Muscles Of The Back
The muscles of the back are disposed in five layers, one beneath the other. The two largest and most superficial muscles are : Trapezius and Latissimus dorsi. The Trapezius is a very large muscle and covers the other muscles of the upper part of the back and neck, as well as the upper part of the latissimus dorsi. The latissimus dorsi muscles act upon the bones of the upper extremities. They elevate the shoulders, move the arms and assist in drawing up the body in climbing.
Muscles Of The Chest
The chief bulk of the anterior muscular wall of the chest is made up of the pectoral muscles, viz., Pectoralis major and Pectoralis minor. The Pectoralis minor is underneath and entirely covered by the Pectoralis major. These muscles move the arms
Muscles Of The Thorax
The muscles of the thorax are chiefly concerned with the movements of the ribs during respiration. They are the intercostals and Levatores Costarum. The intercostals are found filling the spaces between the ribs. Each muscle consists of two layers, one external and one internal. There are eleven internal spaces on each side and two muscles on each space. Therefore there are 44 intercostal muscles. The fibres of these muscles run in opposite directions. The action of the external fibres is to pull the ribs upwards and outwards, thereby increasing the chest cavity. The action of the internal fibres is to depress the ribs. The Levatores Costarum muscles are the lifters of the ribs. They assist in elevation of the first ten ribs and with the other muscles draw the lower ribs backwards.
Muscles Of The Diaphragm
The diaphragm is a thin, musculo-fibrous partition between the thoracic and abdominal cavities. It is dome-shaped, and has three openings for the passage of Aorta, the large artery in the body, the inferior Vena Cava, one of the largest veins in the body and the oesophagus or gullet. The diaphragm is the great respiratory muscle of the body.
The mechanical act of respiration consists of two sets of movements, viz., those of inspiration and of expiration, in which air is successfully drawn into the lungs and expelled from them by the alternate increase and diminution of the thoracic cavity. In the act of inspiration the diaphragm contracts and in contracting flattens out and descends, the abdominal organs are pressed downwards, and the thorax is expanded vertically. The diaphragm is an expulsive as well as the chief respiratory muscle of the body
Muscles Of The Abdomen
The muscular walls of the abdomen are mainly formed by three layers of muscles, the fibres of which run in different directions, those of the superficial and middle layers being oblique and those of the innermost layer being transverse. The chief muscles of the abdomen are external oblique, internal oblique, recuts abdominis and transversalis. Linea Alba or white line is a tendinous band formed by the union of the aponeuroses of the oblique and transverse muscles. It extends perpendicularly down the middle line from the sternum to the pubis.
The abdominal muscles compress the abdominal organs by constricting the cavity of the abdomen. They give assistance in expelling the foetus from the womb, the excreta or faeces from the rectum, the urine from the bladder and the contents of the stomach in vomiting. They are much assisted by the descent of the diaphragm. The abdominal muscles raise the diaphragm and assist in expiration. They draw the pelvis upwards as a preparatory step to the elevation of the lower limbs in the action of climbing.
Muscles Of The Upper Extremities
The most Prominent Muscles found in the upper limbs are the Deltoid, Biceps, Triceps, pronators, Supinators, Flexors and Extensors. The deltoid is a coarse triangular muscle that covers the top of the shoulder. It raises the arms from the sides so as to bring them at right angles to the trunk.
Biceps : This is along fusiform muscle. It occupies the whole of the anterior surface of the arm. It is divided above into two portions or heads, hence the name biceps. The flexes the forearm on to the upper arm.
Triceps : This muscle is situated on the back of the arm. It is of a large size and divided above into three heads. Hence the name triceps. It is the extensor muscle of the forearm. It is the direct antagonist of the biceps. The extensor muscle is the one that helps in the extension of the limbs.
The muscles that cover the forearm are disposed in groups, the pronators and flexors being placed on the front and inner part of the forearm and the Supinators and Extensors on the outer side and back of the forearm. They are antagonistic to one another. The pronators turn the palm of the hand backward and when the elbow is flexed, downwards or prone. The supinators turn the palm of the hand forwards, and when the elbow is flexed, upwards or into the spine position. The flexors and extensors serve to flex and extend the wrist and fingers.
Muscles Of The Lower Extremities
These include the muscles of the hip, thigh, leg and foot. The most important of these muscles are : Glutei or Gluteai muscles, Psoas Magnus, Posterior Femoral, Anterior Femoral, internal Femoral, Tibialis Interior, Extensors, Peroneal, Gestrocnemius Soleus, Flexors, Tibialis Posterior.
If we compare the muscles of the shoulder, arm and forearm with those of the hop, the thigh and the leg, we shall see that the anterior muscles of the former correspond roughly with the posterior muscles of the latter and the muscles of the hip. Thigh and leg are larger and coarser in texture than those of the shoulder, arm and forearm.
Glutei : Three gluteal muscles – Gluteus Maximus, Gluteus medius and Gluteus minimus – from the chief prominence of the buttocks. The Gluteus medius and Gluteus minimus are under the Gluteus maximus. These muscles support the trunk upon the head of the femur and bring the body into the erect position when the trunk is ben forwards upon the thigh.
Psoas Magnus: This the great loin muscle. Its action is flexion and external rotation of the thigh.
Posterior Femoral : Posterior femoral or the hamstring muscles cover the back of the thigh. There are three of these muscles, the biceps, the semitendinosus and the semimembranosus. The chief of these is the biceps, and is somewhat analogous to the biceps covering the front of the arm. The action of the hamstring muscle is to flex the knee and extend the thigh.
Anterior Femoral : The Principal anterior femoral muscles are the Quadriceps and the Sartorius.
Quadriceps : This is a four-headed muscle that covers the front of the thigh, and is analogous to the triceps covering the back of the arm. Each head is described as a separate muscle : (1) Rectus Femoris (2) Vastus Externus, (3) Vastus Internus (4) Vastus Intermedius. The quadriceps is the great extensor of the leg. It also flexes the thigh and antagonizes the action of the hamstring muscles.
Sartorius : This muscle, Sartorius, often called the “Tailor’s muscle”. It is a long ribbon-like muscle. It is the longest in the body and is the muscle principally concerned in producing the posture assumed by the tailor in sitting cross-legged and hence its name.
Internal Femoral : The internal femoral is otherwise known as adductor muscles. These muscles are also adductors of the thigh.
The Tibialis anterior, the extensors and the peroneal muscles cover the front and outer side of the leg. The Gastrocnemius and the Soleus, the Flexors and the Tibialis Posterior cover the back of the leg.
The action of the Tibialis anterior and one of the three peroneal muscles (Peroneus Tertius) is to flex the ankle, while the action of the Tibialis Posterior and the other Peroneal muscles (Peroneus Longus, peroneus Brevis) is to extend the ankle. The flexors and extensors act on the toes.
The Gestrocnemius muscles form the calves of the legs. They possess considerable power, and are constantly in use while standing, walking, dancing or leaping.