DISSECTION OF A SMALL MAMMAL ( NECK, CHEST AND ABDOMINAL REGIONS )

Learning Objectives

After studying or reading this, you should be able to:
1. Dissect a small mammal and display the internal organs.

2. Relate the structure of the internal organs to their functions.








THE ARRANGEMENT OF INTERNAL ORGANS 0F MAMMALS

The dissection of the neck of a small mammal, such as a rabbit, shows two internal organs running through the neck. The gullet or oesophagus and wind pipe or trachea. The trachea is anterior to the gullet and it is a long, tubular structure with the walls supported by incomplete rings of cartilage, which keeps it permanently opened. It connects the buccal cavity with the lungs through the bronchi.

The gullet is muscular and tubular and lies behind the trachea and connects the buccal cavity with the stomach in the abdomen.

The trunk of the rabbit is distinguished into two unequal compartments: the thorax and abdomen, which are separated by a muscular sheet or septum called diaphragm. Within the thorax and abdomen lie the internal organs or viscera. Dissection of the two compartments:

Rabbit cut open



Dissected Rat displaying Part of the Digestive System.



Dissected Rabbit displaying some internal organs.


also known as thoracic or chest and abdominal cavities, clearly shows the viscera neatly arranged.

The thoracic or chest cavity is enclosed by the ribs and intercostals muscles. Within the chest cavity lies the reddish brown and pearshaped heart flanked by two lungs. The lungs are pink, spongy organs which are linked to two whitish, firm tubular bronchi (singular: bronchus). The blood vessels, vena cavae (singular: vena cava) and aorta leading into and out of the heart respectively appear as delicate reddish tubes.

Within the abdominal cavity are most of the organs of the digestive, excretory and reproductive systems. All the organs of the various systems are held in place by sheets of thin transparent tissue called Mesenteries. Lying just beneath the dome-shaped diaphragm is a large, lobular liver. It is dark red in colour and overlaps a large portion of a large, pear-shaped muscular bag called stomach. Attached to the liver and partially embedded in one of its lobes is the gall bladder, which is a transparent thin sac. Attached lengthwise to the ventral or lower surface of the stomach is an elongated brown organ called spleen.

The stomach leads to a long, narrow and coiled tube called small intestine. The anterior portion lying just below the stomach forms a U-shaped loop called duodenum. The duodenum leads to a very long and highly coiled, ileum. Within the U-shaped loop of the duodenum lies a pink. diffuse organ called pancreas. The gall bladder and pancreas are connected to the duodenum by the narrow, delicate bile duct and pancreatic duct respectively. The pancreas is held in position by a sheet of mesentery.

The posterior end of the ileum opens into a large, dark, sac-like caecum (plural: caeca), which narrows into a blind-ending tube called appendix. The open end of the caecum is connected to a relatively long, but shorter than the small intestine, large tube called the colon. The caecum and colon constitute the large intestine. The colon opens into the rectum, which terminates with the anus.

The excretory organs are only visible when the digestive organs are displaced. They include two bean-shaped dark red kidneys (singular; kidney) attached to the dorsal wall of the abdomen, with each of them embedded in a mass of fatty tissue. A fine tube called the urethra, leads out of each kidney and enters a delicate, pink, thin-walled bag called urinary bladder. The urinary bladder occupies the lowest part of the abdominal cavity. It opens into a narrow tube called urethra, which also opens to the outside. The urethra passes through the penis in the male but opens close to the vagina posterior to the clitoris in the female.

The reproductive organs of the female rabbit in the abdominal cavity include two oval bodies called ovaries (singular: ovary), which become visible when the intestines are displaced or removed. A fallopian funnel lies close to each ovary and leads to the fallopian tube or oviduct. Each oviduct runs downwards and inwards on one side of the body. It dilates at the posterior end to form the uterus (plural: uteri). The two uteri join to form a short tube called vagina. The tube extending from the junction of the urethra and vagina is the vestibule, which opens to the outside as the vulva.

The male reproductive organs include a pair of testes (singular: testis), which lie in a sac or pouch called the scrotal sac or scrotum. The scrotum protrudes out of the lowest part of the abdomen and lie between the thighs.

A long tube called vas deferens (plural: vas diferentia) or sperm duct joins each testis to the lowest part of the urinary bladder, where it also opens into the urethra. As already mentioned, the urethra runs through an elongated penis, which projects out of the body at the lowest part of the abdomen.





FUNCTIONS OF THE INTERNAL ORGANS

The internal organs which constitute the respiratory system include the two pink, spongy lungs, which are made up of numerous alveoli (singular: alveolus). The alveoli are thin-walled and highly vascularised tiny airsacs, which greatly increase the surface area of the lungs and make them efficient organs for gaseous exchange in mammals. They are connected to the nasal passages by the trachea (plural: tracheae) which branch into each lung as the bronchi (singular: bronchus). The whitish tubular trachea and bronchi are kept constantly open by rings of cartilage embedded in their walls. This allows air through them easily during breathing. The surfaces of the alveoli are kept moistened by mucus produced by mucus-secreting glands. This makes diffusion of respiratory gases efficient.

The main organ of the circulatory system is the pear-shaped heart, divided into two halves by the septum (plural: septa) which prevents the mixing of de-oxygenated blood and oxygenated blood. It is also divided into four separate chambers by valves: tricuspid valves on the right and bicuspid valves on the left, which ensure that blood flows through the heart in only one direction or prevent back-flow of blood. The heart is a muscular organ made of cardiac muscles which allow it to pump blood through the body throughout the life of the mammal without being fatigued. The arteries except pulmonary artery, connected to the heart ensure that it pumps oxygenated blood to all parts of the body. The veins, with the exception of the pulmonary veins, connected to the heart also transport deoxygenated blood from all parts of the body to the heart.

The oesophagus and intestines of the digestive system are tubular and have longitudinal and radial muscles in their walls, which ensure that food is moved along the alimentary canal by peristalsis. Peristalsis is the rhythmic waves of muscle contraction which moves materials along the digestive tract. The stomach is glandular and secretes gastric juice. it has a thick muscular sac. This enables the stomach walls to contract and relax forcefully, a process known as churning, which mixes the food with gastric enzymes thoroughly. The stomach has cardiac and pyloric sphincters, which control the rate at which food enters and leaves the stomach respectively. The small intestine is very long for complete digestion of food. The endothelium of the ileum is provided with numerous villi and micro-villi for efficient absorption of digested food. The large intestine has an enlarged and relatively thick-walled caecum for the digestion of cellulose. which forms a major portion of the diet of the rabbit. The liver and pancreas. which are accessory organs of the digestive system. are glandular and produce digestive secretions. The large, lobular and dark red liver secretes bile, which breaks down lipids or fats and oil into small droplets by emulsilication. The pink, diffuse pancreas secretes pancreatic iuice, which contains amylases, proteases and lipases which are enzymes that bring the process of digestion to completion. The secretions of the liver and pancreas pass through the bile duct and pancreatic duct respectively into the duodenum of the digestive tract.

The bean-shaped kidneys of the excretory system contain numerous kidney tubules or nephrons, which are long. The kidneys are used for excretion and osmo-regulation. The urinary bladder has a sphincter muscles at the base of the urethra. The sac-like urinary bladder stores urine, formed by the kidneys, temporarily. The urine passes through the urethra into the urinary bladder. The sphincter muscles contract to retain urine in the urinary bladder and relax to allow the urine to pass through the urethra to the outside.

In the reproductive system, the female ovaries release eggs or ova which are captured by the fallopian funnels and transferred to the fallopian tubes or oviducts. The endothelia of the oviducts are lined with numerous villi, which beat rthymically to move the eggs to the uteri. The uteri are thickwalled, mascular sacs used for the storage and protection of the developing embryos. The vagina and vestibule are short tubular organs with elastic walls for the passage of the fully developed or matured embryos. In the male, the testes produce sperms which pass through the long sperm ducts or vas diferens and urethra into the vagina of the female during copulation or mating. The urethra passes through the penis, which is elongated with spongy tissues or many tiny channels which are filled with blood to cause an erection. This allows the penis to penetrate the vagina easily during copulation. The testes hang outside the abdomen in the scrotum between the thighs to control the temperature at which sperms are produced and stored.

The scrotum or scrotal sac is temperature sensitive and helps in regulating the temperature of the testes.





Did you understand what you have just studied or read?

If you really want to UNDERSTAND it so well,  PERFORM the ACTIVITY below:

1. Get a small mammal and dissect the neck, chest and abdominal regions .

2. Draw and label fully the respiratory and digestive systems of the dissected mammal.
script async src="//pagead2.googlesyndication.com/pagead/js/adsbygoogle.js">

Post a Comment

0 Comments