(3) The Digestive system, by which all the food is made into liquid and changed so as to nourish the body and pass into the blood. This system includes the mouth, gullet, stomach, liver, pancreas, intestines, and other organs.
(4) The Secretory, or excretory, system (for they are best grouped as one) manufactures the various fluids of the body, such as bile, urine, sweat, saliva, gastric juice, etc. It consists of various glands or secretory organs in different parts of the body, such as those in the skin, the kidneys, the lymphatic glands, the spleen, etc. It also gets rid of the refuse of the body.
Lastly, there are two in the head and limbs:
(5) The Locomotor system, by which all movement is effected. This includes the bones, joints, and muscles.
(6) The Nervous system, by which all the body is controlled, directed, and regulated. This system includes the brain, spinal cord, and the special senses, such as the ear, the eye, and all the nerves.
The Human Chest, or Thorax.—In it, the blood is purified and circulated. The thorax is closed above and below: above, by the neck, through which the windpipe enters it in front, conveying air to the lungs; and by the gullet behind, conveying food to the stomach. Below, the floor, dividing it from the abdomen beneath, is formed by a very large muscle stretching right across the body, called the Diaphragm, or partition wall; also called the Midriff. The thorax is walled in at the sides by the ribs, and behind by the backbone in which is the other tube that contains the spinal cord. The thorax contains the two organs of respiration and circulation.
The lungs are the organs of respiration. They are like two sponges filling the right and left halves of the chest. Wherever you can feel a rib there is part of the lung underneath. Each of these lungs is contained in a bag, like a skin, that separates it from the ribs, and is called the pleura (from pleuron = a rib), but the lung is not inside the bag.
The outer layer of the pleura is fixed to the side of the chest, the inner layer to the lung, and the two layers move on each other like a joint when we breathe.
The lungs are full of small air-cells with minute tubes leading from them. These gradually increase in size as they join together, till at last they unite in one large tube, or bronchus, for each lung. These two bronchi join together, and form the windpipe, or trachea, which conveys the air through the larynx into the mouth.
The windpipe is kept stretched widely open by a series of elastic rings of gristle. Behind the windpipe is the gullet, leading to the stomach.