THe visual sense is the dominant sense in humans. Around the eyes the eye brows are designed to keep liquid and other things out, as are the eyelashs. The eye is a ball, but we can only see approximately one sixth from the front. The wall is made up of the fibrous, vascular, and inner layers.

The posterior chroid supplies all of the layers with blood. There is the ciliary body, the muscle that surrounds the lens. When light goes through the pupil the disc focuses that light, and protjeccts onto the retina which is the inner layer of the back of the eye ball. The retina’s have photorecetpors which convert the light energy to electrical signals to be transported through the brain.

The photoreceptors are called rods and cones. Cones detect colour, and fine detail. The rods don’t sense colour ?, instead the sense black and whitr The rteina has two layers, the inner layer is neurons. It contains bipolar neurons, and ganglion neoruons. The cones have red, blue and green senstiive types? Rods connect to ganglion cells in many to one which makes it difficult for the ganglion cells to ascetsain whihc rod sent what information.

Each cone connects to a ganglion cell one on one.

THE SKELETAL SYSTEM

An adult human contains 206 bones, reduced from 270 bones at birth as some get fused together. Your bones are made of connective tissue. The bonses are divdied into the axial bones, which occur along the bodies vertical axis, The appendicular bones are the ones that make up everything else.

Bones are also classidied by shape. There are long bones.

The short bones are normally cube shaped.

There are thin bones which normally protect something underneath.

The irregular bones tend to be uniquely shaped.

Despite ths shape of the bone the internal structure is similiar. They all have adense , compact external layer called corticol bone, which is around a spongy bone that looks honeycomb shape. The honey comb is made up of a material called trabeculae. The trneculase is where the bone marrow is. Bone marrow can be red bone marrow, or yellow bone marrow.

The red marrow is meant to be where we make cells, where as the yellow is where we store cells as fat.

The larger end of the bone. It almost looks like the end of the prototypical ” dog” bone, or like bookends are called epiphyses, they surround a caviity called the medullary caivty which has yellow marrow.

The bones are porous. The osteocytes form the cells of the tissue of the bone. They form what we call the mineral matrix of the bones. Inside the compact part of the bone is like tubes running in a tree trunk. These tubes are called lamellae. From lamellae to lamellae the fibres ? run one way and then the other which helps resist the torsion stress that gets placed on bones.

In the middle of the lamellae runs nerves and blood vessels in a central canel.

The bones store calcium and phosphate ? The bone marrow also makes the red blood cells. This is called hematopoiesis. ? Humans make over two million new red blood cells every second. Stop

In contrast to osteoblasts, there are osteoclast cells in the bones. They are responisble for bone reabosbtion, and to regulate the level of calcium. This is sometimes called “bone remodelling”. This process of bone reanorbtion is sometimes called ” resorption”. Exercise is often thought of as being potentially a way to stimulaete bone remodelling.

We know from experiments in space that when there is less gravity the osteoblasts don’t contribute as much to bone formation, and the osteoclasts are more active in their orle of bone resorption.