THORAX. 



1037 



same characters in both sexes. There is little 

 difference in the height of the two apices. The 

 elevation of the liver on the right side does 

 not necessarily cause the right apex to be 

 the higher. The right lung is more shallow 

 than the left; but this is not because it is 

 " pushed up," but because, in order to ac- 

 commodate the liver, there is less lung- 

 substance on the right side. If the mean of a 

 series of observations represents the right 

 side of the thoracic cavity as equal to 151, the 

 left may be given as 182. The same may be 

 said of both sexes. 



(b) Posterior region. If we examine this 

 region upon a cast of the thoracic cavity, we 

 observe in the middle line a deep fossa, formed 

 by the projection, to the very centre of the 

 thoracic cavity, of the bodies of the dorsal 

 vertebrae. 



This fossa is wider towards the base of the 

 thorax, as may be seen by comparing the dif- 

 ferent sections of the cast (compare the 

 notches, 10, 11, and 12, with those of 3,4, and 

 5 \nfig. 668.). 



Fig. 668. ' 



Sections of a cast of the thorax showing the space 

 encompassed by each pair of ribs respectively. 



The bodies of the thoracic vertebra, form 

 almost a complete septum ; they are certainly 

 a central column of support for the whole 

 trunk of the body, bearing an equal distri- 

 bution of the superincumbent weight in all 



directions. There is no part of the skeleton 

 which more strikingly demonstrates that man 

 was destined for the erect posture, than this 

 central position, together with the increasing 

 dimensions from above downwards, of the 

 vertebral column. These conditions exist only 

 in the human subject. The groove in the cast 

 formed by the dorsal vertebra; is directed 

 upwards and forwards, so that at the apex of 

 the thoracic cavity it completely divides the 

 lungs, producing two little cones laterally for 

 the lungs, forming the right and left apex. 



At the base of the thorax this perpendicular 

 column again completely divides the lower 

 lobes of the two lungs. The shape of the 

 posterior part of the cast is that of a curve 

 directed upwards, and sharply forwards, near 

 the apex (fig. 667.). This curve near the apex 

 is, like the form of the apices, ,very various. 

 An inflated lung assumes the same shape 

 as this cast, giving even the marks of the ribs. 

 The student, in order to have a correct idea of 

 the lungs, should remove them from the body 

 with the heart attached ; then inflate them to 

 their utmost, when their shape, their lobes, 

 and relative mass before and behind, are clearly 

 seen. 



If now we inspect the cavity of the thorax 

 itself, we find that the bodies of the vertebrae 

 by their projection as above described, divide 

 the posterior portion of the chest into 

 two vast lateral grooves, which lodge the 

 posterior portions of the lungs; these two 

 grooves, partaking of the form of the thorax, 

 are consequently conical in their configuration. 

 They lodge fully one half of the entire lungs. 

 This is worthy of remembering in reference to 

 diagnosis, particularly when pneumonia is sus- 

 pected; in such cases auscultation of the dorsal 

 region demands as much attention as that 

 of the anterior region. Though less vulnerable 

 to phthisis pulmonalis, yet it may lodge disease 

 in parts comparatively remote, 'and where in- 

 flammation may insidiously gain serious ground 

 upon the patient, particularly in children. 



(c) Lateral region. This precisely corre- 

 sponds with the external lateral view of the 

 thorax (compare fg. 667. with Jig. 680.); it 

 presents indentation of the ribs, resembling a 

 diagonal, curved, grate. 



(d) The base of the thoracic cavity has been 

 described above. 



Conformation as affected by age and sex. 

 Age alters the conformation of the chest. 

 In the earlier periods of existence the thorax 

 is the smallest of the three great cavities, 

 probably from the inactivity of the lungs. 

 In the foetal thorax the antero-posterior dia- 

 meter exceeds the transverse diameter, the 

 sternum projecting forwards, and the heart and 

 thymus gland filling up the middle of the cavity. 

 The ribs in foetal life are less curved, and conse- 

 quently those deep grooves, seen in the interior 

 of the chest on each side the spine, formed 

 by the angle of the ribs, so conspicuous in 

 after life, are almost wanting ; the vertical 

 depth also is much less at this period, because 

 the lungs are unexpanded and unemployed, 

 while the abdominal viscera, particularly the 



