1030 



THORAX. 



ence to the spine. But the relation of these 

 torsions to the spine are different : the pos- 

 terior torsion is relative to the spine late- 

 rally, while the anterior torsion relates to 

 the spine more in the antero-posterior direc- 

 tion : they both conspire to increase the obli- 

 quity of the rib in one given direction, from 

 above downwards. 



The torsion of the 1st rib, we have noticed, 

 is directed in a contrary direction to that of 

 other ribs ; and we have observed that the 

 presence of torsion in general favours 

 muscular traction : but the 1st rib is an 

 exception to this; here the torsion exists 

 only between its two chief articulating pro- 

 cesses, the head and the tubercle : in the 

 other ribs the torsion is between the tu- 

 bercle and the body of the bone. The pos- 

 terior torsion of the 1st rib appears to be 

 merely destined to afford the head a more 

 complete attachment to the body of the one 

 vertebra (the 1st dorsal) to which that rib 

 is fixed. A posterior torsion, in this short 

 rib, is not needed for muscular traction, be- 

 cause here the scaleni are placed in the most 

 favourable position nearly at an angle of 

 90 with reference to the body of the bone 

 in question, while their other insertion into 

 the cervical vertebrae facilitates the most ex- 

 tensive and favourable means for its mobility, 

 independently of any favouring twist in the 

 rib for that purpose. 



(4<) Surfaces (special differences). The 

 thorax being conical, or somewhat barrel- 

 shaped, it follows that the surfaces of 

 the ribs, like the hoops of a very spherical 

 barrel, must gradually change their direc- 

 tion ; thus the surfaces of the 1st rib are 

 nearly superior and inferior, this bone forming 

 the lid to the thorax, while the surfaces of the 

 6th or 7th rib are external and internal, and as 

 we proceed downwards to the 10th, llth, and 

 12th ribs, the surfaces are again slightly 

 tending towards a superior and inferior po- 

 sition, so that the internal surfaces of the 1st 

 and 12th ribs are directed somewhat towards 

 each other. The body of the rib, or that 

 part which covers the lung laterally, and the 

 anterior and posterior extremities, have also 

 their surfaces inclined in different directions. 

 Thus, take a perfect rib, say the 7th, laterally 

 to the thorax the two surfaces are internal 

 and external, while at the anterior end they 

 are directed external surface, forwards and 

 downwards ; internal surface, upwards and 

 backwards ; at the posterior end, external 

 surface, upwards and backwards ; internal 

 surface, downwards and forwards. This is 

 produced by their respective torsions. In 

 some of the lower animals the ribs overlap 

 each other like the tiles of a house,- this 

 sometimes threatens in man, particularly in 

 diseases of the spine (fig. 666.), when they 

 closely approach each other. 



lowed out for their cartilage. As the ribs 

 become more perfectly developed, for in- 

 stance, the 5th, 6th, 7th, and 8th, the an- 

 terior extremity is broader, but not more 

 deeply hollowed out than some of the other 

 ribs, which are less perfectly developed, as in 

 the 2nd and 3rd, or llth ribs. This extremity 

 is most pointed in the 12th rib. 



Fig. 666. 



Relation of the ribs to the spine in angular cttrvature. 



(b) Posterior extremity. The posterior ex- 

 tremity of the rib is more complicated, and 

 has certain named parts, as the head, neck, 

 tubercle, and angle, all of which become modi- 

 fied as we pass from above downwards. Their 

 differences may briefly be noticed. 



1st Of the head. On the head of the rib, 

 articulating with the vertebrae, a surface or 

 facet is formed. The 1st, the llth, and the 

 12th ribs articulate each with the body of one 

 vertebra, and therefore they have one arti- 

 culating surface. All the rest articulate each 

 with the bodies of two vertebrae, and they 

 consequently have two such articulating sur- 

 faces as already described. The head of the 

 1st rib is relatively larger than that of the 

 others. For the most part, as the ribs in- 

 crease in size, the head likewise increases, 

 so that in the best developed rib the head and 

 its surfaces are most perfectly formed, dege- 

 nerating again to the 12th rib. 



2nd .Of the neck. The neck being that 

 part of the rib between the articulation of 

 the rib with the bodies of the vertebra, and 

 that with the transverse process, and these 

 points differing but little in their distance 

 from each other in the dorsal vertebrae, it 



(5) Specific differences of the extremities of follows that the absolute length of the neck 



the ribs. The greatest difference is in the of the different ribs is nearly the same. The 



posterior end of the rib. The anterior pre- necks of the ribs differ in tiieir thickness, ac- 



senting little difference. cordingly as their respective ribs increase or 



(a) Anterior extremity. These are all hoi- diminish in size ; therefore, in the middle 



