BOXES OF THE THORAX. STERXCJM. 25 



accessory or aiinpopJii/sis. directed downwards or backwards. Several circumstances 

 in the ana,tomy both of the bones and muscles, as well as the form and laosition of 

 occasional or supernumerary lumbar ribs, seem to indicate that the outer part of 

 the lumbar transverse processes is serially' homologous with the first part of the 

 ribs, but so intimately combined with both diapophysis and parapophysis, and 

 the part lying between them, as to leave no arterial passage ; but a groove 

 on the upper side is regarded by some as an indication of the place of separation, 

 lu the sacral part of the column still greater departui'e from the form of 

 the transverse process of the upper vertebras takes place by the large develop- 

 ment and ossific union of the lateral parts. Tliroughout the whole five vertebraj 

 recogiiised as sacral in man. this may be looked vipon as occurring to some extent 

 in portions of the bones which are serially homologous with the combined 

 diapophysis and parapophysis ; but in two at least, and perhaps in three of the 

 upper sacral vertebras, another lateral element appears to be interposed between 

 that combined transverse process and the iliac surface of articulation, constitut- 

 ing the gi-eatcr portion of the large lateral mass of the sacrum. This element 

 is looked upon by some as serially homologous with part of a rib. (Retzius, 

 Miiller's " Archiv," 1849 : Miiller '• Vergleich. Anat. der Myxinoiden ; "" Owen. 

 '• On the Archetype. .Sec., of the Vert. Skeleton : " Aug. Miiller. Miiller s " Ai-chiv,"' 

 18.")8 ; Hiunphry. '• Treatise on the Human Skeleton ; " Goodsir, in '' Edin. Xew 

 Philos. Journ.."' 1857 ; Cleland. in " Xat. Hist. Review," 18G1 and 1SG3 ; Huxley, 

 " Hunterian Lect. in Brit. Med. Journ.." 18G9 ; St. George Mivart, in " Linn. 

 Trans.," 1870. and " Elementary Anatomy." 1873 ; Flower, " Osteology of the 

 Mammalia," 1870 : Gegenbaur, " Grundzuge der Yergleich. Anat.," 1870.) 



"With the exception of the anterior arch of the atlas akeady referred to, there 

 are no parts developed in the human skeleton which correspond with the ^ii/j/- 

 aiiojihyscs which occur in connection with the vertebral column of animals, such 

 as the " chevron " bones below the caudal vertebra3 of cetacea, and the haemal 

 arches enclosing the main arteiy of fishes. 



The similarity of some of the parts composing the skull to vertebrae, or the 

 serial homology of the bones of the head and vertebraj, supposed by some 

 anatomists to exist, will be referred to hereafter. 



IL— THE THORAX. 



The thoracic part of the skeleton cousists of the dorsal vcrtebrse 

 already described, the sternum, the ribs, and the costal cartilages. 



THE STERNUM OR BREAST-BONE. 



The sternum is situated in the median line at the fore part of the 

 thorax. It is connected witli the rest of the trunk by the cartilages of 

 the first seven pairs of ribs, and gives attachment to the clavicles. It 

 consists originally of six segments. The first of these usually remains 

 distinct up "to middle life, and is called the mamihrium or jn'esfcr/unn ; 

 the succeeding four are united into one in the adult, and form the bodt^ 

 or me so si em urn : the sixth generally remains cartiLaginous up to the 

 period of pubert}', and sometimes partially so even to advanced age, 

 con'&iiivi'dxig i\\Q ensiform process, or metasternum : in middle life it is 

 most frequently ossified and united by bone to the body. 



The sternum is flattened fi-om before backwards, and presents a slight 

 vertical curve with the convexity in fi'ont. It is of unequal vridth, 

 being broad at the upper part of the presternum, considerably narro\Yer 

 at the lower end of that portion and in the first segment of the meso- 

 sternum, somewhat wider near the lower end of that portion, and finally 

 compressed and narrowed near the junction with the metasternum. It 

 consists of light cancellated texture, with a thin covering of compact 

 bone. 



