Class notes HSC A Truly NCERT Biology

Exercise 33











Procedure
(i) Observe the different types of bones and joints present in a
human skeleton.
(ii) Draw labeled diagram of your observations.

Observation
Frontal
(a) Human Skull
(i) It is composed of two sets of bones - cranial and facial
Parietal
(Fig. 33.1).
(ii) Cranial bones are occipital, parietal, frontal, temporal, Temporal
sphenoid and ethmoid bones. Orbit
Nasal
(iii) Corresponding to their location in the body, the cranial passage
bones have strong bone case for eyes called orbit.
Maxilla
(iv) Facial bones form the front part (i.e., face) of the skull.
Mandible
(v) A single U-shaped bone called hyoid is present at the
base of the buccal cavity. Fig. 33.1 Human Skull
(vi) A nasal passage formed by nasal bones is present just
below the orbit.
(vii) Maxilla and pre-maxilla bones form the upper jaw, and the mandible
bone forms the lower jaw. These two bones also form the face, and
into them are lodged teeth in special sockets. Teeth are not bones.
(viii) Distinct sutures in zig-zag fashion are present at the junctions of
the frontal with the two parietals, as well as between the
two parietals.



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, Exercise 33

(ix) The occipital bone has a very big foramen at its posterior base, the
foramen magnum, through which the brain is continued posteriorly
as a spinal cord.
(x) The skull is dicondylic, i.e., it has two occipital condyles for
articulation with the first cervical vertebra.
Note: The cranium forms the hard protective outer covering for the brain.
All the bones of the cranium are articulated by fibrous or fixed or
immovable joints. Mandible is the strongest bone of the body.

(b) Vertebral Column
(i) It consists of 26 serially arranged Neural spine
units (Fig. 33.2) called vertebrae
(singular: vertebra). Transverse process
(ii) Each vertebra has a central hollow
portion called neural canal through
which the spinal cord passes. The
Neural canal
first vertebra is the atlas and it
articulates with the occipital
condyles of skull. Centrum
(iii) Vertebral column has several types
of vertebrae: cervical (7), thoracic (12), Fig. 33.3 A typical vertebra
lumbar (5), sacral (1 which is fused),
and caudal or coccygeal (1which is fused).
(iv) A typical vertebra (Fig. 33.3) has a — (i) centrum, the modified
notochord (ii) two laterally projecting transverse process (iii) a neural
canal through which passes the spinal cord (iv) a mid dorsal neural
spine formed by the union of neural arch. Depending upon their
location in the body, secondary modifications are seen in the length
of transverse process and the length of neural spine. The two
Fig. 33.2 Vertebral neighbouring vertebrae articulate with each other through their
column anterior and posterior zygapophyses. Intervertebral discs are present
between the centra of two neighbouring vertebrae.

Note: The vertebral column forms the central axis of the body and keeps it
erect. It encloses and protects the spinal cord and provides surface for the
attachment of skull, ribs, pectoral and pelvic girdles, as well as several muscles
in the neck, thorax, abdomen and waist.


(c) Rib Cage and Sternum
(i) Sternum forms the floor of branchial basket. It bears 7 (seven) notches
for articulation with ribs. It has hexagonal disc at the top called
manubrium. Lower end has a reduced bone called xiphoid process
(Fig. 33.4).



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