Vertebra - One of the bones making up the spinal or vertebral column. Vertebrae are present in almost all vertebrates replacing the embryonic notochord (the living lung fishes and the cyclostomes have no vertebrae but retain the primitive notochord). The vertebra consists of a solid bony centrum and a hollow neural arch on the dorsal side. The spinal cord runs through the neural arch. Each vertebra is linked to its neighbours by a cushion of cartilage which allows a little movement and thus enables the animal to bend. All the vertebrae of fishes are similar but the land living vertebrates (tetrapods) show a number of variations.

The first two vertebrae (those nearest the skull) are specially fied for the carriage of the head. The first vertebra is the atlas which articulates with the back of the skull. The centrum is hollow. Nodding the head involves movement between the skull and the atlas. The second vertebra is the axis. On the front there is a peg which fits into the hollow centrum of the atlas. The peg is, in fact, formed from the missing part of the atlas). The rotation of the head involves a swivelling of the skull and atlas on this peg of the axis.

Behind the axis there are a number of cervical of neck vertebrae (in mammals there are normally seven a giraffe has no more vertebrae in its neck than a mouse has). Behind the neck region there are a number of thoracic vertebrae which have projections for the articulation of the ribs. Vertebrae of the lumbar region behind the ribs are normally larger than the rest. They are followed by the sacral vertebrae which often fuse into one structure called the sacrum. The hip girdle is attached to one or more sacral vertebrae. Behind the sacral region there are a number of small tail or caudal vertebrae.

Vertebrata - Sub phylum of Chordata which includes all the backboned animals fishes, amphibians, reptiles, birds, and mammals. They differ from the other chordates in the possession of bony or cartilaginous skull surrounding the brain. In most of them the notochord is replaced by vertebrae of bone or cartilage. The skin is several layers thick.

Vestigial - Of very reduced form when compared with the ancestral structure. For example, the hind limbs of whales have been reduced during the course of evolution until they are now represented only by tiny bones inside the body. These bones are the vestigial limbs. Able to live or develop. Used for embryos.

Vibrissae - The stiff whiskers of the face of most mammals, but especially noticeable in cats, which act as touch receptors. Cats are thought to use their vibrissae for, among other things, judging the width of gaps in relation to the size of their bodies.

Villus - (plural villi). Projection in wall of intestine.

Visceral Arches - The partitions and skeletal rods separating the gill slits of fishes. The rods and the associated muscles are important in the respiratory movements of the pharynx. Modern fishes have five pairs of visceral (or branchial) arches, one behind each gill slit. It is though that the earlier, jawless fishes had more than five pairs of visceral arches and that jaws evolved from one of the more anterior pairs. In the earliest jawed fishes, the jaws were not connected to the arch behind them the hyoid arch and there was a complete gill slit behind the jaws extended further back and become supported by the hyoid arch.

This involved compression of the first gill slit behind the jaws and it is now represented in many fish by the small spiracle. In land living vertebrates the visceral arches are not required for respiratory movements and the upper jaw becomes fixed to thed base of the skull. This fress the upper part of the hyoid arch which becomes part of the middle ear, transmitting sound form the eardrum to the inner ear. The other arches are reduced and incorporated as small cartilages into the hyoid apparatus and larynx in the throat.

Vitamins - Complicated organic compounds that must be provided in an animal's dief if it is to remain healthy. Only tiny amounts are needed but if they are not supplied, certain abnormalities occur. Such abnormalities caused by vitamin deficiency are called deficiency disease and it was through investigation of the these diseases that the vitamins were first discovered. Many diseases once thought to be caused by germs are now known to be caused by lack of vitamins and it is possible to cure them by giving the required vitamin. The importance of a mixed diet in providing vitamins is now realised.

One of the chief effects of vitamin deficiency is a slowing down of growth so it is important that children particularly should be well supplied with vitamins. It is during this early period that the human body is growing most actively and consequently the effects of vitamin deficiency will be most pronounced. McOllum and Davies in 1915, working in the United States, introduced a system of naming vitamins, dividing them into two groups. Those in the one group that dissolved in fat they called the fat-soluble A’ group and the others, that dissolved in water, the called the 'water-soluble B' group. Within these groups many vitamins were recongnised. Each became known by a letter (e.g. Vitamin A, Vitamin C) or by a letter with a subscript for vitamins each with similar properites (e.g. vitamins BI-B12). Many vitamins have now been made synthetically and, though the naming of them by lettrs has not been dropped completely, there is an increasing tendency to use the chemical names thus vitamin c is known as ascorbic acid, vitamin B1as aneurin (Britain) or thiamin (U.S.A.), and vitamin B2 as riboffavin.

Vitelline Membrane - Membrane immediately surrounding the egg cell or ovum.

Vitreous Humour - Fluid in chamber behind of eye .

Viviporous - Giving birth to active young which have been nourished inside the mother usually by means of a placenta.

Vocal Cords - (See Voice).

Voice - The mammalian voice is produced in the larynx, or voice box, which is a special part of the wind-pipe in the throat. Around the larynx there are various cartilages the large one at the front (thyroid cartilage) forms the Adam's Apple in Man. Inside the voice box are two sheets of tissue called the vocal cords. These are responsible for the actual sound production.

During normal breathing the vocal cords are relaxed and there is a triangular opening between them. During sound production the cords are tightened by their muscles and as air is forced up from the lungs the cords vibrate the opening between the cords opens and closes rapidly and sets the air vibrating with a fundamental frequency corresponding to the rate of vibration of the cords. This fundamental frequency determines the pitch of the note. There are also many overtones or harmonics produced. These are of less intensity than the fundamental note but of higher pitch. They give the note quality. A pure note, without overtones, sounds tinny.Notes of different fundamental pitch are produced by changing the position and tension of the vocal cords. The volume of sound is controlled by the pressure of the air pushed through the voice box by the lungs.

A man's voice is deeper than that of a woman or child. This is because during adolescence the larynx enlarges the Adam's Apple gets bigger and the vocal cords grow. Just as a double bass produces a lower note than a violin, so the vocal cords of a man produce a lower note than the shorter cords of a woman or child.

All mammals have this basic arrangement for making sounds although the size of the cords varies a great deal, The sound is produced in the voice box but its quality is determined by the mouth and nasal region of the breathing tract. These regions are cavities in which some of the overtones are lost, while others are emphasised. Man has fine control over his facial muscles and, just by changing the shape of the mouth, he can produce a very different sound. This is called articulation. The sound 'oo' is altered to 'ee' simply by altering the shape of the mouth. The vocal cords are still vibrating at the same frequency but a different set of overtones is being emphasised and the quality is therefore changed. Other animals have not this fine control and their voices lack the variety of sounds.