Practical Course in Zoology: Study Guide - T. A. Dauda 2014

Multicellular Animals
Chordates
Subphylum Vertebrates, or Craniates — Superclass Tetrapods — Class Mammals, or Beasts

General characteristics

Mammals are the highest-organized Class of vertebrate animals. The main Progressive features of mammals are as follows:

✵ highly developed Central Nervous System, primarily the Cerebral Cortex of the Forebrain, which serves as the center of Higher Nervous Activity. Consequently, the adaptive responses of mammals to environmental conditions are highly complex and sophisticated;

✵ viviparity and feeding of the young with milk;

✵ a highly developed capacity for thermoregulation, ensuring a constant body Temperature. Morphologically, mammals are characterized

by the following features. The body is covered with Hair (exceptions are rare and of a secondary nature). The Skin is rich in glands. Teeth are differentiated into incisors, canines, and molars, and are set in alveoli. The Heart is four-chambered, and a single left aortic arch is retained. Erythrocytes are non-nucleated.

The Class Mammalia is divided into two subclasses: Prototheria (egg-laying mammals) and Theria (viviparous mammals), comprising 19 orders.

To study the external Structure and internal Organization of mammals, a laboratory white rat can be used.

White rat (Rattus norvegicus).

The white laboratory rat represents a semi-domesticated white race of the brown rat bred for scientific purposes (Order Rodentia, family Muridae).

Materials and equipment. For every one to two students, the following are required: a freshly euthanized white rat (anesthetized with chloroform 10–15 minutes before THE START OF the practical session); a dissecting pan or dissecting board; forceps; a scalpel; scissors; dissecting needles; pins (10–15 pcs.); absorbent cotton; gauze wipes; charts: "Internal Anatomy of a Mammal," "Diagram of the Circulatory system," and "STRUCTURE OF THE Brain."

Fig. 140 HEAD of a rat:

1 — eyes; 2 — nostrils; 3 — External ear; 4 — vibrissae; 5 — incisors; 6 — hair coat.

Assignment. Study the External structure of the rat. Dissect it and examine the arrangement of Internal Organs. Isolate and sketch individual Organ Systems. Examine and sketch the brain. Note the characteristics typical of the class Mammalia; Features of the rat's biological adaptation (as a representative of the order Rodentia) to its lifestyle; features showing affinity with reptiles. Make the following drawings: "Internal Structure of the Rat," "Diagram of the Circulatory System," and "Structure of the Brain."

Study of the external structure on an anesthetized rat. Pay attention to the elongated body with shortened limbs, adapted to burrowing and moving through narrow passages, and to the hair coat—it is absent only on the paws, while the tail is covered with sparse hairs and keratinized scales (a similarity to reptiles).

When examining the head, note the external cartilaginous pinnae, as well as the facial and cranial regions (Fig. 140). Examine the fleshy Lips. The upper lip is cleft, which is typical of rodents and facilitates feeding on roots, tree bark, etc. Find the long, stiff whiskers (vibrissae) on the upper lip, which serve as an accessory tactile organ. Part the lips—the rat's teeth are heterodont. They vary in Structure and function; the lower jaw bears lower incisors and molars, while the upper jaw bears molars and chisel-like upper incisors that protrude from the Mouth, as in all rodents. The teeth are set in alveoli only in the jaw bones (compare with the teeth of reptiles!).

In female rats, the nipples are clearly visible on the ventral side of the body (especially in lactating females); their number varies, most commonly 6 or 7 pairs. The presence of Mammary Glands is one of the Characteristic Features of mammals.

Note that the rat's limbs are asymmetrical—the hind limbs are longer than the fore limbs.

Dissection (Fig. 141): study the internal anatomy on a dissected rat, using both a preserved wet specimen of the rat's internal anatomy and an injected specimen along with a diagram of the circulatory system.

Place the rat used for the external structure study into the dissecting pan or onto the dissecting board, ventral side up, stretch out its limbs, and pin them to the bottom of the pan or tie them with twine to the hooks at the corners of the board. Using forceps, lift the skin on the abdomen, make a longitudinal skin incision from the anus to the chin along the midline using scissors, and make transverse incisions at the level of the fore and hind limbs. Reflect the skin flaps to the sides and pin them to the bottom of the pan. Taking care not to damage the Diaphragm, make an incision through the musculature from the anus to the posterior edge of the Sternum, followed by transverse incisions to the right and left along the lower boundary of The thoracic cage. Reflect the Muscle flaps and body tissue to the sides and pin them down. Soak up any emerging Blood with cotton swabs. Observe the diaphragm separating the thoracic cavity from the Abdominal cavity (absent in other vertebrates) and examine the general arrangement of internal organs in the abdominal cavity (see Fig. 141).

Fig. 141 Dissected female rat:

1 — heart; 2 — Larynx; 3 — Trachea; 4 — lung; 5 — diaphragm; 6 — Cytology/practical/97.html">Parotid salivary gland; 7 — Esophagus; 8 — Stomach; 9 — duodenum; 10 — Pancreas; 11 — Small Intestine; 12 — Large Intestine; 13 — cecum; 14 — rectum; 15 — anus; 16 — Liver; 17 — Spleen; 18 — Kidney; 19 — Ureter; 20 — Urinary Bladder; 21 — Ovary; 22 — oviduct; 23 — Uterus; 24 — Vagina; 25 — excretory opening; 26 — genital opening.

Locate the large brown liver just beneath the diaphragm, partially covering The Stomach, and a small red spleen in the left side of the abdominal cavity.

The urinary bladder is located in the posterior part of the abdominal cavity; it is inconspicuous when empty, but large and transparent when filled. The loops of the intestine, which cover the other organs, are clearly visible.

Pierce the diaphragm with a scalpel: its apex will immediately drop downward. Lift the xiphoid process of the sternum with forceps and open the thoracic cavity. To do this, use scissors and a scalpel to make oblique incisions starting from the lower lateral angles of the thoracic cavity, cutting through the Ribs and Muscles toward the neck, and remove the anterior chest wall.

In the thoracic cavity, locate the heart enclosed in the pericardial sac, as well as the pale pink Lungs with the Bronchi and trachea. The esophagus is situated behind and to the side of the trachea.

The Digestive System of the rat has a more complex structure than that of amphibians and reptiles. It begins with the Oral Cavity, which is surrounded by lips. Inside the mouth, food is ground by the teeth (compare with the teeth of amphibians and reptiles) and moistened with saliva secreted by the Salivary Glands. Food passes through the Pharynx and Esophagus into the stomach. Note that the esophagus pierces the diaphragm and transitions into the stomach, which is located beneath the diaphragm. To examine the intestine, use scissors to cut the mesentery, untangle the intestinal loops, and spread them out along the bottom of the dissecting tray or on a board next to the animal's body. Find the duodenum extending from the stomach; the ducts of The Liver and pancreas empty into it. Locate the large, loose pancreas lying within the loop of the duodenum. Rats lack a Gallbladder. Note that the duodenum transitions without a sharp boundary into the small intestine, which in turn continues into the large intestine. Locate the cecum, which terminates in a blind pouch (the Appendix); it is situated at the junction of the small and large intestines. The cecum in the rat is quite large. The terminal section of the large intestine gradually narrows into the rectum, which ends at the anus. The total length of the rat's entire intestine exceeds its body length by 5–6 times.

The respiratory organs are more differentiated compared to those of reptiles. Locate the trachea, which begins with the larynx formed by cartilages: the cricoid, thyroid, epiglottis, and two arytenoid cartilages. Find The Thyroid Gland on the sides of the trachea; it lies on the thyroid Cartilage and consists of two connected halves. Pulling the trachea forward with forceps, locate the point where it divides into two bronchi. In the lungs, the bronchi branch extensively. Cut the upper part of the trachea, insert a Glass tube into it, and inflate the lungs with air. The inflated lungs clearly reveal their cellular structure. Examine the membrane covering each lung externally—the Pleura—which also lines the inner wall of the thoracic cavity. Note that the lungs are permeated by a network of blood capillaries. The strong Development of the lungs and the presence of a diaphragm ensure a more advanced Respiration than that of amphibians and reptiles.

Circulatory system. Lift and incise the pericardial sac (Pericardium) with forceps. Examine the heart—its apex points backward. The two dark-red atria are easily distinguished from the pale-pink ventricles, which point posteriorly and form the apex of the heart. Examine the vessels emerging from the heart: the aortic arch with its branching Arteries, and the pulmonary artery. Carefully cut the vessels and remove the heart from the pericardium, which comes away together with the Thymus gland, resembling a fatty formation (the thymus is highly developed in young animals).

Take the heart in your left hand, and with your right hand use a scalpel to make an incision through the walls of the left atrium and ventricle, followed by an incision through the right atrium and ventricle. The atria are thin-walled, whereas the ventricles have thick muscular walls. Insert forceps into the cavities of the left and right ventricles and compare the thickness of their walls. The wall of the left ventricle, which drives blood through the systemic Circulation, is significantly thicker than that of the right. Note that the right half of the heart, containing venous blood, is completely isolated from the left half, which contains arterial blood.

Trace The pathway of blood through the SYSTEMIC AND PULMONARY circulations using an injected specimen and the circulation diagram (Fig. 142). The Pulmonary Circulation begins at the right ventricle with the pulmonary artery. Shortly after leaving the heart, it branches into the right and left pulmonary arteries, which carry venous blood to the lungs.

Gas exchange takes place in the lung capillaries. Then, via the Pulmonary Veins, arterial blood enters the left atrium and, during its systole, flows into the left ventricle, from where it is directed into the systemic circulation. When the left ventricle contracts, it pumps blood into the aorta, which forms a sharp leftward arch. Unlike birds, which have a right aortic arch, mammals retain the left aortic arch. Three vessels emerge from the aortic arch: a short brachiocephalic artery (which divides into the right Subclavian Artery and the right common carotid), the left common carotid artery, and the left subclavian artery. Having arched over the heart, the aorta extends backward along the spine. Vessels branching off the aorta supply blood to the internal organs: the intestine, liver, Kidneys, and others. In the pelvic cavity, the aorta divides into the common iliac arteries, which extend further into the hind limbs and supply them with blood. Through a dense, branching network of vessels, arterial blood delivers oxygen to the Cells of the animal's body Tissues.

Fig. 142 Diagram of the mammalian circulatory system:

1 — External Carotid Artery; 2 — Internal Carotid Artery; 3 — subclavian artery; 4 — aortic arch; 5 — pulmonary artery; 6 — left atrium; 7 — right atrium; 8 — left ventricle of the heart; 9 — right ventricle of the heart; 10 — dorsal aorta; 11 — visceral artery; 12 — renal artery; 13 — iliac artery; 14 — jugular vein; 15 — subclavian vein; 16 — left hemiazygos vein; 17 — right azygos vein; 18 — posterior vena cava; 19 — hepatic vein; 20 — HEPATIC PORTAL VEIN; 21 — liver; 22 — kidneys; 23 — iliac vein.

Venous blood collected from the entire body flows into the right atrium via the venae cavae: from the anterior part of the body through the paired anterior venae cavae, and from the posterior part through the unpaired posterior vena cava. Venous blood from the intestine flows into the portal vein, which enters the liver and forms the hepatic portal system. In the liver, the capillaries unite to form two hepatic veins that empty into the posterior vena cava near the heart, which collects blood from the entire posterior region of the body.

Excretory system. Locate the paired kidneys—they are positioned asynchronously on either side of the spine; the right kidney lies higher than the left. The Kidneys in the rat are secondary—metanephric, similar to the kidneys of reptiles and birds.

Examine the Ureters extending from the inner side of the kidneys: they appear as whitish threads and empty into the urinary bladder. The urinary bladder opens to the exterior via the Urethra.

In the female, locate the two Ovaries in the lumbar region near the kidneys, on the dorsal wall of the body cavity. Examine the oviducts—highly convoluted tubes that have no direct connection to the ovaries: one end of the oviduct begins with a funnel embracing the ovary and opens into the abdominal cavity, while the other end opens into the uterus. Note that the uterus consists of two horns, which serve as a continuation of the oviducts. Embryonic development of the embryos takes place within the uterine horns. Locate the unpaired vagina into which the uterine horns open; the vagina opens externally through the genital opening. Locate the Testes of the male rat: they are located in the Scrotum—a specialized pouch at the Base of the tail representing an outpocketing of the body wall. If they are not visible, gently press your fingers on the scrotum to push the testes into the body cavity. Examine the Epididymis adjacent to the testes and the vas deferens extending from them. Note the accessory sex glands in the body cavity—highly branched Seminal Vesicles (located near the urinary bladder) and the multi-lobed Prostate Gland situated at the neck of the urinary bladder. The vas deferens receives the ducts of these glands and empties into the urethra.

Fig. 143 Rat brain:

1 — cerebral hemispheres; 2 — olfactory lobes; 3 — Pineal Gland (epiphysis); 4 — Midbrain; 5 — Cerebellum; 6 — Medulla Oblongata.

Study The structure of the rat's brain using a preserved specimen. Examine the small olfactory lobes—behind them lie the cerebral hemispheres (cerebrum). Note that the hemispheres are covered with Gray matter, which forms the cerebral cortex. The Diencephalon is not visible from above—it is covered by the hemispheres, is relatively small, and lies between the posterior edge of the cerebral hemispheres and the cerebellum. Examine the midbrain—it is almost entirely covered by the cerebral hemispheres. The cerebellum is highly developed and almost completely covers the medulla oblongata (Fig. 143).

The rat's Sensory Organs are well developed. The higher development of the brain and Sense Organs determines the more complex behavior of mammals compared to more simply organized vertebrates.

Make a dorsal sketch of the rat's brain.