492 HEALTH AND DISEASE 



of cartilage, to the sternum or breast-bone. The remaining ten, false or 

 asternal ribs, whilst attached above like the true ribs to the vertebrae, 

 end below in cartilages which are only indirectly connected with the 

 sternum. 



The first and last ribs are the shortest, the ninth is the longest. They 

 form a series of highly elastic arches, the sharpness of the curvature being 

 greatest in the first and diminishing to the last. The articulations formed 

 by the heads of the ribs with the vertebrae permit a slight degree of 

 motion upon them. In complete expiration the ribs of the opposite 

 sides of the thorax fall towards one another, and at the same time approxi- 

 mate to those of their own side by their edges, thus diminishing the 

 capacity of the chest; whilst in full inspiration the ribs are raised, form 

 wider curves, separate from those of the opposite side and from each other, 

 and thrust the sternum forwards, in this way effecting enlargement of the 

 chest both from side to side and from before backwards. The move- 

 ments of the ribs are effected by muscles placed between them, named the 

 inter costals. These consist of two layers of short muscular fibres which 

 extend between the edges of adjoining ribs, the external running down- 

 wards and backwards, the internal running downwards and forwards, the 

 former serving to raise the ribs, and thus to effect inspiration, which is 

 aided by those fibres of the internal intercostals which extend between the 

 cartilaginous portion of the ribs. 



The front aperture of the thorax is occupied by the trachea, OBSO- 

 phagus, large vessels, and nerves, together with firm connective tissue. 

 The posterior opening of the thorax is very wide, and is closed by the 

 great muscular arch of the diaphragm or midriff. This important muscle 

 divides the thorax from the abdomen. Its posterior concave surface is 

 covered with the peritoneum or lining membrane of the belly, and is in 

 contact with the liver, stomach, and organs of the abdomen; the anterior 

 convex surface is covered with the pleura or lining membrane of the chest, 

 and is in contact with the lungs. 



The diaphragm is the most powerful muscle of inspiration, for in the 

 act of contraction the arch it naturally forms falls back and tends to become 

 a plane surface, and thus the cavity of the chest is enlarged from before 

 backwards at the expense of the abdomen. In forced respiration the chest 

 is acted on by many muscles which are attached to its outer surface, and 

 which tend to raise the ribs and effect inspiration. On the other hand, the 

 muscles of the abdominal wall tend to pull down the ribs, compress the 

 abdominal organs, and force forward the diaphragm; they consequently 

 cause expiration. 



The enlargement of the chest during tranquil respiration is smaller 



