132 PROFESSOR STRUTHERS, 



lowest part. The sixth column of the Table shows the amount of 

 the slope from the top of the tubercle to the most projecting part of 

 the end. 



The articular surface or cartilage-covered area^ begins at the top, 

 or, it may be, a little external to the top of the tubercle. On the 

 1st rib it occupies the whole end. On the 2nd and 3rd it occupies 

 the inward slope of the tubercle, or what might also be called the 

 broad part of the beak, for 4 to 4| inches ; leaving a nan-ow beak 

 proper, 5 inches long on the 2nd rib, 4 inches long on the 3rd rib. 

 On the 4th rib the articular area extends over the whole sloping end, 

 and on the ribs behind the 4th goes also a little below at the 

 rounded-off part. 



Adaptation of the Ribs to the Fossce on the Transverse Processes. — 

 It is not easy at first to see how the two somewhat narrow surfaces 

 are adapted, the rib-ends being mainly vertical, the fossfe mainly 

 antero-posterior in direction. The measurements of the end of the 

 7th rib are — height, 3| inches ; breadth, upper part 1 inch, at 

 middle 1^, lower part 1|. Those of the fossa on the 7th transverse 

 I^rocess are — antero-posteriorly 4 inches, vertically 3, depth | inch. 

 If a middle rib be so placed that the long axis of the two surfaces 

 shall correspond, the rib will be nearly horizontal. The mode of 

 articulation, by fibrous cushion, may render exact adaptation of sur- 

 faces less necessary than in the case of diarthrodial joints, but the 

 adaptation becomes evident on close examination. If the 7tli rib is 

 placed naturally, the lower end carried back to about opposite the 3rd 

 transverse process behind its own, the articular end is seen to be 

 directed downwards and backwards at an angle of about 25°, and to 

 fit against the anterior i or |- of the costal fossa. That is the part on 

 which the fossa is buttressed by the thick ridge bounding it in front, 

 thus offering resistance to the rib, and only the anterior | or -| of the 

 fossa have been covered with cartilage. The cartilage has been con- 

 tinued down upon it from the end of the transverse process. When 

 the 12th is reached the Avhole area is cartilaginous, being placed on 

 the end of the transverse process. But the rib is longer, vertically, 

 than the fossa. Part, about ^ or ^, projects above the fossa, and, 

 corresponding to this, is the concavity on the extreme edge of the 

 transvei'se process, above the anterior J of the costal fossa. The 

 rounded-off part below may project under the fossa. 



Looking now to the exact form of the articular end of the rib, the 

 terminal torsion is seen to give it the downward and backward direc- 



^ The parts which have been covered by cartilage are easily recognised by their 

 roughness and perforations. The costo-transverse articulation in finners appears 

 to be, not by regularly formed diarthrodial joints, but by fibrous cushion. 

 AVithin this I have found an irregular synovial cavity (this Jov,rna,l, 1872, 

 p. 48), but in B. rostrata I found no synovial cavity at any of the costo-ti'ans- 

 verse articulations. The cartilage on these tubercles and ends may be only 

 the growing cartilage of the end, but it may be regarded as also articular in 

 function. 



