Introduction : That branch of science which deals with bones and cartilage is called Osteology. Bones are made up of a protein called ostein and cartilage are made of a protein called chondrin. Hence study of bones is called osteology and study of cartilage is called chondrology. Body of animals (vertebrates) is supported by skeleton.

 Skeleton                                                                                                                                                                  

The hardened tissues of the body together form the skeleton (sclero = hard). Organism will remain small and slow moving if there had been no skeleton for support and to serve as levers on which muscles can act. Skeleton of invertebrates is most often secreted on the surface, forming a lifeless or dead exoskeleton. Whereas skeleton of vertebrates develops most often underneath the surface forming a living or growing endoskeleton. Three types of skeletons develop in vertebrates :

1. Epidermal horny exoskeleton : These include hard and horny of keratinized derivatives of epidermal layer of skin, such as claws, reptilian scales, bird feathers and mammalian hairs, horns, nails and hoofs, etc. All living amphibians lack an exoskeleton.
2. Dermal bony skeleton : Dermal bony skeleton is derived from the dermis of skin. It includes bony scales and plates or scutes (osteoderms), finrays and antlers of fishes, reptiles and mammals. In fishes, dermal scales become exposed due to wearing out of epidermis, and form exoskeleton.
3. Endoskeleton : Greater part of vertebrate skeleton lies more deeply, forming the endoskeleton. It develops from mesenchyme. Endoskeleton is formed by bones in vertebrates. Skeleton in different animals are as follows –

Invertebrate –

• Protozoa – No skeleton.
• Porifera – Calcarius spicules + silicious spicules + spongin fibre.
• Spicules in porifera represent endoskeleton.
• Coelentrata – Calcareous corals.
• Helminth – No skeleton, cuticle present.
• Annaelida – No skeleton, cuticle present.

In earthworm and ascaris is hydrostatic skeleton is found that is fluid is filled in coelom and form turgid skeleton.

• Arthropoda – Chitinus exoskeleton.
• Chitin is modified cuticle.
• Mollusca – Calcarius shell.
• Echinodermata – Calcareous plates are present.
• In vertebrates exoskeleton may be epidermal or dermal.

Vertebrates : In vertebrates dermal skeleton is formed by bones. Bone is the connective tissue with

intercellular spaces filled with ossein matrix composed of calcium salts 2/3  (Ca Po4 , Ca2 (SO4 )3 ), CaF2 CaCO3   and

calcium oxalate) and organic matter 1/3. In the ossein osteocyte cells are found and outer surface of bone is covered by periosteum. The inner most region is full of bone marrow having various types of cells. In mammals the bone is full of haversian canals. The bones are of three types :

 

 

 

1. Cartilage bones : The bones which are formed by the ossification of preexisting cartilage are called cartilage bones or replacing bones.
2. Membrane or dermal bones : The bones which are formed by independent ossification in connective tissue are called dermal, membrane or investing bones.
3. Sesmoid bone : Ossification takes place on Ligament and Tendons.
   1. Functions of endoskeleton : Chief function of vertebrate endoskeleton can be enumerated as follows –
      1. To provide physical support to body by forming a firm and rigid internal framework.
      2. To give definite body shape and form.
      3. To protect by surrounding delicate internal organs like brain, heart, lungs etc.
      4. To permit growth of huge body size (whale, elephant, extinct dinosaurs), since it is living and growing.
      5. To provide surface for attachment of muscles.
      6. To serve as levers on which muscles can act.
      7. To manufacture blood corpuscles in bone marrow.
      8. To help in breathing (tracheal rings, ribs). Endoskeleton in vertebrate made up of :

1. Axial endoskeleton : (Skull + Vertebral column + Sternum + Ribs)
2. Appendicular endoskeleton : (Girdle + Limb bones)

 

 

SKULL (ANTERIOR MID DORSAL)

 

VERTEBRAL COLUMN (BACK SIDE)

 

 

STERNUM (ABDOMINAL SIDE)

 

 

HEAD

 

 

 

 

 

PECTORAL GIRDLE

 

 

FORE LIMB

RIB (LATERAL)

PELVIS GIRDLE

 

 

 

HIND LIMB

 

(a) Axial skeleton : It occupies the body's main longitudinal axis. It includes four structure : skull in the head, vertebral column in the neck, trunk and tail if present, sternum and ribs in the thorax. It form the upright axis of body and includes 80 (87 in children) bones are as follows in man –

Cranium – 8                                             Hyoid – 1

Face – 14             28 Skull                         Vertebrae – 26 (33 in children)

Ear ossicle – 6                                            Sternum – 1

Ribs – 24

 

 

 

(1) Skull (General structure) : It is anterior most axial skeleton. It is divisible into two main parts –

(i) Chondrocranium                (ii) Splanchocranium

1. Chondrocranium : Chondrocranium is formed by

1. brain box or cranium proper and (b) two sense capsules –

 

 

 

 

 

 

PARIETAL

 

 

 

 

 

 

FRONTAL

 

 

 

 

 

SPHENOID ETHMOID

 

 

 

 

 

 

 

 

FACE

Orbit or optic capsule (eye) and auditory or otic capsule (ear).

1. Cranium proper : It is a strong and firm bony box with a helmet-like covering over the brain, called vault of skull, and a relatively thicker and stronger floor of base upon which the brain rests. Its cavity is called cranial cavity. Size of cranial cavity averages 1475 cubic centimetres (cm³) in adult

 

 

 

OCCIPITAL

 

 

 

EXTERNAL AUDITORY MEATUS

 

TEMPORAL

 

 

 

 

 

STYLOID PROCESS

 

LACRIMAL NASAL

EYE ORBIT MALAR

(ZYGOMATIC)

MAXILLA            CORONOID PROCESS

 

MANDIBLE

men. At about the middle of the floor of cranium, there is a

 

BASE OF SKULL

CONDYLE

large opening of cranial cavity called foramen magnum. The brain is connected to spinal cord at this foramen. Cranium proper of mammal has four distinct zone –

 

Fig. – Human skull viewed from right side

1. Occipetal zone : Occipital zone has one supra-occipital bone on dorsal side, one basioccipital on ventral side and two exoccipital on both lateral side of foramen magnum. Foramen magnum is present in ventral side of skull, which fills on Ist atlas vertebra. Two occipital condyles forming dicondylic skull at the junction of supra and exo-occipital.
2. Parietal zone : In the dorsal side of cranium parietal zone has three bone, that is two parietal, one inter parietal and ventral side of cranium has 3 bone i.e. one basisphenoid with pituitary foramen and two alisphenoid bone.
3. Frontal : Frontal part of cranium has two frontal bone in dorsal side, each frontal bone has one process called supra orbital process of frontal. Two orbitosphenoid, one presphenoid bone in ventral side.
4. Ethmoidal : Ethmoidal part has one circular plate called cribriformplate. That plate is having two perforation for exit of I cranial nerve.

 

 

 

SUPRA ORBITAL PROCESS OF FRONTAL

 

 

FRONTAL (2)

 

PERIETAL (2)

 

ETHMOIDAL ZONE WITH ONLY ONE BONE i.e. – CRIBRIFORM PLATE

 

ORBITAL CAPSULE (EYE)

 

INTER PARIETAL (2)

 

 

 

SUPRA                              OCCIPITAL (1)

 

EXTERNAL AUDITORY METUS

 

                         TYMPANIC

BULLA

 

 

TYMPANIC BULLA

 

 

 

 

 

 

Occipital region (of cartilage bones)

Parietal region                                        Frontal region                      Ethmoidal region

Temporal region

 

 

Exoccipitals (on side)

Supra occipital (on roof)

Basioccipiatal (on floor)

Orbitosphenoids (on sides & floor) (Cartilage bone)

Frontals (on roof

membrane bone)

Presphenoid or Parasphenoid

(on floor – Median) (Cartilage bone)

 

 

Basisphenoid (On floor Median) (Cartilage bone)

Parietals (on roof)

(Membrane bone)

(Inter-parietal)                    Alisphenoids (on floor & sides) (Cartilage bone)

 

 

 

2. Sense capsule : Chondrocranium contains two sense capsule.
   1. Optic or orbital capsule      (2) Otic or auditory capsule

Ethmoid

(on floor median)

Ectethmoids or lateral ethmoids (on sides)

1. Optic capsule : One pair of optic or orbital capsule are present in frontal zone of chondrocranium. It is

made up of 7 pairs of bones which are –

I – Pre frontal                II – Post frontal                     III – Anterior orbital                IV – Posterior orbital V – Infra orbital                VI – Supra orbital               VII – Lacrymal

In frog optic capsules are absent but in place of optic capsule eye-orbit are present in same position. In between two eye capsule, a separating bone is present in mammals only. This separating bone is called inter-orbital septum. This septum is

absent in frog between two eye orbits.

When inter orbital septum is present in any skull then such

SEPTUM IN FROG

skull is called tropibasic skull. In tropibasic skull both eye face the front and can be directed at the same object, this type of vision is called binocular vision. When inter-orbital septum is absent in skull, then such skull is called basic platy skull with monocular vision. Inter-orbital septum is formed by 5 bones which are 2 orbitosphenoid, 2- alisphenoid and one basissphenoid.

2. Auditory or Otic capsule : Auditory capsule located between occipital and parietal zone. It has two parts – Tympanic bulla and External auditory metus. Auditory capsule in vertebrates is formed by 5 pairs of otic bones.

(I) Preotic                              (II) Epiotic                                    (III) Opisthotic

(IV) Sphaenotic                       (V) Pterotic

Out of these 5 pairs only I pair i.e. preotic participate in formation of auditory capsule of frog i.e. amphibian. In mammals e.g. rabbit I, II & III pair fuse to form a fusion bone called periotic, which forms the auditory capsule. In reptiles and birds (aves) all 5 pairs bone combinedly constitute auditory capsule. Membranous labyrinth is enclosed in the pro-otic and tymanic bulla. Auditory capsule has two distinct part – Outer spongy part called patrus part and Inner bony part called mastoid part.

 

 

 

 

 

Auditory capsule                                          Orbital capsule (membrane bones)

Olfactory capsule

 

Epiotic (Superior) & Posterior

Opisthotic (Posterior) & inferior

Prootic (Anterior) & inferior

Sphenotic present

Pterotic present

 

Nasals (above)

 

Vomers (below)

 

Form periotic in birds and mammals                               Present in Teleosts fishes only

In amphibians and reptiles, however, only prootic is present.

Membrane bones

 

Post-orbital (Postero-dorsal side of orbit)

Pre-frontal (Infront and on

outer side of frontal)

Supra-orbital (Above the orbit)

Pre-orbital (Antero-dorsal side of orbit)

Lacrymal (In front)

Post-frontal (Behind and on

outer side of frontal)

3. Skull of man : In man however the skull remain erect at top of vertebral column because of perfectly erect posture of body it is divisible into the large and hollow cranium and the facial region.
   1. Cranium (Brain case) : In skull of man all eight bones are articulated with each other to form the cranium as follows –

 

Name	No.	Description
Frontal	1	Forms the forehead (anterior or front part of the top of cranium) and some upper parts (roofs) of eye orbits or sockets and nasal cavities. A newborn infant displays a faint suture in midline of frontal, indication that adult frontal is actually formed of two completely fused frontal.
Parietals	2	Articulated to and situated just behind frontal. Form the main parts of bulging top and sides of cranium.
Occipital	1	Articulated to and situated just behind parietals. Forms posterior (back) and lower (base) parts of cranium. Foramen magnum is a large perforation in this bone. On each side of the foramen, the occipital bears a prominent elevation called occipital condyle. The condyles articulate the skull with first vertebra (atlas). Thus, human skull is dicondylic.
Temporals	2	Form lower parts of right and left sides of cranium, as well as, the floor of cranial cavity. These house structures of internal and middle ears and form a part of external auditory meatuses. The middle ear of each side encloses the three small ear ossicles – malleus, incus and stapes.
Sphenoid	1	A typically butterfly-shaped bone that forms the middle and anterior parts of base of cranium in front of occipital in the middle and temporals on the sides. It articulates with all skull bones, keeping these firmly together. It also forms parts of lateral walls and floors of eye orbits.
Ethmoid	1	A small, irregular bone in front of sphenoid and behind nasal bones. It fashions the front (anterior) extremity and closer of cranial cavity. It also contributes to the architecture of eye orbits and proximal parts of nasal chambers.

2. The facial region : This is the front or anterior part of our skull comprised of 14 bones as follows –

HARD PLATE

 

 

PARIETAL

 

 

 

SPHENOID

NASAL MIDDLE

CHONCHA

 

 

 

LACRIMAL

 

 

 

FRONTAL ETHMOID

EYE ORBIT

MAXILLA MALAR

(ZYGOMATIC)

 

 

INTERNAL

 

 

 

INTERNAL NARIS

MAXILLA

 

ZYGOMATIC ARCH

SPHENOID

 

VOMER

STYLOID PROCESS

MASTOID PROCESS

INFERIOR CHONCHA

VOMER

MAXILLA

NASAL SEPTUM                         TEMPORAL

 

OCCIPITAL CONDYL

 

OCCIPITAL                       PARIETAL

 

Fig. – Human skull viewed from in front

 

Fig. – Human skull viewed from below

 

 

 

 

 

 

Name	No.	Description
Nasals	2	Small, oblong bones in middle of upper part of face, forming proximal part of the bridge of our nose. The remaining, lower part of our nose is formed of cartilage.
Inferior nasal conchae (Turbinales)	2	Two highly coiled, scroll-like processes of ethmoid bone, called conchae project into each nasal cavity from lateral wall of the proximal bony part of concerned nasal chamber. One ethmoidal concha is superior (uppermost). The other one is called middle concha, because it is followed by a thin, separate scroll-like bone which is named inferior nasal concha or turbinate.
Vomer	1	A thin, elongated, platelike bone, forming a part of the septum which separates the two nasal cavities.
Lacrimals	2	Small and thin, finger-shaped bones, each located in front part of the medial (inner) side of corresponding eye orbit. these form a part of the passages of corresponding tear ducts.
Zygomatics (Malars)	2	Cheek-bones; form the prominences of our cheeks and parts of the floor and side walls of eye orbits.
Palatines	2	L-shaped bones that form the back (posterior) part of our hard palate (roof of mouth). Also contribute to the framework of nasal cavities and floor of eye orbits.
Maxillae	2	Large, upper jaw bones that form the major part of our face and upper jaw. Comprise entire front (anterior) part of our hard palate. Also contribute to the architecture of eye orbits and nose. Bear the teeth of upper jaw.
Mandible	1	Largest bone of our face, and strongest of all bones of the body. Forms entire lower jaw and bears all lower jaw teeth. Articulated with temporal bones of skull.

 

2. Splanchnocranium : It is also known as facial. It includes following two parts –

(a) Visceral skeleton        (b) Olfactory or Nasal capsule

1. Visceral skeleton : Visceral skeleton is formed by pairs of visceral arches which are –
   1. I pair – Mandibular arch
   2. II pair – Hyoid arch
   3.  

The mandibular arch

III to VII pair – Branchial arches (remaining 5 pairs)

 

 

Palato-pterygo-quadrate bar forming                      Meckel's cartilage (forming lower jaw)

 

Suspensorium                                  Upper jaw

 

 

Quadrate (Autostylic suspensorium

in amphibia, reptiles and birds)

Squamosal (Craniostylic suspensorium in mammals)

 

 

 

Premaxilla            Maxilla              Palatine            Pterygoid           Jugal

 

 

Angular                     Articular                      Dentary                         Splenial                          Supra

angular

Coronary

 

 

 

(1) Mandibular arch (one pair) : It is made of two arches one is upper forming upper jaw and second is lower called lower jaw. In tadpole stage upper jaw i.e. upper part of mandibular arch is formed by the fusion of three cartilage called palatine, pterygoid and quadrate. These all fused to form palato-pterygoquadrate.

Lower jaw or II part of mandibular arch is cartilagenous initially and is called Meckel's cartilage which soon changes into bony structure.

(i) Upper jaw : The upper jaw is made of 14 bones i.e. 7 pairs of bones which are –

(i) Premaxilla                        (ii) Maxilla                         (iii) Jugal                 (iv) Squamosal

5. Pterygoid                         (vi) Palatine                       (vii) Quadrate.

Out of these 7 pairs of bones only quadrate are not visible because they constitute II ear ossicle i.e. – incus. In man the nasal cavity is separated from the buccal cavity by bone called palatine complex. Palate of birds is identical in animal kingdom, which is used for birds classification.

Process of upper jaw

1. Premaxilla

• Nasal process on dorsal side which are covered by Nasal.
• Palatine process of premaxilla.

(2)Maxilla

• Nasal process of maxilla.
• Palatine process of maxilla.
• Zygomatic process of maxilla.
  3. Squamosal : Only zygomatic process of squamosal.

(ii) Lower jaw : It is composed of 6 pairs of bone i.e. 12 bones maximum. These are articular, angular, splenial, dentary, coronoid and supra angular.

In frog out of 6 pairs only 4 pairs of bones are present. Only 3 pairs form lower jaw and one pair forms I ear ossicle i.e. collumella aures. Remaining 3 pairs i.e. Angular, splenial and dentry combine to form lower jaw of frog. In mammals only one pairs of bones are present of which only one pair i.e. dentry from lower jaw. In the process development cartilaginous dentry become offified due to osteocytes. Articular of lower jaw forms malleus i.e. I ear ossicle. All the 6 pairs of bones are present in lower jaw of reptilies and aves.

Upper jaw in vertebrate is completely ossified with skull but lower jaw is always free from chondrocranium and hangs downwardly. A bone hangs lower jaw from upper jaw. This bone is called suspensorium. A skull in which suspensorium is formed by quadrate is called autostylic skull e.g. frog skull. A skull in which suspensorium is formed by squamosal is called craniostylic skull e.g. rabbit skull (all mammal).

2. Hyoid arch (II pair of visceral arch) : It is also one pair of

which is called Hyoid proper and Hyomandibular.

1. Hyoid proper : It is a horse-shoe shaped bone in our neck

Fig. – Human hyboid bone viewed from above

between lower jaw and sound box or larynx. It is not articulate to any bone but is simply suspended from temporal bones by means of ligament. It consists of an elliptical main part or body and two

processes on each side of body, called greater and lesser cornua. It supports our tongue and provides insertion to some tongue muscles. In colloboration with branchial arches forms hyoid apparatus in terrestrial vertebrates. It is absent in fishes because branchial arches form gill rackers which support gills.

2. Hyomandibular : It is second part of hyoid arch which constitutes ear ossicles in vertebrate. In frog hyomandibular forms stapidial plate which is IIear ossicle which is dot or lid like bone. In rabbit hyomandibular forms stapes which

 

 

 

is III ear ossicle. That is stirrup like bone.

Ear ossicles

 

I	II	III
Mallius	Incus	Stapes
Articular	Quadrate	Hyomandibular
Hammer	Anvil	Sterip

3. Branchial arches : These are five pairs, which constitute III to VII pair of visceral arches. These constitute gill racker in fishes but terrestrial animals then form hyoid apparatus in collaboration with hyoid proper. Five pairs of branchial arches are as follows –
   1. III pair ceratohyle.                          (ii) IV pair i.e. is epihyal.                   (iii) V pair i.e. is stylohyal.

(iv) VI pair i.e. tympanohyal.                 (v) VII pair i.e. thyrohyal.

 

NASAL PROCESS OF PREMAXILLA

 

NASAL PROCESS OF MAXILLA

 

 

 

 

 

EXTERNAL

 

 

 

N          N

 

 

 

F      F

 

 

S_q                           S_q

 

PREMAXILLA (U.J)

 

MAXILLA (U.J)

ZYGOMATIC PROCESS OF MAXILLA

JUGAL (U.J.)

ZYGOMATIC PROCESS OF SQUAMOSAL

PALATINE PROCESS OF PREMAXILLA

 

PALATINE PROCESS OF MAXILLA

PREMAXILLA

 

 

PALATINE MAXILLA

 

 

 

 

JUGAL

 

AUDITORY CAPSULE

AUDITORY MEATUS

 

TYMPANIC BULLA

P        P

 

INTER PARIETAL

S_o

 

PTERYGOID SQUAMOSAL

Fig. – Dorsal view of skull of mammal

 

Fig. – Ventral view of skull of mammal

Difference between skull and frog of rabbit

 

S.No.	Frog	Rabbit
1.	Skull somewhat triangular and dorsoventrally flattened; broader than long; not bent in the snout.	Skull longer than broad, not flattened, but bent in front in the snout.
2.	Cranial skeleton not distinguished into cranial and facial regions; divisible into occipital, cranial and olfactory segments.	Cranial skeleton distinguished into cranial and facial regions. Cranial region divisible into occipital, parietal and frontal segments.
3.	Eye orbits form as depressions of skin and not as sockets in skull; separated from each other by cranial skeleton itself (platybasic)	Eye orbits form as sockets in skull; separated from each other only by a thin interorbital septum ( = tropibasic)
4.	Auditory capsules not lodged in cranial segment of skull; each is formed by a pro-otic bone.	Auditory capsules lodged in cranial skeleton; each is formed by a tympanic bone and reinforced by a compound periotic bone.
5.	Occipital segment small; has only two cartilage bones; the exoccipitals, around foramen magnum; no paroccipital processes.	Occipital segment large; has 4 cartilage bones around foramen magnum – dorsal supraoccipital, ventral basioccipital and lateral exoccipitals having paroccipital processes.
6.	Brain box or cranium small due to smaller brain; mainly consists of cylindrical sphenethmoid reinforced only by three investing bones – two frontoparietals dorsally and one parashpenoid ventrally.	Brain box larger and more complex due to large brain; its posterior, parietal segment has two parietals dorsally, two alisphenoids, laterally and one basisphenoid ventrally; anterior, frontal segment has two frontals dorsally, two orbitosphenoids laterally and presphenoid ventrally.

 

 

 

7.	No prefrontal and lacrimal bones.	These bones present.
8.	Cranial cavity not anteriorly covered by cribriform plate.	Cranial cavity anteriorly covered by a cribriform plate.
9.	Vomers not fused together; bear vomerine teeth.	Vomers fused medially; bear no teeth.
10.	Olfactory capsules without turbinal bones.	Olfactory capsules have turbinal bones.
11.	Septomaxillary bones present.	These bones absent.
12.	Jaw wuspension autostylic.	Jaw suspension craniostylic.
13.