110 



OSTEOLOGY. 



Head Neck 



Tubercle 



Angle 



Articular part 

 of tubercle 



-Costal groove 



the seventh and eighth, which are usually the longest, are from two and 



a half to three times the length of the first and twelfth ribs respectively. The 



bodies are curved so as to adapt them to the form of the thoracic wall. More acute 



in the upper members of the series, where the shafts are shorter, the curve opens 



out in the middle and lower parts of the thorax, where the diameters of that cavity 



are greater. The curve, however, is not uniform. Including the whole length of 



the bone, it will be seen to be most accentuated towards the posterior part, where, 



in correspondence with the point at which the bend is most pronounced, there 



is a rough ridge placed obliquely across the outer surface of the shaft for the 



attachment of the 



slips of the ilio-cos- 



talis muscle ; this 



bend is the angulus 



costse. The distance 



between the angle 



and the tubercle is 



greatest on the eighth 



rib ; above that, the 



width between these two points gradually decreases until, in 



the case of the first rib, the two coincide. Below the level 



of the eighth rib the distance slightly diminishes in con- 



formity with the general narrowing of the thorax below 



that level. Towards the anterior extremity of the rib 



where the digitations of the serratus anterior and external 



oblique muscles are attached to its outer surface the curve 



of the body is somewhat more pronounced, and is referred 



to as the anterior angle. 



Combined with the curve, there is in many of the ribs 

 a twist. This may best be understood if the student will 

 take a strip of stiff paper and bend it in the form of the 

 curve of the rib. If, after he has done this, he pulls down 

 the anterior end and turns up the posterior end of the strip, 

 he will have imparted to the strip of paper a twist similar 

 to that met with in the rib. This appearance is best 

 seen in the middle members of the series, notably in the 

 seventh and eighth ribs, above and below which it gradually 

 becomes less marked. It is the occurrence of this twist 

 which prevents the extremities of the ribs, together with 

 the body, from resting on the same plane surface. To this 

 rule there are certain notable exceptions, viz., the first and 

 second, the twelfth, and not infrequently the eleventh. 



The body has two surfaces, internal and external, 

 and two borders, a superior and an inferior. The external 

 surface, which is smooth, conforms to the general vertical 

 convexity of the thorax, being directed upwards in the 

 first rib, upwards and outwards in the higher ribs, out- 

 wards in the middle series, and outwards and slightly down- 

 wards in the tenth, eleventh, and twelfth. The internal sur- 

 faces are arranged conversely and are covered with the parietal 

 pleura. Towards the sternal end of the middle ribs, at the 

 anterior angle where the downward twist is most marked, 

 there is often an oblique line across the outer surface. 

 The upper border of the body is thick and rounded be- 

 hind, thinner and sharper in front; to it are attached the 

 fibres of the internal and external intercostal muscles. 

 The lower border is grooved behind at the expense of the inner surface, and is 

 overhung laterally by a sharp margin. Anteriorly this sulcus costalis (costal groove) 

 fades away, and its lips coalesce to form a rounded edge. The intercostal vessels 

 and nerve are lodged in this groove, whilst its lips afford attachment to the 



Nutrient foramen. 



Shaft 



i 



For costal cartilage 



