ANATOMY. 



223 



Ribs, twenty-four in number; the true ribs (No. 16. 

 fig. 1.; tbe false, 17. fig. 1.) those well known semi- 

 lunar bones are situated at each side of the chest, or 

 thorax. They are connected with the vertebrae of the 

 back, and, in connection with those vertebras, form 

 tbe posterior and lateral part of the chest. As 

 we wish to include in this section the osteology 

 or bony boundaries of that part of the trunk 

 called the chest, or thorax, we remark, in gene- 

 ral, that the chest resembles an arched bony cavity, 

 narrow above, broad below, flat anteriorly, or in 

 the front, hollow posteriorly, and convex laterally, 

 and is composed of the twelve dorsal vertebras al- 

 ready described, the sternum or breast bone, and 

 the twenty-four ribs just noticed. 



The Sternum, or Os pectoris, or breast bone, 

 and its situation, are well known; it is an oblong 

 flat bone, somewhat of the shape of a dagger, of a 

 very spongy texture, and mostly consists of two, 

 and sometimes of three, portions. A sharply point- 

 ed cartilage, situated at the pit of the stomach, is 

 attached to the inferior extremity of the breast 

 bone, and, from its supposed resemblance to a sword, 

 called the zyphoid, or sword-like cartilage, and 

 sometimes the ensiform cartilage. The bones then, 

 above named, viz. the dorsal vertebrae, the twenty- 

 four ribs, and the breast bone, constitute the thorax, 

 and the obvious uses of this bony casement, are to 

 defend those important organs, the heart and lungs, 

 and their more immediate appendages, the blood- 

 vessels and air-vessels ; and to give adhesion to the 

 mediastinum, that membranous partition which 

 divides the chest into two cavities, and also attach- 

 ment to the other muscles, especially those con- 

 cerned in respiration. 



In addition to what is stated above, respecting 

 the situation, number and designation of the ribs, 

 &c. we may here observe, that they extend oblique- 

 ly, from tbe dorsal vertebrae round towards the 

 sternum, to which they are connected by strong 

 cartilages. The seven ribs on each side, whose 

 cartilaginous extremities are affixed to the breast 

 bone, are those called the true ribs ; and the five on 

 each side whose extremities do not reach the 

 breast bone, are the spurious, false or floating ribs. 



The first rib is very short and much curved, 

 broader than the other ribs, and its external extre-i 

 mity is united to the breast bone by a short por- 

 tion of cartilage at a right angle. The second is 

 longer than the first but shorter than the rest. 

 The other ten differ chiefly in their length, which 

 gradually increases from the first or uppermost to 

 the seventh, or last true rib, from which they 

 gradually diminish to the twelfth. Their obli- 

 quity, in respect to the spine, increases as they 

 descend, as does the distance between the head and 

 angle of each rib from the first to the ninth. 

 The two lowest ribs differ from all the rest, hav- 

 ing no tubercle for the articulation or union with 

 the transverse processes, and are much shorter. 

 Although the views of the skeleton in the plate 

 give a pretty plain representation of the relative 

 situations and articulations, it may be more clearly 

 understood by stating that while the true ribs 

 articulate with the breast bone, the eighth rib, or 

 the first of the false. ribs, is attached by its carti- 

 lage to the lowei edge of the cartilage of the 

 seventh, or last of the true ribs, the ninth in the 

 same manner to the eighth, and the tenth' to the 

 ninth, the cartilage of each rib being shorter than 

 that of the rib above it, and the two last, as may 

 be seen in the plate, are not fixed at their anterior 



extremities, but hang loosely, being supported by 

 ligamentous fibres. 



The only other circumstances of particular in- 

 terest are, the longitudinal groove on the inferior 

 and interior surface of each rib, for the passage of 

 the intercostal artery, vein, and nerve, and the 

 kind of connection formed between the spine ribs 

 and sternum. The joints formed by the connection 

 of the ribs with the vertebrae and sternum are of 

 the ginglymus kind, allowing the alternate motion 

 of the ribs rising and falling, as we draw in and let 

 out our breath, while the great head of each rib is 

 hinged in the invertebral substance, touching two 

 vertebrae, and surrounded by a capsular ligament; 

 and tbe cartilages of the ribs which join the ster- 

 num are similarly connected by the same kind of 

 ligaments and ligamentous fibres which expand 

 over the breast bone. 



The ribs, at birth, differ very little in substance 

 and form from those of adults, and it need scarcely 

 be observed that they unite with the other parts 

 of the thorax, for the same purposes of giving form 

 to the chest, to act in respiration, to defend the 

 vital viscera, and give attachment to muscles. 



The bones of the thorax, like the others, are 

 subject to accident and disease, great portions of 

 the ribs being absorbed by the pressure of aneu- 

 risms, as stated in our paragraph on the vertebrae, 

 while caries sometimes attacks tbe sternum, and 

 the bone crumbles away insensibly. The sternum is 

 likewise liable to be operated upon by the trephine, 

 to afford exit to the matter of an abscess in the an- 

 terior space of the mediastinum. The xyphoid 

 cartilage attached to the inferior part of the sternum 

 occasionally takes on a disposition to curl outwards, 

 and produces considerable pain and uneasiness. 



"We next descend to the loins, which have been 

 already described, in the article Vertebree, in 

 the Encyclopedia. The Pelvic Region forms, as 

 it were, the basis of the trunk of the skeleton, and 

 has received its name from its resemblance to a 

 bason formerly used by barbers. The fore part 

 and sides of the pelvis, or bason, and lower part of 

 the sides of the abdomen, are constituted by the 

 two large, broad and irregularly shaped bones, 

 called ossa innominata, or hip bones, and the upper 

 edge is frequently called the hip. Each of these 

 bones is usually described as three bones, from its 

 having been composed of three distinct portions or 

 pieces, in the first period of life, and these portions 

 yet retain the same name, though yet united into one 

 solid broad bone, and are referred to by figures in 

 the plate. They are the ilium, (No. 23. fig. 1.) the 

 ischium, (25. fig. 1.) and the os pubis, (26. fig. 1.) 

 There is likewise an eminence called the cresta, or 

 spine of the ilium, (24. fig. 1.) and other eminences, 

 spines, and tuberosities on these bones. There is 

 one cavity, however, in each of the hip bones 

 which deserves particularnotice.viz. the acetabulwn, 

 which receives tbe head of the os femoris, or thigh 

 bone. The os sacrum, (10. fig. 2.) forms the back 

 part of the pelvis, and is situated at the bottom of 

 the spine, and of course a posterior part of the 

 pelvic region. It derives its name either from its 

 being offered in sacrifice by the ancients, or from 

 its supporting the organs of generation, which they 

 considered as sacred. It is of a triangular shape, 

 bent forwards, and by many anatomists considered 

 as a bone of the spine, and from its irregularities, 

 processes, and foramina, it seems to have some 

 claims to be considered as such. The only other 

 bone of the pelvis is the os coccygis, (11. fig. 2.) 



