THORAX. 



1025 



of a pair of cervical or lumbar ribs : in this 

 case the supernumerary ribs are formed 

 from the anterior parts of the transverse 

 processes of either the seventh cervical or first 

 lumbar vertebra ; which affords a strong 

 proof of the analogy existing between a 

 transverse process and a rib. Sometimes 

 the usual number is diminished to 22 : this is 

 more rarely the case. When t^is occurs, we 

 sometimes find two adjacent ribs united 

 throughout their entire length. Sometimes 

 the first rib is in a rudimentary state, being 

 properly formed posteriorly, but having its an- 

 terior extremity lost among the muscles, or 

 united to the 2nd rib. Mr. Quain has lately 

 seen an instance in which this diminution of 

 the number of the ribs was accompanied with 

 the absence of a dorsal vertebra.* 



Classification of the ribs. The ribs are nu- 

 merically designated 1st, 2nd, 3rd, and so on, 

 counting from above downwards. In the 

 living or in the undissected subject it is easier 

 to count the ribs from below upwards. 



The seven superior ribs are united by 

 means of their own cartilaginous prolongations 

 (fig. 661. b, c, d, e,f,g, and h ; and fig. 662.) 



to the sternum, and are called true ribs or 

 sternal ribs, or vertebro-sternal ribs ; the re- 

 maining five are not so immediately prolonged 

 to the sternum, and are denominated false ribs, 

 or asternal ribs, or vertebral ribs. 



We think it would be more judicious to 

 classify them otherwise, and consider the five 

 superior ribs as sternal, true, or thoracic ribs ; 

 the five next inferior, as diaphragmatic ribs ; 

 the two last, being floating or false ribs. Be- 

 cause, the first five especially encompass the 

 cavity of the thorax ; the five next a portion 

 only of this space, together with a large por- 

 tion of the abdominal viscera ; and, lastly, be- 

 cause the two last do not touch the sternum 

 through the medium of any cartilage. The 

 transverse shade (fig. 682.) represents the arch 



* Elements of Anatomy by Mr. Quain and W. 

 Sharpey, M. D. London, 8vo. 1843, p. 105. 

 VOL. IV. 



of the diaphragm, or the floor of the thorax. 

 Every rib articulates with the dorsal vertebrae 



Fig. 662. 



Position of ilie ribs and spdM after deep expiration. 



the spine is their fixed point, or centre of 

 motion, the main pillar upon which they 

 act. The superior ten ribs articulate through 

 the medium of cartilages, the first seven 

 through the medium of their own, the next 

 three through that of those of their superior 

 neighbours, (fig. 661.) with the sternum. 



The ribs have certain general characters 

 which distinguish them from all other bones ; 

 and likewise certain proper or special charac- 

 ters, by which one is known from another. 



I. Of the general characters of the ribs. The 

 ribs resemble flattened bony hoops, varying in 

 breadth from *4 to '7 of an inch, and from ! 

 to *4 of an inch in thickness; they once at- 

 tain a maximum, and twice a minimum length 

 (fig. 662.) 



They are of a very irr^ular shape. Their arch 

 or curve is neither uniform relatively to each 

 other, nor yet relatively to itself at different 

 parts of the bone ; moreover they are twisted 

 in different degrees upon themselves so that 

 the two extremities of the same bone point 

 in different directions, and cannot simulta- 

 neously touch an horizontal surface. 



Surfaces. These bones present two sur- 

 faces : an external or cutaneous surface, which 

 is convex and smooth ; and an internal or pul- 

 monary surface, which is concave and likewise 

 smooth. The anterior end is comparatively 

 flat, the posterior is more cylindrical and 

 truncated, and is rough, particularly at the 

 extremity. 



Borders. The ribs have two borders, 

 the one superior, and the other inferior. The 

 superior border is smooth and rounded, and 

 gives attachment to the intercostal muscles ; 



3u 



