173 



lie in opposite directions. Besides, several muscles cover the outer part 

 of the thorax, and pass from the ribs to the neighbouring bones ; as the 

 subclavian muscles, the great and lesser pectorals, the serrati, the lastissi- 

 mi dorsi, the scaleni, the longissimi dorsi, the sacro lumbales, and the 

 serrati minores, posterior, superior, and inferior. But of all the mus- 

 cles which form the anterior, posterior, and lateral parietes of the chest, 

 the most important is the diaphragm, a fleshy and tendinous partition, ly- 

 ing horizontally between the chest and the abdomen, which it separates 

 from each other ; it is attached to the cartilages of the false ribs, and to 

 the lumbar vertebrae, and has three openings to transmit the oesophagus 

 and the vessels which pass from the abdomen to the chest, or from the 

 latter into the abdomen. 



In health the chest dilates only by the descent of the diaphragm. The 

 curved fibres of that muscle straightened in contraction, descend towards 

 the abdomen, and compress the viscera. The descent of the viscera 

 thrusts forward the interior parietes of that cavity, and these recede, 

 when on expiration taking place after inspiration, the diaphragm now re- 

 laxed, rises, pressed upwards by the abdominal viscera, compressed them- 

 selves by the large muscles of the abdomen. But when it is necessary to 

 take into the chest a great quantity of air, it is not sufficient that it should 

 be enlarged merely by the descent of the diaphragm it is required be- 

 sides, that its dimensions should be increased in every direction. The inter- 

 costal muscles then contract, and tend to bring together the ribs between 

 which they are situated. The intercostal spaces, however become wider, 

 especially at their anterior part, for, whenever lines falling pbliquelyona 

 vertical line, change their direction, approaching to a right angle, the in- 

 termediate spaces receive the greater increase, as the lines, more oblique 

 at first, become at last more nearly horizontal. Besides, as the ribs are 

 curved in the course of their length, in two directions, and both the di- 

 rection of their faces, and edgewise, the convexity of the first curvature is 

 outwards, the ribs recede to a distance from the axis of the chest, whose 

 cavity is enlarged transversely, while the second curvature (in the direc- 

 tion of their edge) being increased by a real twisting of these bones, and 

 which reaches to the cartilaginous parts, the sternum is heaved forward 

 and upward, so that the posterior extremity of the ribs is removed from 

 their sternal end. But as the ribs are not all equally moveable, as the 

 first is almost always invariably fixed, and as the others are moveable in 

 proportion to their length, the sternum is tilted in such a way that the 

 lowermost extremity is thrust forward*. The diameter of the chest from 

 the fore to the back part increases, therefore, as well as the transverse 

 diameter. This increase of dimensions has been estimated at two 

 inches, to each of these diameters; the dimensions of the vertical dia- 

 meter, which are regulated by the depression of the diaphragm, are much 

 greater. 



LXXII. Professor Sabatier, in his memoir on the motion of the ribs, 

 and on the action of the intercostal muscles, maintains that during the 



* The opinion originally advanced by Haller, of the immobility of the first rib, has 

 been very properly contested by Magendie, (Precis. Elem.) In certain states of dis- 

 ease where FORCED inspirations are frequently made, we can readily satisfy ourselves 

 that the upper part of the chest is very freely dilated by the action of the scaleni ser- 

 rati, &c. In asthma we have repeatedly had occasion to remark this mobility. 

 Godman. 



