- THE STRUCTURE AND FUNCTION OF THE EAR
The ear is concerned with hearing and the sense of balance. It is divided into three parts, the external ear, the middle ear, and the inner ear, as shown in the figure 2 on next page
The external ear in mammals is able to move in most directions in order to pick up sounds more quickly and efficiently. It is separated from the middle ear by the tympanic membrane.
The middle ear contains three fine bones, or ossicles; these are the malleus (hammer), the incus (anvil), and the stapes (stirrup), and these provide a link between the tympanic membrane and the oval membrane. The middle ear is separated from the inner ear by the two membranes which close the oval and round windows.
The inner ear consists of the cochlea, which is the part sensitive to sound, and the semi-circular canals, which are sensitive to balance. If these become affected in any way, the mammal will tend to lose its balance. These canals contain fluid and a detailed diagram of their arrangement is shown below.
Figure 1: The Ear Arrangement
Sound waves strike the tympanic membrane, or ear drum, causing it to vibrate. These vibrations are transmitted to the inner ear through the three fine bones, the ossicles and the oval window. In the inner ear the vibrations are picked up by the fluid in the cochlear, and its sensitive, hair-like cells transmit impulses to the brain where they are interpreted as sounds.
The way in which the impulses are transmitted to the brain and interpreted is not fully understood. One theory, called the telephone theory, is that when an impulse is picked up by the fine hairs of the cells in the cochlear, they send nerve impulses to the brain. The problem arising from this theory is that the lumen ear can receive and transmit to the brain, sound waves of up to 20 000 per second. Nerve fibres can transmit impulses only at a maximum rate of 1000 per second.
The second theory, the Resonance Theory, is based on the fact that the cell fibres in the cochlear are long at one end and short at the other. It has been shown that some sounds striking these fibres cause only some of the fibres to vibrate.
The Resonance Theory is based on the fact that if you strike a note on a musical instrument held near a piano, the strings of the piano will vibrate and sound the same note.
The theory is that a vibration striking the fibres in the cochlea causes that sound or note to be passed on to the brain. The arrangement of the fibres is shown in the diagram below.
Figure 2: The Arrangement of Fibres in the Ear
2. THE STRUCTURE AND FUNCTION OF THE EYE
The eye is associated with the sense of sight. The eye ball is a round organ. The front part, known as the cornea, is transparent in order to allow light to enter the main part of the eye. The inside of the eyeball is lined with specialised cells called the retina, which are very sensitive to light impulses.
Impulses are received on the retina, transmitted through the optic nerve to the cerebrum part of the brain where are turned into visual images.
Figure 3: The eye
The lens of the eye is a bi-convex structure attached to the eye by very fine fibres of ligament. At the other end they are joined to the ciliary body which can be moved by the ciliary muscle. The attachment of the lens is shown in the diagram below.
Contraction of the ciliary muscles causes the plastic choroid, which is attached to the lens to stretch and the lens to bulge and become wider. This happens when you try to look at something close to you. When you look at something in the distance, the ciliary muscles relax, and the choroid becomes shorter, pulling the lens and causing it to flatten.
This is the coloured part of the eye that controls the amount of light entering the eye through the aperture in the middle of the iris, called the pupil. In the dark, the pupil enlarges and in strong light it becomes very small. The opening and closing of the pupil is controlled by muscles in the iris. Study the illustrations below.
Figure 4: The Opening and Closing of the Pupil
- 3. THE ENDOCRINE SYSTEM
Throughout a mammal’s body there are groups of specialised cells which form quite large glands and produce secretions. These glands are unlike other types of glands (the exocrine glands) because they have no duct and pour their secretions directly and by diffusion into the blood vessels. Due to this they are called ductless glands, or endocrine glands. Their secretions are called hormones, and these belong to two main chemical groups, the steroids and the amino acid derivatives.
|Immunological: the branch of medicine and biology concerned with immunity.
The secretions of the endocrine glands play an important part in regulating the functions of the body, and some of them act directly on muscles and other glands in a way that is very similar to the actions of nerve impulses. Other hormones are concerned with the regulation of the body’s metabolism and growth. Some endocrine glands are themselves stimulated by hormones from other glands, while some are under the control of the nervous system and thus are able to respond very rapidly to changes.
THE PITUITARY BODY:
This is a fairly large gland situated in the base of the skull just below the hypothalamus. The hormones produced by the pituitary are:
- The thyroid-stimulating hormone (T.S.H.) which influences the development of the thyroid gland;
|Metabolism: the chemical processes that occur within a living organism in order to maintain life.
The follicle stimulating hormone (F.S.H.) which stimulates the growth of graafian follicles in the ovary and the seminiferous tubules of the testes. Its secretion tends to be stopped by hormones produced by the sex organs, an example of negative feedback;
- The luteinising hormone (L.H.) which induces the formation of the corpus tuteum in the ovaries and stimulates the development of some secondary sexual characters;
- The adrenocorticotropic hormone (A.C.T.H.) which stimulates the activity of the adrenal body;
- The lactogenic hormone (L.T.H.) which starts milk production in the pregnant female;
- The somatropic hormone (S.T.H.) which stimulates growth. In excess, it causes the formation of giants. It also stimulates the release of glucagon from the pancreas;
- The oxytocin, which is released when the mammary glands are stimulated by suckling, and which causes the ejection of milk; and
- The antidiuretic hormone (A.D.H.) which increases the sodium from the uriniferous tubules in the kidneys and reduces the amount of urine produced by increasing the re-absorption of water.
This is a relatively large gland lying in the region of the throat. The main hormone produced is thyroxin, which is formed in the thyroid by accumulating iodine and combining it with an amino acid. The function of thyroxin is to promote growth in the young animal and to speed up most of the body activities in the adult. A deficiency of thyroxin causes a general slowing down and sluggishness in the animal while an excess has the opposite effect.
Are found next to the thyroid and their hormone works together with vitamin D to maintain an adequate concentration of calcium in the blood. Its effect is thought to be one reason for ‘milk fever’ in newly calved cows. This is caused by a lack of calcium in the blood due to the demands of the milk being produced by the animal.
Is found next to the heart. Its main function is to produce lymphocytes in the young animal. Is essential for the development of immunological reactions which play an important part in the defenses of the body against bacterial and viral infection. As the animal grows, the thymus degenerates.
The adrenal bodies are found near the kidneys. The important hormones produced are:
Is responsible for the “fight or flight” reaction which happens when an animal is alerted or given a fright. It increases the rate of the heartbeat and the output of the heart bringing more blood to the muscles. It increases the amount of glucose in the blood by increasing the rate of breakdown of glycogen in the liver. This is accompanied by increased oxygen consumption and an increase in the production of heat and energy. Adrenalin causes symptoms of anxiety in humans and animals.
Has very similar effects as adrenalin, but increases the blood pressure by constricting the arteries and arterioles through stimulation of the smooth muscle in their walls.
- Other hormones produced are the steroids called corticosteroids, of which the main ones are corticosterone, cortisol and cortisone. Their actions are to increase the level of glucose in the blood by increasing the rate of breakdown of protein and fats. Other hormones of this type also maintain the balance of mineral ions in the blood. Another effect is to induce male secondary sexual characters. These hormones, steroids, are sometimes taken by athletes, particularly weightlifters, javelin and hammer throwers in order to increase their muscle and so their strength. Women who take these steroids are likely to develop male characteristics such as hair on the face and chest.
The pancreas is situated beside the liver. It produces two protein hormones, insulin and glucagon. Insulin lowers the level of glucose in the blood by increasing the rate at which it is converted to glycogen in the liver, and glucagon acts in the opposite way by increasing the rate at which glycogen in the liver is converted into glucose. Between them, these two hormones control the amounts of glucose in the blood.
Produce a series of steroid hormones called androgens, the chief one being testosterone. Their
functions are to maintain the male ducts and glands in good condition, the development of male characteristics (horns, hair, voice, etc.), the stimulation of sperm production, and various metabolic effects such as an increased rate of protein build-up.
Ovarian hormones are secreted by the follicle cells and the corpus luteum. They are called oestrogens, progestins and relaxins, the first two being steroids and the last a protein hormone. Oestrogens activate the mucous glands of the reproductive tract preparing it for pregnancy. They also produce many secondary sexual characteristics, enlargement of the mammary glands at puberty, the high- pitched voice of the female, etc.
Progestins also prepare the reproductive tract for pregnancy and activate the mammary glands to secrete milk. Relaxins cause dilation of the cervix of the uterus at birth and inhibit uterine contractions. Most of these activities follow a cycle, and if fertilization does not take place, they die down and then repeat themselves in a following cycle.
THE PINEAL BODY
Is found in the brain and produces a hormone called melatonin. This appears to be sensitive to light and is thought to control seasonal sexual activity such as animals coming on heat in the spring, as a result of the lengthening daylight hours.
The mucous membrane of the stomach:
Produces the hormone gastrin which affects the breakdown and digestion of foodstuffs.
The mucous membrane of the small intestine:
Produces the hormone secretin which travels to the pancreas via the bloodstream and stimulates the production of the pancreatic juices.
The following table gives a brief outline of the various hormones produced by the endocrine glands, how their production is stimulated in the body.
Table 1: Endocrine Glands
|GLANDS STIMULATED BY:
|ACTIONS OF HORMONE SECRETED
|Elevated levels of glucose (G) in blood entering pancreas (Feedback
|INSULIN (from B Cells)
|Increase G utilisation in muscle, pituitary and mammary gland and storage in the liver
|Lower G levels in the blood
|GLUCAGON (from A Cells)
|(Anti-insulin). Release of G from liver into the blood
|T.S.H. released from pituitary
|Accelerates metabolic rate (by increasing efficiency of O2 consumption in cells)
|Increase in Ca++ blood levels in thyroid
|Inhibition of resorption of Ca++ from bone (Anti P.T.H.)
|GLANDS STIMULATED BY:
|ACTIONS OF HORMONE SECRETED
|Lower Ca++ blood levels in parathyroids
|PARATHYROID HORMONE (P.T.H.)
|Raises blood Ca++ level by resorbing Ca++ from bone. Absorption of Ca++ from intestines (Vit D). Increases
|4. STOMACH MUCOSA
|Distension of stomach wall
|Secretion of gastric juice
|5. DUODENAL MUCOSA
|Release of chime from stomach
|Stimulates flow of pancreatic juice.
|Entry of fats and partially digested proteins into duodenum
|Stimulates bile release and increases enzyme content in pancreatic juice
|A C T H from pituitary. Stress
|Increase blood G levels and protein build up
|A C T H Fall in blood pressure or Na+ level in blood. Stress
|Increase in Na+ and water re- absorption from kidneys
|A C T H from pituitary
|OESTROGEN or TESTOSTERONE
|Regulate secondary sex characteristics and sexual behaviour
|ADRENALIN (from medulla)
|Increased heart rate, stimulation of C.N.S. and raised blood G levels
|NOR-ADRENALIN (from medulla)
|Increased peripheral blood pressure, stimulation of C N S
|L H from pituitary
|TESTOSTERONE (from Leydig Cells)
|Regulates male secondary characteristics and sexual behaviour
|F S H or L H from pituitary
|OESTROGEN (from follicles)
|Regulates female secondary characteristics and sexual behaviour (oestrus)
|F S H and L H from pituitary
|PROGESTERONE (from corpus luteum)
|Prepares uterus for pregnancy and mammary gland for lactation
|Onset of labour
|Relaxes cervix and widens birth canal
|GLANDS STIMULATED BY:
|ACTIONS OF HORMONE SECRETED
|Stress. (Negative feedback of corticoids on hypothalamus)
|A C T H (adreno-corticotrophic hormone)
|Stimulates release of gluco corticoids and mineralo-corticoids from adrenal cortex
|Low energy levels, increase in blood amino acids, sleep; all act via hypothalamus
|Stimulates body growth (by increasing efficiency of cell metabolism and energy mobilization) Lactation and synergist
|Stress via hypothalamus and negative feedback of thyroxine
|T.S.H. (Thyro-stimulating Hormone)
|Stimulates thyroid to secrete thyroxine
|Negative feedback of L.H. via hypothalamus
|Maternal instincts and milk secretion. Synergist
|Releasing factor and negative feedback of F.S.H. on hypothalamus
|F.S.H. (Follicle-stimulating Hormone)
|Development of ovarian follicle (female). Maturation of Leydig cells (males).
|Oestrogen via hypothalamus. Negative feedback of L.H. on hypothalamus
|L.H. (Luteinizing Hormone)
|Ovulation and formation of corpus luteum (female). Secretion of testosterone from Leydig cells and formation of sperm (male).
|Decrease in blood pressure or higher osmotic pressure via hypothalamus
|Increased re-absorption of water from kidney
|Suckling and genital stimulation via hypothalamus
|Uterine contraction during fertilization and labour. Milk “let- down”