CHAP. VI.] THE MUSCLES. 65 



widened into a flat circular space, in the centre of which is sit- 

 uated the cicatrix of the umbilicus. 



The abdominal muscles perform a threefold action. When 

 the pelvis and thorax are fixed, they compress the abdominal 

 viscera by constricting the cavity of the abdomen, in which 

 action they are much assisted by the descent of the diaphragm. 

 By these means the foetus is expelled from the uterus, the 

 faeces from the rectum, the urine from the bladder, and its con- 

 tents from the stomach, in vomiting. If the spine is fixed, 

 these muscles raise the diaphragm by pressing on the abdominal 

 viscera, draw down the ribs, compress the lower part of the 

 thorax, and assist in expiration. If the thorax is fixed, and 

 the trunk and arms raised and fixed, they draw the pelvis 

 upwards as a preparatory step to the elevation of the lower 

 limbs in the action of climbing. 



The diaphragm is a thin musculo-fibrous partition, placed 

 obliquely between the abdominal and thoracic cavities. It is 

 fan-shaped, and consists of muscle fibres arising from the whole 

 of the internal circumference of the thorax, and of an aponeu- 

 rotic tendon, shaped somewhat like a trefoil leaf, into which 

 the muscle fibres are inserted. ( Vide Plate V. for illustration 

 of diaphragm.) It has three large openings for the passage 

 of the aorta, the large artery of the body, the inferior vena 

 cava, the largest vein of the body, and the oesophagus or gullet ; 

 it has also some smaller openings, of less importance, for the 

 passage of blood-vessels, nerves, etc. The upper or thoracic 

 surface of the diaphragm is highly arched; the heart is sup- 

 ported by the central portion of the arch, the right and left 

 lungs by the lateral portions, the right side of the arch being 

 slightly higher on the right than on the left side. The lower 

 or under surface of the diaphragm is deeply concave, and covers 

 the liver, stomach, pancreas, spleen, and kidneys. 



The action of the diaphragm modifies considerably the size 

 of the chest, and the position of the thoracic and abdominal 

 viscera, and it is essentially 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 successively drawn into the lungs and expelled 

 from them by the alternate increase and diminution of the 

 thoracic cavity. The changes in the capacity of the thorax are 



