The head, neck, thorax and abdomen are known collectively as the trunk. The arms and legs are known as the limbs - upper limbs and lower limbs. They contain parts of the key body systems such as the skeletal system, muscular system, nervous system, and circulatory system, but not large organs of the sort we have been describing above. There will be some smaller structures, such as lymph nodes, that belong to the lymphatic system, but nothing comparable to the heart, lungs, liver or spleen, for example.
This is a good time to introduce some technical words about relative position when we talk about the body. We are trying to avoid jargon as much as possible at this time, but these terms will be essential later and the best way to take them in is to begin using them now and become familiar with them through repetition.
The reason why we need these special terms is this - by using every-day language such as ‘above’, ‘below’, ‘in front’ or ‘behind’ as we have been doing up until now, it all depends on the position of the person at the start. If they are standing up normally, then the head is above the chest. But if that same person then does a handstand, then their head is now below their chest.
Patients can be in all kinds of positions, such as standing or sitting, or lying face down or face up on the operating table. Their limbs can also be in different positions in relation to the rest of their body. As you can appreciate, describing the position of one part of the body in relation to the others in an unambiguous way so that everyone in the health team understands what is meant will be impossible unless everyone agrees to use a standard system of naming.
For this reason, we always imagine the person to be in the ‘anatomical position’ before using relative terms of position or movement: the person is standing upright, facing forwards, with the feet reasonably close together. The arms are at the side of the body, with the hands turned outwards until the palms face forwards.
In this position, we can now talk about those things that are higher than something else as being ‘superior’, while those that are lower are ‘inferior’. These terms simply mean above or below - they do not have any other connotations about whether something is better or worse than something else. Something that lies in front of something else is said to be ‘anterior’, while something that lies further back is ‘posterior’. Something that is closer to the midline of the body is ‘medial’, while something that is further to one side is ‘lateral’. By using these terms of relative position it is possible to define the 3-dimensional position of any structure in the body in relation to any other. Try filling in the missing word in the following sentences:
If something is nearer the surface of the body it is said to be ‘superficial’, while if it further in it is said to be ‘deep’.
If we are considering a structure that has an extended shape, for example the upper limb or a blood vessel, we can refer to different parts of it in relation to a reference point by using the terms proximal and distal. Think, for example, of the upper limb. We could say that the elbow joint is proximal to the wrist joint when taking the shoulder joint as the point of reference. Or we could talk about the branches of the aorta (a major blood vessel leaving the heart). The first branches that arise, closer to the heart, are called proximal branches, while branches that arise later, at a greater distance from the heart, are called distal branches of the aorta.
So these are the main terms of position - superior/inferior, anterior/posterior, medial/lateral, superficial/deep, and proximal/distal. Notice how they come in pairs. Later, we shall look at the special terms that describe movement.