ANATOMY AND PHYSIOLOGY
Pictures (4 B&W & 3 Color) showing egg, embryos and stages of growth at BOTTOM
These two subjects are so closely related that they will be discussed together. Anatomy is concerned with the structure of the body and physiology with the function of the organs. Two branches of these subjects are biochemistry--the chemistry of the processes of life--and embryology, which is concerned with how the complex differences between species evolve from similar, single-celled eggs. It is impossible to explain evolution in a few paragraphs, but it must be remembered that through the ages, as changes occurred in climate, the availability of water, food supply, etc., those animals which adapted most efficiently to the new conditions were those most likely to survive. Freaks or "sports" as they are called, flourish if they prove to be more successful than the normal members of the species. The majority of "sports," however, are less suited to the environment and perish without trace. These accidental variations have produced the vast variety in birds we know today. In bird-keeping, it will be noted that some species, usually the rarer types, are not suited to a wide range of temperatures, diets, and environments, whilst others can flourish in an exposed aviary on a north wall and on the most unnatural diet. It is the rare and delicate birds which are probably destined to become extinct and which represent therefore such a challenge to keep and to attempt to breed in captivity. Before acquiring such birds we should study their biology and aim to meet as many of their natural requirements as possible. A well balanced, palatable diet acceptable to the bird is particularly important and if this cannot be maintained, then the bird should not be kept. It would be tedious and of little value to describe in detail the anatomy of each species. An appreciation, however, of a few of the variations of form and function among species is helpful in understanding the problems of disease. It is assumed that the reader possesses some general knowledge of the anatomy of vertebrates and has an idea of the function of the main organs of the body.
When one looks at a bird or any animal, it is difficult to realize that it is made of millions of tiny bits of living matter or cells, each entirely covered with its own membrane. The simplest complete forms of life are one-celled; that is, they are minute blobs of jelly or protoplasm enclosed in a skin. The make-up of the protoplasm within this cell, varies in different areas. There is a central nucleus or nerve center in a contractile, clear jelly containing undigested and partly digested food and waste material, but there are no obvious divisions between the various areas. Such unicellular organisms, of which the amoebae is one, are called protozoa which simply means "first animals." Amoebae are primitive but highly successful forms of life and are numerous throughout the world, especially in fresh water. They obtain food by flowing around particles of organic material, which are totally enveloped and slowly digested. They travel in a similar way by pushing a part of themselves forward as a sort of foot (pseudo-podium), and then "pour" themselves into this foot. Simple as their structure seems, they are attracted to suitable food and moderate warmth; they avoid heat, cold, and strong light; and they generally respond to their environment as do many more advanced creatures. Some protozoa which are parasitic in animals and man, are little different from their free-living relations. Plasmodiurn, the cause of malaria, and trypanosomes, which cause the tropical sleeping sickness in man, are two important members of the group. After protozoa, the next step in evolution was for numbers of these simple and identical cells to fuse into solid and then hollow spheres, thus producing organisms made up of some hundreds of cells. In these more advanced organisms some cells became modified and adapted for special purposes, such as conducting messages from one part of the body to another.
Evolution from these simple creatures has taken millions of years and has resulted in the development of vast numbers of species, of both invertebrates such as insects, and vertebrates such as fish, amphibians, reptiles, birds and mammals. All evolutionary development has resulted in an increase in cell specialization: but all body cells are developments of the basic type of cell. Cell specialization is bewilderingly complex; yet a few cells of very primitive types persist. White blood cells are little different from the simple amoebae and feed in a similar manner. Most white cells, however, are much less mobile than the amoebae of the duck pond and it is the circulation of the blood which provides movement to new areas. In contrast there are nerve, muscle, bone and gland cells, all of which are very different from amoebae. Body cells also vary considerably in size, the longest being muscle and certain nerve cells, whilst the largest cell is the egg.
The body cells are arranged in an organized fashion and gathered into sheets or masses called tissues. A tissue may contain several cell types, but a unit which contains groups of various types with one overall function is called an organ. The skin, mucous membranes, and other covering and lining tissues protect against attacks by mechanical, chemical, or microbial agents. The skeleton helps to hold the body together; it is a mixture of different cells which make up its bone, cartilage, and fibrous tissue. In these tissues many of the cells are fixed in a large amount of protein and lifeless mineral. Skeletal muscles, mainly associated with bones, are tissues generally gathered in parallel bundles of long, narrow cells. Because these contract on impulse from the brain, they are called voluntary muscles, although they are also known as striated muscle from their striped appearance. Involuntary muscles are automatic and are found in the intestinal or oviduct walls and some other internal organs. A short piece of intestine, detached and placed in salty water will continue to shorten and lengthen rhythmically: this automatic action comes from the alternate contraction of encircling and longitudinal muscles. Such involuntary muscle, also known as smooth muscle, is paler than skeletal muscle, and its cells are long and spindle-shaped. A third type of muscle is found in the heart. Although composed of interlinked short fibres showing the striations of voluntary muscle, its action is automatic. Heart muscle contraction (the heart rate) is governed by various factors including hormones. A conscious effort on the part of a bird or other animal cannot cause its heart to stop or restart. Glandular tissues (such as the liver and thyroid) are in reality chemical factories. They are usually grouped into two main types, the typical glands with ducts or tubes, e.g. salivary glands, which are under nervous control; and the ductless glands, e.g. the adrenals, which are controlled by hormones.
The blood and lymph systems are pipelines conveying warmth, moisture, gases, salts, food and waste materials to the appropriate parts of the body. Blood is considered to be a tissue. Lymph is a colorless, alkaline liquid which has similar functions to the blood. It has no red corpuscles and therefore does not convey oxygen. Nerve tissue is the most complex and specialized of all, both in structure and function. The most important organ in the body is the brain and this controls all the organs either directly or indirectly. Although these cell types and tissues look and act differently from one another, they all derive from the primitive cell symbolized by the simple avian egg. They still retain many common characteristics, although there is a great degree of variation. The power of preventing damage to a threatened part is demonstrated by a rapid increase of blood to the area which carries white cells to devour the antagonist, antibodies to neutralize it and finally the walling off of the damaged part. The sensitivity of the response to damage varies with different organs. White cells and sex gland cells are very susceptible to damage by radiation, for example, while skin, bone, and brain cells are amongst the least affected.
The egg is very large in relation to the adult bird, but it is similar in many ways to the microscopic ovum of a woman or other female mammal. The incubation period of the fertilized avian egg corresponds to the period of pregnancy in the mammal. The main difference is that with the mammal, nourishment of the embryo is carried by blood, whereas with birds it is derived from the egg albumen. In birds the periods of greatest stress are prior to laying, and during the feeding of the nestlings. Thus the two periods of strain on the bird's resources are separated by a pause for partial recuperation, the sitting or incubation period. The ovum is budded off from the ovary after it has acquired its full complement of yolk layers. These are deposited in response to hormones developed and liberated from the pituitary. The ovary is also influenced by the adrenal cortex, thyroid and ovarian glandular cells. Changes occur in the yolk-laden ovum which prepare it for fertilization by a sperm whilst still in the upper part of the oviduct. Irrespective of whether or not fertilization takes place, the ovum passes down the oviduct wrapped in its transparent membrane, picking up layers of albumen or egg white, the parchment-like shell membranes, and finally the shell.
After fertilization, the ovum--now with its full complement of chromosomes--begins to grow by dividing into two, four, eight, sixteen cells, etc., until by the time the egg is laid a few hours after ovulation, a minute speck or patch of cells on one face of the yolk indicates the beginnings of the embryo. At this early stage the embryo is called a blastoderm. If the egg is incubated, cell division proceeds at different rates in different parts of the blastoderm so that variations in size and distribution begin to appear. A slight separation becomes apparent between the surface and deeper layer of cells forming the ectoderm and endoderm or outer and inner skins. In the middle of this double sheet of cells a denser line of cells develops, visible from the surface of the yolk as a line and known as the primitive streak. From here the development is best described by surface diagrams or plans of the embryo, from the primitive groove and fold to the beginning of the organ formation. The embryo proper develops in front of the primitive groove, which then forms a tail-like extension to it. In the embryo, three layers of cells are recognizable, the ectoderm, mesoderm, and endoderm. The ectoderm forms the covering layers of the body--the skin, feathers, horny tissues, also brain, spinal cord, retina and lens of the eye. The endoderm forms the mucous linings of the alimentary canal from oesophagus to cloaca and certain glands and tubes budded off from the alimentary tract. The mesoderm produces the greater part of the bird's body, including muscle, bones, connective tissues, blood, heart, blood vessels, and the bulk of the reproductive, genital, and respiratory tracts.
The embryo's power of internal organization with each stage of development of an organ or structure is one of the miracles of nature. Not only do the parts grow, but some--like the notocord or primitive spine, and the atavistic forerunners of the tail, kidneys, and "gills"--change in form and even completely disappear by the time the chick hatches, all in harmony with its overall development. By about one-eighth of the way through the incubation period, all major organs can be recognized, even the stumpy limbs. The embryo as a whole is approaching the appearance of a vertebrate. By one-quarter of the incubation period, it is recognizable as a bird, but with a massively disproportionate head. It is during development, that the organs are most susceptible to infection, dietary deficiencies and hereditary influences. As the embryo develops, it is gradually elevated above the surface of the yolk, but becomes attached to the yolk sac by the formation of an umbilical cord connecting it with the intestine. Before hatching the yolk sac is withdrawn into the abdomen and after the bird hatches, yolk passes through the yolk stalk remnant of the umbilical cord to the intestine. This arrangement provides nourishment for the bird during the first few days of its life.
THE SURFACE ANATOMY
The main external features of birds are easy to recognize and understand. The terms used to describe them are for the most part well known. Structures which are peculiar to one or a group of birds are of special interest. A difficulty is that the terms used for these landmarks by anatomists differ in certain respects from those used by ornithologists or aviculturists. For example, the terms knee, ankle, hock and tibiotarsus-tarsometatarsal joint are all synonymous, being used by the layman, the ornithologist, the veterinarian and the anatomist respectively. The outer covering of the body is largely composed of skin. Areas where the ectodermal tissues are modified occur in the eyes, beak, cloaca, shanks and claws. Typical skin is composed of two main layers. The outer one or epidermis, is a dying layer of flattened cells covered with flakes of a protein called keratin. Feathers, scales, claws, and beak are all largely constructed of this tough inert keratin also known as horn. The second and deeper layer, the dermis, consists of cells which generate the epidermis. It contains numerous tiny blood and lymph vessels, nerve-fibre endings and usually fat. The dermis also initiates the production of special structures such as feathers and scales. The skin and feathers are important as a protection against the elements and injury, and they also act as a barrier to disease. When large areas of skin are damaged or removed as in a severe burn, not only does infection enter readily, but shock may result from loss of heat, blood protein and minerals.
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Hamilton & District Budgerigar Society Inc.