Endoskeleton of vertebrates
1.Axial endoskeleton:(Skull + Vertebrate column + Sternum +Ribs)
2.Appendicular endoskeleton:(Girdle + Limb bones)
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.If form the upright axis of body and includes 80(87 in children) bones as follows in man28 bones in skull:
What is skull
It is the anterior most axial skeleton.It is divisible into two main parts-(i) Chondrocranium (ii)Splanchnocranium
Chondrocranium is formed by brain box or cranium proper and two senes capsule-Orbit or optic capsule(eye) and auditory or otic capsule(ear).
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 upo which the brain rests.Its cavity is called cranial cavity.Size of cranial cavity averages 1475 cubic centiments (cm3 )in adult men.At about the middle of the foor of cranium.there is a large opening of cranial cavity called foramen magnum.The brain is connected to spinal cord at this foramen. Cranium proper of mammal has following distinct zones-
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 fits on 1st atlas vertebra.Two occipital condyles forming dicondylic skull at the junction of supra and exo-occipital.
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.
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.
Ethmoidal part has one circular plate called cribriformplate.That plate is having two perforation for exit of I cranial nerve.
Chondrocranium contains two sense capsule.(i)Optic or orbital capsule. (ii)Otic or auditory capsule.
One pair of optic or orbital capsule is 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-Lacrimal.
In frog optic capsule 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 orbit.
Auditory or Otic capsule
Auditory capsule is located between occipital and parietal zone.It has two parts- Tympanic bulla and External auditory meatus.Auditory capsule in vertebrates is formed by 5 pairs of otic bones.(i)Preotic
Out of these 5 pairs only I pair i.e. Preotic participate in formation of auditory capsule of frog i.e.,Amphibians. In mammals eg., 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 together constitute auditory capsule.Membranous labyrinth is enclosed in the preotic and tympanic bulla.Auditory capsule has two distinct part -Outer spongy part called petrous part and inner bony part is called mastoid part.