies to structures above the carpus and

tarsus.

7. ROSTRAL: applies to the head region only. A surface towards or relatively

near to the nose.

8. ORAL: applies to the mouth region only. A surface towards or relatively

near to the mouth.

9. ABORAL: applies to the surface opposite to or away from the mouth

region.

(B) TERMS APPLIED TO THE LIMBS (see Fig. 1-D)

1. PROXIMAL: refers to relative distances of different parts from the long

axis of the body; viz., those parts of the limb or limb structures that are

nearest to the body or main mass. Thus, we have the proximal extremity of

limb bones being the upper extremity and the proximal part of bone structures

being mostly the upper parts.

2. DISTAL: refers to that part of a structure that is furthest away from the

main mass of tissue. In the appendages, it applies to the lower end of say a

limb bone or even the free end of the limb.

With reference to the thoracic limb (pectoral limb) or forelimb (see Fig. 1-D)

3. DORSAL: refers to the cranial face of the distal part of the forelimb. In

addition, it can refer to the dorsum of the manus (homologue of the hand).

4. PALMAR: (the older term is VOLAR) refers to the face opposite the

dorsal face.

5. RADIAL (EQUIVALENT TO MEDIAL): that side of the forearm in

which the radius is located.

6. ULNAR (EQUIVALENT TO LATERAL): that side of the forearm in

which the ulna is located.

7. BRACHIUM (OR ARM): specifically the region from the shoulder to

the elbow. Also, a general term used to designate an arm-like process or

structure.

8. AXILLA: is the space between the thoracic limb and the thoracic wall.

With reference to the pelvic limb or hindlimb (see Fig. 1-D)

9. DORSAL: the anterior face of the distal part of the pelvic limb. In

addition, it can refer to the dorsum of the pes (foot).

10. PLANTAR: refers to the face opposite the dorsal face.

11. TIBIAL (EQUIVALENT TO MEDIAL): that side of the leg on which

the tibia is located (medial).

12. FIBULAR (EQUIVALENT TO LATERAL): that side of the leg on

which the fibula is located (lateral

(C) TERMS TO INDICATE RELATIVE DISTANCES FROM THE CENTRE OF

THE LIMB (see Fig. 1-E)

AXIS: is the centre line of the body or any of its parts.

In ARTIODACTYLA (RUMINANTS and PIGS) and in CARNIVORA

(DOGS AND CATS), the functional axis of the limb passes between the

3rd and 4th digits. In PERISSODACTYLA (HORSE), the functional axis

of the limb passes along the centre line of the only digit present.

1. AXIAL and 2. ABAXIAL

are terms meaning pertaining to or being relative to the axis. e.g. the

AXIAL SURFACE of a digit faces the axis while the ABAXIAL SURFACE

faces away from the axis.

(D) TERMS TO INDICATE RELATIVE DISTANCES FROM THE SURFACE

OF THE BODY

1. SUPERFICIAL: relatively near to the surface of the body, or to the

surface of a solid organ.

2. DEEP: relatively near to the centre of the body or the centre of a solid

organ.

3. EXTERNAL OR OUTER: away from the centre of a hollow organ.

4. INTERNAL OR INNER: close to, or in the direction of the centre of a

hollow organ

Figure 1-E: Axis of the limbs in diffeenrt orders of animal

(E) TERMS WHICH APPLY TO THE BASIC MOVEMENT OF THE PARTS OF

THE BODY (see and complete Fig. 1-F)

1. PROTRACTION: taking the whole limb forward.

2. RETRACTION: taking the whole limb backward.

3. EXTENSION: the movement of one bone upon another in such a way

that the angle formed at their joint is increased. Thus, the limb reaches out

or is extended; the digits are straightened. Referring to the back it means

that it is straightened.

4. FLEXION: the movement of one bone in relation to another in such a

way that the angle formed at their joint is reduced. Thus, the limb may be

retracted or folded; the digits are bent. Referring to the back it is arched.

5. PRONATION: as applied to the manus (hand or paw), the act of turning

the palm backward (posteriorly) or downward, performed by medial

rotation of the forearm. This is the normal position of the manus in

quadripeds.

6. SUPINATION: as applied to the manus (hand), the act of turning the

palm forward (anteriorly) or upward, performed by lateral rotation of the

forearm. Dogs have some limited ability to supinate the manus; horse

forelimbs are fixed in pronation and the manus cannot be supinated at all.

7. ABDUCTION: the movement of a part away from the median plane.

8. ADDUCTION: the movement of a part towards the median plane.

Figure 1-F: Terms applied to basic mveoments

General Considerations in Anatomy of Animals

Anatomical terminology

Differences between human and animal anatomical terms

Much of the terminology used in describing anatomy of animals is the same as that used

in human anatomy. The principle differences can be explained by remembering that an

animal is generally considered standing on all fours. As a result

a) Structures that in the human would be referred to as anterior, are referred to as

ventral, i.e. directed towards the ground.

b) Structures that in the human are referred to as posterior, are referred to as dorsal.

An additional but crucial difference, is that in the human anatomical position, the palms

of the hands are directed anteriorly, such that the ulna and 5th digit are medial. In

animals, the palms face posteriorly (inevitably, if the animal is to walk on them!), so as

a result, the radius and 1st digit are medial. This is an important distinction to remember

in considering the anatomy of the forelimb in animals, for those with training in

human anatomy.

Finally, in the quadripedal position in which the vertebral column is parallel with the

ground, the direction of the head is referred to as cranial, and the direction of the tail is

caudal. When referring to structures within the head, the term rostral, meaning towards

the nose, is substituted for cranial.

Another possible source of confusion may come from the terms upper and lower limb.

In humans, “upper limb” refers to the arm, and “lower limb” to the leg. The terms tend

to be applied more loosely in animal anatomy, as essentially all four limbs are regarded

as legs. Hence, “upper limb” tends to refer to the part of the limb above carpus or hock

(ankle), while “lower limb” refers to the part of the limb below these points.

Although physiotherapists will be familiar with anatomical terminology, the following

glossary is included to help overcome any confusion that may arise from differences in

human and animal anatomy.