During evolution the animal kingdom split into two main groups: the invertebrates and the vertebrates. The invertebrates possess no internal skeleton but instead have a tough outer shell or exo-skeleton. The various parts of the limbs and trunk articulate with one another and movement is provided by muscles attached on the inside of this outer case. Such a structure is found in countless species, including insects and shellfish. In some the exo-skeleton has become thick and tough, providing protection for the internal tissues and acting as a suit of armour. However, the rigidity of this system produces considerable problems. For example a rigid system such as this cannot accommodate growth. A change of skeleton must take place at regular intervals. The very nature of the joints between rigid segments allows the animal to produce stereotyped movements but severely restricts its ability to adapt and evolve in relation to altered environmental conditions. These constraints have left the invertebrates relatively lowly members of the animal kingdom.
In contrast the vertebrates have an internal skeleton. In the trunk there is a backbone or vertebral column and in the limbs the various long bones. This skeleton grows with the animal and is flexible, as its components are connected by various types of joints designed according to the requirements at each site. Muscles, ligaments, and other structures are attached to this internal skeleton and can produce a subtlety and complexity of movements that are lacking in invertebrates. This type of structure can be seen in primitive animals such as the lancet, coelacanth, and even the modern dogfish. Natural selection has allowed the more successful species to flourish at the expense of those less well adapted to their environment. This process of evolution has led to the development of modern species of fish, amphibians, reptiles, birds, and mammals. The internal skeletons of all these diverse groups show similar fundamental structures but with extreme adaptations to the species' way of life.
Nearly all animals walk on all four limbs with the trunk horizontal. Man is almost unique in standing upright balanced on two limbs. The advantages of this posture are obvious but clearly it also leads to considerable stresses on the skeleton. This alone could be responsible for many of the back problems that we suffer today. That is not to say that animals do not suffer from backache. Communication is difficult but we do know that dachshunds with their long and relatively poorly supported spines are very prone to this problem.
It has been argued that if the upright posture were responsible for producing spinal problems, natural selection would not have favored the vertical position and we would all be walking on four legs. However, the span of life during effectively all the period of man's evolution was only some 20 to 30 years. As the majority of back problems occur at an older age, back pain would hardly affect the evolutionary process.
You can learn all about the human spine at out Free Online Anatomy Course. Another fantastic way to learn about the human spine is by studying anatomical models and anatomical charts of the human spine.
Brendon Mitchell owns and operates VisuMedical.com. He is dedicated to the advancement of quality medical education.
ไม่มีความคิดเห็น:
แสดงความคิดเห็น