1042 



THORAX. 



internal measurements (cubic or diameter) 

 clearly bear no relation to the height or weight 

 of the man, whilst vital capacity * does so in 

 an exact ratio. 



If we take a cast of the thoracic cavity and 

 view the base, it presents the shape of the 

 figures in diagram 'fig. 669. which gives a mean 

 measurement, in the males of forty-six, and 

 in females of thirty-five, superficial inches 

 In the diagram (reduced by scale), we affix 

 the height of each case, and the area in su- 

 perficial inches of such section. 



But the base of the thorax presents another 

 measurement, that of the absolute area of the 

 diaphragm. By the Jigs. 2. and 3., art. DIA- 

 PHRAGM, this muscle will be noticed as a large 

 muscle of a vaulted form. G.fig. 670. repre- 

 ents a section of a thorax, the measurement of 



fig. 670. 



101 outside. 



74 inside 



Dimensions of the diaphragm ^n three stages. 



C, ordinary stage. B, spread out A, completely 

 extended. D, relative difference between expiration 

 and inspiration. 



which is forty superficial inches ; the figure B, 

 next above, is the diaphragm of the same per- 

 son spread out, which is extended to 110 

 square inches, being nearly three times the 

 area of the former figure ; even in this con- 

 dition the centre is quite free, and not upon 

 the stretch, though the circumference is so. 

 The full measurement is nearly obtained, by 

 slitting up the sides, as shown in figure A. 

 and this condition gives it an increase of 

 twenty-two superficial inches, making alto- 

 gether 133 square inches ; but even in this con- 

 dition the entire arched muscle is not perfectly 

 spread out. This was the diaphragm of a man 

 five feet six inches high, with an exceedingly 



* By " vital capacity " is meant the measure of 

 the mobility of the chest, as more fully explained 

 hereafter. (P. 1056.) 



small chest, only twenty-nine inches in ex- 

 ternal circumference, and whose vital capacity 

 was 188 cubic inches. The section, therefore, 

 of the thorax to the area of the diaphragm is 

 as 40 to 133, or 1 to 3'3. This renders the 

 base of the thorax highly mobile. 



There is, perhaps invariably, a difference in 

 the dimensions of the two sides of the thorax, in 

 favour of the left side. The least difference 

 which we observed was one cubic inch, but 

 we think there must have been some error in 

 this observation. Passing this over, we may 

 say that the difference between the two sides 

 varies from 10 to 77 cubic inches, and that, in 

 all cases examined by us, the left side was 

 larger than the right. This difference, also, 

 we have found not to bear any relation to sex 

 or stature. One female of 5 feet 4 inches in 

 height had a difference of 77 cubic inches, 

 which exceeds by 10 inches any of the males 

 which we examined, up to 5 feet 10 inches 

 high. 



OF THE RESPIRATORY MUSCLES. There 

 are certain muscles especially destined to expand 

 and contract the thoracic cavity, and there are 

 others which act in different degrees as ac- 

 cessories, they may be classed as direct and 

 indirect respiratory muscles. 



The direct respiratory muscles are, inter- 

 costales externi and interni, levatores costarum, 

 infra costales, triangularis sterni, and the dia- 

 phragm. The indirect respiratory muscles are 

 all those which straighten the spine or aid in 

 fixing the whole body for the thoracic mus- 

 cles to act from as a fixed point, whilst 

 by their other attachment they elevate or 

 depress the ribs ; these are particularly the 

 muscles of the neck and upper extremities, 

 and those composing the walls of the abdo- 

 men. More indirectly still, the muscles of 

 the limbs assist in respiration ; in difficult 

 respiration, the patient seizes hold of any 

 fixed object, whilst he employs his whole 

 muscular force to assist in inspiration, or, as 

 Boerhaave has expressed it, " scarcely any 

 particle remains in the body which is not 

 more or less concerned in the business of 

 respiration."* The indirect respiratory mus- 

 cles, in fact, comprise nearly all the muscles 

 of the body; therefore we shall only no- 

 tice the direct respiratory muscles. The di- 

 aphragm has already been described (art. 

 DIAPHRAGM). 



I. The intercostal muscles are arranged as 

 two thin laminae between the ribs ; one lamella 

 is external to the other, hence they are named 

 external and internal. The fibres of each layer 

 are oblique in their direction in reference to 

 the ribs, and each layer has its fibres disposed 

 in a contrary direction to those of the other ; 

 thus they are said to decussate. The twelve 

 ribs form eleven intercostal spaces, conse- 

 quently there are eleven such decussating 

 lamellae on each side of the thorax, and twenty- 

 two in all. Their attachments are to the in- 

 ferior border of one rib, and to the superior 



* Praelect. ad Instit. 601. Morgagni. By Alex, 

 ander, vol. i. p. 357. 



