Respiration occurs in two phases namely inspiration and expiration. During inspiration, thoracic cage enlarges and lungs expand so that air enters the lungs easily. During expiration, the thoracic cage and lungs decrease in size and attain the preinspiratory position so that air leaves the lungs easily. During normal quiet breathing, inspiration is the active process and expiration is the passive process.


Respiratory muscles are of two types:

1. Inspiratory muscles

2. Expiratory muscles.

However, respiratory muscles are generally classified into two types:

1. Primary or major respiratory muscles, which are responsible for change in size of thoracic cage

during normal quiet breathing

2. Accessory respiratory muscles that help primary respiratory muscles during forced respiration.

Inspiratory Muscles

Muscles involved in inspiratory movements are known as inspiratory muscles.

Primary inspiratory muscles

Primary inspiratory muscles are the diaphragm, which is supplied by phrenic nerve (C3 to C5) and external intercostal muscles, supplied by intercostal nerves (T1 to T11)

Accessory inspiratory muscles

Sternocleidomastoid, scalene, anterior serrati, elevators of scapulae and pectorals are the accessory

inspiratory muscles.

Expiratory Muscles

Primary expiratory muscles

Primary expiratory muscles are the internal intercostal muscles, which are innervated by intercostal nerves.

Accessory expiratory muscles

Accessory expiratory muscles are the abdominal muscles.


Inspiration causes enlargement of thoracic cage. Thoracic cage enlarges because of increase in all diameters, viz. anteroposterior, transverse and vertical diameters. Anteroposterior and transverse diameters of thoracic cage are increased by the elevation of ribs. Vertical diameter is increased by the descent of diaphragm. In general, change in the size of thoracic cavity occurs because of the movements of four units of structures:

1. Thoracic lid

2. Upper costal series

3. Lower costal series

4. Diaphragm.

Thoracic Lid

Thoracic lid is formed by manubrium sterni and the first pair of ribs. It is also called thoracic operculum.

Movement of thoracic lid increases the anteroposterior diameter of thoracic cage. Due to the contraction

of scalene muscles, the first ribs move upwards to a more horizontal position. This increases the anteroposterior diameter of upper thoracic cage.

Upper Costal Series

Upper costal series is constituted by second to sixth pair of ribs. Movement of upper costal series increases the anteroposterior and transverse diameter of the thoracic cage.

Movement of upper costal series is of two types:

i. Pump handle movement

ii. Bucket handle movement Pump handle movement

Contraction of external intercostal muscles causes elevation of these ribs and upward and forward movement of sternum. This movement is called pump handle movement. It increases anteroposterior diameter of the thoracic cage.

Bucket handle movement

Simultaneously, the central portions of these ribs (arches of ribs) move upwards and outwards to a more horizontal position. This movement is called bucket handle movement and it increases the transverse diameter of thoracic cage.

Lower Costal Series

Lower costal series includes seventh to tenth pair of ribs. Movement of lower costal series increases the

transverse diameter of thoracic cage by bucket handle movement.

Bucket handle movement

Lower costal series of ribs also show bucket handle movement by swinging outward and upward. This

movement increases the transverse diameter of the thoracic cage.

Eleventh and twelfth pairs of ribs are the floating ribs. These ribs are not involved in changing the size of

thoracic cage.


Movement of diaphragm increases the vertical diameter of thoracic cage. Normally, before inspiration

the diaphragm is dome shaped with convexity facing upwards. During inspiration, due to the contraction,

muscle fibers are shortened. But the central tendinous portion is drawn downwards so the diaphragm is flattened. Flattening of diaphragm increases the vertical diameter of the thoracic cage

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