Practical no.

3 ___ Comparative study of mammalian skeleton

Procedures:

1. Read introductory material.

2. Observe the models of different mammalian skeleton and compare to illustrations
provided.
3. Diagram several of the mammalian skeleton specimens available.

SKELETAL
SYSTEM

The skeletons of different species of vertebrates share many similarities. Since the rat
skeleton is small you may find it easier to understand the basic organization if you
compare the articulated skeleton of the rat to that of a cat. It may also be helpful to
examine individual cat bones and note the many similarities and differences between the
two species.

The skeleton of vertebrates is composed primarily of bone. Cartilage covers articular

surfaces between bones and connects the ribs to the sternum. The skeleton is divided into
two major parts:
• The axial skeleton includes the skull, mandible, hyoid, ribs, sternum, and vertebrae
• The appendicular skeleton includes the girdle, limb and feet bones

Skull. The skull which is part of the axial skeleton has two major regions (Fig 2.1). The
facial region houses the nose, eyes, and provides support for the jaw. It is elongate in
mammals that depend on their sense of smell. The more posterior cranial region of the
skull houses the brain and ear. The size of this region depends on brain volume. At the
back of the skull is the foramen magnum through which the spinal cord passes into the
vertebral column. On either side of the foramen magnum are the occipital condyles,
which articulate with the first cervical vertebra. Examine the rat skull and trace the lines
or sutures between the bones. Sutures make it possible to identify individual bones.
Identify the following:

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Premaxillary and maxillary. These bones are associated with the teeth. The large
incisors are located in the premaxillary bones and the molars are in the maxillary bones.
Nasal. These bones form the dorsal wall of the nasal cavity.

Figure 2.1. Left, lateral view of a rat skull.

Frontal. These bones form the roof of the anterior portion of the skull and the lateral wall
of the eye socket.

Zygomatic arch. The arch encloses

the ventrolateral side of the orbit. It is composed of three bones: the maxillary at the
anterior end, the zygomatic bone in the middle and the temporal bone forming the
posterior margin. An expanded tympanic bulla with an opening to the inner ear lies at
the base of the temporal bone.

Parietal & Interparietal. These bones and the temporals form the dorsal and lateral wall of the
cranium.
Occipital. This bone has the occipital condyles and foramen magnum on its caudal surface.

Palatine. These bones form the roof of the mouth and the ventral surface of the facial
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 portion of the skull.
Sphenoid. These bones form the ventral wall of the cranial cavity, that houses the brain.

Mandible. Two dentary bones fuse anteriorly to form the lower jaw or mandible which
provides support for two types of teeth: incisors for gnawing and molars for grinding.

The many openings in the skull bones provide for the passage of nerves and blood
vessels. Flat spaces and ridges provide surfaces for attaching muscles that support the
skull and jaw.
Vertebral Column, Ribs and Sternum are part of the axial skeleton. The vertebrae provide
support for the head and body. All vertebrae have a similar basic design. A solid disc of bone
forms the ventral body or centrum. Above this is a space enclosed by the vertebral arch. The
spinal cord passes through this space. Associated with the arch are spines and processes of
various sizes. These serve as points of attachment for the muscles and ligaments that control
movement. The vertebrae at the anterior end overlap those at the posterior end and the
articular processes arise from the vertebral arch. Examine the vertebral column and identify the
vertebrae shown in figure 2.2. Note how the shape of the vertebrae changes as you move from
the anterior to the posterior end of the column. The disarticulated cat vertebrae will help you
more easily see some of the major differences.

Figure 2.2. Rat Skeleton - lateral view.

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Cervical vertebrae have a small transverse processes containing a hole through which nerves
and blood vessels pass. The first two vertebrae in this region, the atlas and axis are structurally
modified to provide for head movements. (Explore this with some of the skulls and vertebrae on
the side bench). What types of movement are possible at each point of articulation?

Thoracic vertebrae have articular surfaces where the ribs attach. The head of a rib fits
between two adjacent vertebrae and the tuberculum articulates with the transverse
process. The ribs are attached to the bones of the sternum by costal cartilage. The rib
cage provides support of the body and protects the heart and lungs. Ribs also serve as a
point of attachment for muscles that assist in respiration. The articulation of the ribs can
be more easily seen by examining the cat skeleton.

Lumbar vertebrae have flattened, enlarged, anteriorly oriented transverse processes.

Sacral vertebrae are fused to provide additional support for the hindlimbs.
Caudal vertebrae are variable in number among different species depending on the length
of the tail.

Appendicular Skeleton
Compare the anterior and posterior limb and girdle bones (Fig. 2.3, 2.4). Note the similarities in
the cranial and caudal long bones. The ridges on the long bones and girdles serve as points of
attachment for the muscles that control the limbs. Why might these bones differ in size?

Figure 2.3. Rat left forelimb.

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 Figure 2.4. Rat left hindlimb.

Each side of the pelvic girdle is made of three fused bones that meet to form the hip
socket. The elongate illium extends cranially, the ischium is caudal, and the pubis
extends ventrally from the ischium. The two pelvic girdle bones attach to the sacrum and
meet ventrally along the pubic symphysis. The canal formed by the two sides of the
pelvic girdle and sacrum houses the caudal regions of the digestive, urinary and
reproductive systems.

Note that the pectoral girdle bones are only attached to the axial skeleton by the clavicles,
which extend between the scapula and the cranial end of the sternum (Fig. 2.5). The
clavicles articulate with the finger-like acromion, which is the terminal portion of the
scapular spine. The spine serves as a major point of attachment for the muscles that
control the rotation of the scapula and forearm.

How do the differences in the two girdles relate to differences in the functions of the two
limbs?

Figure 2.5. Dorsal view of the rat

forelimb and clavicle (arrow).

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 Lion skeletons.

Figure 1. Horse skeletons

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 Horse skull.

Elephant skull.

Mole skeleton. 7
 Figure 11. Deer skeleton.

Skeletal Systems

a. human & cat skeletons

 Know:

 axial skeleton, skull, vertebrae, ribs, sternum,

 appendicular skeleton, pectoral girdle, shoulder blade, clavicle, humerous, radius,

ulna, carpals, metacarpals, phalanges, pelvic girdle, femur, tibia, fibula, tarsals,

metatarsals, phalanges

b. bat skeleton-Identify the bones of the pectoral appendages of the bat and note how

they have been modified to form the support for the wings. -Compare the skeletal

structure of the bat wing with that of the bird.

c. mole skeleton-Compare the structure of the forelimb of a mole with that of the cat,

human and bat. Moles spend their lives digging underground tunnels searching for

food -Be able to identify the bones and note the modifications for digging.

d. various skulls; secondary palate-Note how the bony extension of the maxilla bone

forms a secondary palate in mammals which separates the nasal passages from the

mouth. This makes respiration more efficient in these warm blooded animals which

require plenty of oxygen. -In the sectioned skulls, note the relative size of the nasal
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 cavity versus the brain cavity; also note the dramatic change in the relative size of the

brain cavity in primates (including humans) compared to the nasal cavity-Compare

this with skulls from amphibians, reptiles and birds as available.

e. malleus, incus, stapes; Ear bones-In mammals the sense of hearing is well

developed due partly to the presence of three ear ossicles the malleus

(hammer),incus(anvil) and stapes(stirrup) instead of just one, the

columella(homologous to the stapes) common in amphibians, reptiles and birds.

These additional ear bones evolved from some of the extra jaw bones found in

reptiles.

f. mammalian peinis bone

g. mammal skulls; feeding adaptations

Just as the beaks of birds are modified for various feeding types, the teeth of mammals are

variously modified for various types of foods.

 Carnivorous mammals typically have large canines to hold onto prey, sharp incisors

to cut pieces of flesh and pointed premolars to help chew meat.

 Herbivores typically have small canines or no canines, nipping incisors and broad flat

premolars and molars for grinding the tough plant fiber before swallowing.

 Omnivores 'teeth share some of the characteristics of both feeding types. -Observe the

teeth in the various skulls and determine whether the animal is a carnivore (a meat

eater), an herbivore (a plant eater) or an omnivore(eats both plant and animal foods)

References:

http://users.rowan.edu/~holbrook/MLM.pdf
http://biology.kenyon.edu/courses/biol10/110pdfs/8skeleton.pdf
http://digimorph.org/navcommon.phtml
https://www.austincc.edu/sziser/Biol%201413/Zoology%20Lab%20Manual.pdf
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