ASSIGNMENT 2: FIRST DISSECTION OF THE RAT
Aim
To reveal the major organ systems of a mammal.
Note
The rat dissection is usually run over three weeks of labs. However, as we cannot work in the labs
this year, we have converted these practicals to online assignments using a virtual rat dissection. You
will still be familiarised with
- The process and method of dissecting a rat
- The internal and external features of male and female rats
- The position and appearance of organs you will be asked to identify in the virtual rat
Objectives
At the end of these practicals you should be able to
a) locate any of the major organs of a mammal in a dissected specimen.
b) give the function of any major mammalian organ.
c) relate the structure of an organ to its function.
d) be able to draw any part of the virtual dissection using standard line drawing techniques.
Introduction
This series of three practicals is designed to familiarise you with the gross anatomy of a mammal,
the laboratory rat, Rattus norvegicus. The virtual dissection of the rat should not be seen in
isolation. Male every attempt to relate it to all of the theoretical work you have covered to date. For
example, note the differences in the appearance of the arterial and venous vessel and relate this to
differences in the structures vessels that were considered in the presentations.
Submissions:
Similar to last week’s submission, please upload your written tasks as a word/text document or pdf
documents and NOT as pictures.
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RAT DISSECTION 1: External Features, Viscera, Alimentary Canal and Associated Blood
Vessels
Objectives
1. To become familiar with some aspects of the general anatomy of a typical mammal.
2. To improve your biological drawing skills.
Tasks
- Read through the virtual dissections and watch the associated dissection videos (links
provided in text)
- Answer all the questions (questions are framed in blue)
- Complete all required line drawings:
o Make a line drawing of the superficial glands of the neck (p. 7)
o Make a fully labelled drawing of the digestive tract and its associated structures of
the virtual rat (p. 14)
o Name the structures 1-7 in the image below and list their function(s) (p. 15/16)
External features
The rat’s vibrissae (whiskers) and the pinna (ears) are easily identifiable. Rats are gnawing
mammals and two large incisors should be visible. Eyes are usually pink or black with a black pupil
and covered by a nictitating membrane.
b)
Figure 1a-b: The rat head, with vibrissae and pinnae. Also note the eyes and teeth (source:
www.biologycorner.com; OER)
The sex of the rat can be determined by looking for external testes, found on males and the
presence (or absence) of teats, which are only found on female rats (running your fingers through
the fur on along the animal’s abdomen would allow you to find the teats).
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a)
Male Rat
TEATS
b)
Female rat, urogenital opening visible between legs.
Figure 2a-b: The urogenital openings – note the differences between male and female rats
(source: www.biologycorner.com; OER)
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Initial dissection
The rat (and all vertebrates) has anatomical regions to help locate structures:
cranial region – head
cervical region – neck
pectoral region - area where front legs attach
thoracic region - chest area
abdomen - belly
pelvic region - area where the back legs attach
Figure 3: Regions of the vertebrate body (source: www.biologycorner.com; OER)
To start the dissection, the skin on the belly is lifted with a pair of forceps and, with the use of fine
scissors, a cut is made (through the skin only) along the midline (Fig. 4a-c). The cut extends from
the lower jaw anteriorly to the anus posteriorly. The skin is then separated from the underlying
muscle. In the neck region at the base of both forelimbs, the external jugular veins are superficial
and care must be taken not to damage them.
a)
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b)
c)
Figure 4a-c. Dissection of the laboratory rat. c) Ventral surface indicating lines of initial
dissection (bottom left) Exposing the abdominal cavity and ribcage (bottom right).
To get a better understanding of the dissection, please view the video ‘Rat Dissection 1 –
Opening the Rat’ in your Resources Folder.
The Superficial Glands
The glands in the neck region are usually encased in connective tissue and are thus more or less
hidden from view. Once all the connective tissue is dissected away with fine forceps and scissors, the
glands and ducts are exposed.
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Figure 5. Superficial glands of the neck region of the rat
There are two main types of superficial glands in the neck region: the salivary and lacrimal glands.
The parotid salivary glands are large and irregular in shape (diffuse) and lie just below the ear.
They are often a dull light-pink colour and should not be confused with fat. Lying just anterior to
the parotid, and on the lateral aspect of the head, is the exorbital lacrimal gland. This gland is
brownish in colour. The exorbital lacrimal gland covers the beginning of the parotid duct, which
runs parallel to a large nerve and enters the mouth opposite the molar teeth.
Lying ventrally in the neck region are two large, deep-red, oblong, submaxillary salivary glands,
the duct of which leave anteriorly and ran to the floor of the mouth. The major sublingual salivary
glands are two smaller glands, each closely applied to the antero-lateral parts of the submaxillary
glands. At first these look like lobes of the submaxillary glands but can be distinguished due to
their lighter coloration. The ducts of the sublingual glands are short and run parallel to those of the
submaxillary glands. They open separately into the floor of the mouth.
a)
b)
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c)
Figure 6a-c: The glands (source: www.biologycorner.com; OER)
The salivary glands should not be confused with the three pairs of smaller, oval, dark red lymph
nodes which lie in the neck region.
1. To what system do the lymph nodes belong and what is their function?
2. Task: Make a line drawing of the superficial glands of the neck. Use Fig. 5 as a guide
for style, but do not copy this drawing. Your TA will grade your drawing according to how
well it represents the virtual dissection. Please note that it is very important that you learn
and hone your biological drawing skills.
There are three pairs of lacrimal glands, one set serving each eye. The exorbital lacrimal gland has
already been noted (Fig. 5 and 6). It is brownish-green and its duct leads forward to the posterior
edge of the eye. Posterior to, and slightly below, the eye is a region of tough white connective
tissue. Remove this carefully with a pair of fine forceps. The intraorbital lacrimal gland should be
visible (Fig. 5 and 7). This gland similar in colour to the exorbital gland (i.e. brownish-green), but
is smaller and triangular in shape. The duct of the intraorbital lacrimal gland is short and joins that
of the exorbital. Secretions of these glands provide lubrication for the eyelids. The Harderian gland,
which is off-white or yellowish in colour, is visible between the intraorbital gland and the eyeball.
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a)
b)
Figure 7a-b. Superficial glands associated with the eye
The Body Cavity
In the abdominal region, identify the thin strip of muscle (rectus abdominis) running down the
centre of the abdomen, the oblique muscles on either side of it, and the femoral blood vessels
running down the ventral surface of each thigh.
a)
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b)
Figure 8: The abdominal muscles surrounding the body cavity (source:
www.biologycorner.com; OER and SpringerLink)
In the thoracic region (see Fig 8b), note the large fan-shaped pectoralis muscles extending from the
mid-ventral part of the thorax to the upper arm, and the cutaneous blood vessels. Note the salivary
glands in the neck region.
3. What is the function of the pectoralis muscles?
The Alimentary Canal and its Associated Glands
To get a better understanding of the dissection, please view the video ‘Rat Dissection 2 –
‘The Abdominal Cavity’ in your Resources Folder.
The rat liver has four lobes; the left lobe is large and undivided while the right lobe is large and
divided. The median or cystic lobe has a deep fissure and the small caudate lobe is wrapped around
the oesophagus.
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a)
b)
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c)
d)
Figure 9a-d: The alimentary tract, including the liver (source: www.biologycorner.com;
OER)
Locate the stomach in Figure 9c on the left side of the animal, just posterior to the liver. The
stomach is divided into two parts: the cardiac region is the anterior part and leads into the pyloric
region posteriorly. The small intestine leads from the pyloric region of the stomach. At the junction
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of the stomach and the small intestine is the pyloric constriction, which marks the position of the
pyloric sphincter.
4. What is the function of each region of the stomach?
The coils of the intestine are gently unravelled without damaging the mesentery (which contains
blood vessels and nerves; see Figure 10a). The small intestine is divided into three regions, the first
of these, leading from the stomach, is the duodenum. The bile duct enters the duodenum at a point
12-20 mm from the pyloric sphincter. The bile duct lies in the mesentery, and it is formed by the
junction of a number of small hepatic ducts from the liver. The duodenum leads on to the jejunum,
the next part of the small intestine, and this leads to the ileum. There are no sharp macroscopic
demarcations between these regions and so you will only be able to estimate where each is. The
small intestine is coiled, and has flattened lymph nodes (Peyer's patches) embedded in its walls.
The small intestine joins the large intestine, the junction being marked by the first part of the large
intestine, the caecum (Figure 10b). This is a large sac-like structure, with the ileum leading in at
one end and the colon leading out from the same end (The other end of the caecum is blind and is
termed the appendix). The colon is short, and leads into the longer, narrower rectum, which is often
dilated in parts by the presence faecal pellets. The rectum opens to the exterior via the anus whose
aperture is controlled by sphincter muscles.
a)
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b)
Figure 10a-b: The alimentary tract, showing the mesentery and caecum (source:
www.biologycorner.com; OER)
The spleen (which is not part of the digestive system) is an elongated structure, the same colour as
the liver, lying alongside the stomach.
Figure 11: The spleen (source: www.biologycorner.com; OER)
The pancreas is a diffuse pinkish structure, lying in the duodenal loop. Many pancreatic ducts carry
digestive enzymes from the pancreas to the duodenum.
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Figure 12: The panchreas (source: www.biologycorner.com; OER)
Figure 13. Alimentary canal displayed. From Wells (1964), p 42.
5. Task: Make a fully labelled drawing of the digestive tract and its associated structures
of the virtual rat. Use Fig. 13 only as a reference and DO NOT COPY Fig. 13.
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Blood Vessels Associated with the Alimentary Tract
There are three main arteries supplying the alimentary tract. Each of these is unpaired and arises
from the dorsal aorta.
The anterior mesenteric artery runs from the dorsal aorta into the mesentery. The anterior
mesenteric artery branches to the intestine, pancreas, caecum and colon.
The posterior mesenteric artery supplies the colon and rectum. It is usually very thin and arises
from the posterior end of the dorsal aorta, just anterior to the point where the aorta bifurcates to the
hind limbs.
The alimentary tract is drained by the veins of the hepatic portal system. Three main vessels form
the hepatic portal vein:
3.
the splenic vein drains the stomach, pancreas and spleen;
4.
the anterior mesenteric vein drains the intestine, caecum, pancreas and stomach;
5.
the pyloric vein drains the stomach, pancreas and duodenum.
To locate the hepatic portal vein and its associated blood vessels, you would need to deflect the gut
to the right (animal's left), grip the duodenum and colon and carefully separate them to locate the
principle tributaries of the hepatic portal vein while tracing it to the liver.
6. What is the function of the hepatic portal vein?
7. Task: Name the structures 1-7 in the image below and list their function(s).
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