54 



Anatomy of Skeleton 



there is no true angle in the form of a bend of the shaft, yet there is frequently a 

 secondary marking in series with that on the eleventh. 



There is another thing to be noticed in connection with the angle : the pleural 

 surface of the rib at this part of the bone shows a tendency to narrow somewhat in 

 the upper ribs, and this reaches such a degree in the second that the pleural surface 

 for a little distance is nearly linear on this bone, widening on the neck and on the under 

 surface of the shaft. The meaning is not clear, but the result appears to be attained 

 by the encroachment of the internal intercostal above, and this muscle with the 

 subcostal groove below, and it is one of the minor points which, taken together, can 

 be used to distinguish upper from lower ribs. In connection with the hinder parts 

 of the ribs it remains to point out that the upward curve formed by these with their 

 shafts is most marked in the seventh rib and decreases from this. This is best shown 

 by putting the bones in a row, resting each bone on the lower border of the shaft as 

 far as possible, when the vertebral ends will be seen to be raised from the table and 



try Inferco&l ' mi/sc/es 



FIG. 44. Lower surface of first rib. 



a line joining them forms a curve with its highest point made by the seventh head 

 and the lowest by the second and tenth. 



There is not much to be added to what has already been said about the shafts 

 beyond the angles (pp. 48 50). 



There is a tendency to the formation of a supra-costal groove between the attach- 

 ments of the intercostal muscles on the upper ribs. The subcostal groove is perhaps 

 better marked in the lower ribs except 10, n, and 12 than in the higher : the 

 floor of the groove is pierced by several small foramina directed towards the vertebral 

 end of the bone. The lower margin of the shaft is sometimes drawn down to form a 

 prominent flange, at and outside the angle, especially in the eighth and ninth ribs, a 

 condition well marked in the fetus : it is interesting to note that downgrowths from 

 the lower margins of the middle ribs, forming uncinate processes, are found among the 

 Sauropsida, in crocodiles, some lizards, and many birds. 



The general relations of the upper surface of the first rib have been already pointed 

 out (p. 50). The extent of insertion of Scalenus medius is shown in Fig. 43. There 

 is no distinct Levator for this rib, the muscle being continuous with the mass of Scalene 

 fibres, but the area that receives what are apparently corresponding fibres is indicated 

 in the figure : it is seen to extend in beyond the level of the tubercle. 



