THORAX. 



1035 



outer garments even exhibit a correspondence 

 to this base of the thorax. 



II. Contents of the thorax. Not only does 

 the thorax contain organs proper to itself, 

 but it lodges and protects others passing 

 to the abdomen ; thus, the food we eat passes 

 through the thorax to enter the stomach, 

 while the eighth nerve both supplies the lungs 

 and then passes onwards to the stomach. 

 Like other cavities, the thorax is lined with a 

 serous membrane (pleura costalis], which like- 

 wise invests the lungs (pleura pufniona/is) 

 (See PLEUKA). The heart and its blood- 

 vessels, the trachea, and the lungs, almost 

 completely fill the thorax. The remaining 

 organs, viz., the origin of the sterno-hyoid 

 and -thyroid muscles, remains of the thymus 

 gland, lymphatic glands, mammary vessels, 

 triangulares sterni muscles, the phrenic, the 

 eighth pair, and the splanchnic nerves, the 

 oesophagus, thoracic duct, vena azygos, and 

 venae cavae, occupy very little space. 



Thus the thorax is completely filled. The 

 older anatomists, however, even as late asBenj. 

 lloadley's time, considered that there was a 

 considerable space containing air, between the 

 lungs and the walls of the thorax. Now, we 

 know that any considerable accumulation of 

 air in this situation would be destructive to 

 the respiratory function. 



HI. Shape of the thorax. The shape of the 

 thorax is subject to many varieties which 

 may be conveniently studied under the fol- 

 lowing headings : 



External thorax. The thorax in the 

 perfect subject is somewhat conical, broader 

 above than it is below ; but when deprived 

 of the upper extremities and their appen- 

 dages, the contrary obtains, for it is nar- 

 rower above than it is below (see Jigs. 662. 

 and 630. et seq.). Therefore the notion we 

 commonly connect with broad shoulders or 

 broad back, has but a feeble relation with 

 the absolute breadth of the thorax. In 

 fact the diameter of the neck corresponds more 

 correctly with the diameter of the upper 

 part or true apex of the thorax. Freeman the 

 American giant measured 26 inches from tip to 

 tip of his shoulders, while the diameter at the 

 lower part of his neck, in the same direction, 

 was only 6 inches. In women the mammae 

 materially add to the dimensions of the chest 

 towards the apex. We have found the 

 measure of a female round the thorax over 

 the mamma; 45^ inches, while the root of 

 the neck was 15 inches. Allowing two inches 

 for muscle, &c., the true thoracic apex would 

 not exceed 13 inches, whilst 45^ inches was 

 the apparent circumference of the thorax. 

 The true apex of the thorax is loaded with 

 large muscles, sometimes a vast quantity 

 of fat, the upper extremities, and in the 

 female with large maninuu besides. The true 

 base of the chest is comparatively little covered, 

 particularly below the axilla, where theserrati 

 are seen. This is the weakest and thinnest 

 part of the thorax, but it is well protected by 

 the arms. 



(a) The anterior or slcrnal region of the tho- 



rax. In a well-developed thorax the sternum 

 appears to lie in a fossa formed by the pecto- 

 rales and mamma;. This region assumes more 

 or less of the perpendicular, according to the 

 carriage of the person ; a perpendicular drawn 

 to the external plane of the sternum would, 

 however, be directed upwards. Laterally to 

 this, the costal cartilages articulate with the 

 sternum ; and, still more laterally, the ribs and 

 their cartilages unite, forming an oblique ridge 

 from above downwards. (See Jigs. 080. and 

 681. the line on each side of the sternum.) 

 Still more externally, and parallel to this, 

 may be noticed the projections formed by the 

 anterior angles of the ribs, which bound the 

 anterior region. 



(b) The posterior or vertebral region. In the 

 well-developed thorax the spines of the dorsal 

 vertebrae lie in a deep groove formed by the 

 great mass of the erector spins on either 

 side. These masses extend outwards to about 

 the posterior angles of the ribs, which are 

 nearly in a plane with the spinous processes 

 of the vertebra 1 , and may be easily distin- 

 guished, bounding the posterior region. 



(c) The lateral or costal region is composed 

 of the bodies of the ribs and the intercostal 

 spaces. Much difference has prevailed 

 amongst writers upon various points con- 

 nected with the ribs, and the spaces be- 

 tween the ribs. Probably this may have 

 arisen from their having made their observa- 

 tions upon the dead subject, wherein the 

 ribs have collapsed to their minimum, or the 

 thorax of a thin subject with the ribs expanded 

 by, perhaps, 200 cubic inches of air, or the ribs 

 when artificially articulated in the clean skele- 

 ton, wherein they are generally arranged too 

 wide apart and too horizontal. 



In the collapsed state of the ribs, i. e. after 

 the most ample expiration by death, we no- 

 tice 



1st, That the intercostal spaces are not of a 

 uniform width from the vertebra; to the ster- 

 num. They are most narrow behind, and be- 

 come wider as they approach the sternum 

 (fig. 680.). The 1st, 2d, and 3rd spaces upon 

 the whole are broader than the five next in- 

 ferior, and the 10th and llth spaces are the 

 widest (fig. 682.). 



2dly, Their length differs with the length 

 of the ribs ; thus the two first and two last 

 spaces are the shortest. 



3dly, The position of the intercostal spaces 

 is very oblique (fig. 684.), roughly speaking, 

 about mid-way between the horizontal and 

 the perpendicular. 



4thly, They are moveable, and in the dif- 

 ferent stages of respiration they can assume 

 different positions and different perpendicular 

 widths. 



It has been found necessary for the con- 

 venience of auscultation, to subdivide the 

 thorax more minutely. The subdivision pro- 

 posed by Laennec has been generally adopted, 

 although some of the terms, like those of the 

 respiratory sounds, have been modified by 

 writers of later date. 



The anterior and lateral superior division, 



