SERIAL HOMOLOGY OF THE VERTEBRA. 23 



costal facet ; cc, costo-central facet : in 6, v, vascular interval ; ct, costo-transverse, and cc, costo-central 

 articulations ; co, first rib : in 7, in, mamillary, and a, accessory tubercles ; t, transverse process : in 8, 

 CO, co', supernumerary ribs: in 10, I, lateral mass: in 11, I', place of the lateral mass, remaining 

 undeveloped in tins instance : in 12, tl, the transverse process and lateral mass which unite with the 

 corresponding parts above ; f, f, anterior and posterior sacral foramina thus formed : in 13, c, the 

 centrum, which alone remains in the lower coccygeal vertebrae. 



4. It is in the comparison of the parts known in human anatomy under the general name 

 of transverse processes that the main difficulty of establishing homologies exists. In the 

 cervical vertebras the processes so called are pierced by a vertebrarterial foramen, and 

 most of them have two tubercles. Those of the dorsal vertebras are for the most part simple, 

 but articulate with the tubercles of the ribs, whence they are known in comparative anatomy 

 as the tubercular processes (diapophyses). At the place of articulation of the head of the rib 

 with the vertebra there is in some animals a projection, called the capitular process 

 (parapophysis) ; this is represented in man only by the articular facet on the body, which is 

 separated however from the proper central ossification by the neuro-central synchondrosis. 

 It is generally admitted that the part of the cervical transverse process in front of the verte- 

 brarterial foramen corresponds to the first part of a rib, as is illustrated by the separate ossification 

 of that piece of bone in the seventh cervical vertebra in man. and by the occasional occurrence 

 of a more fully developed cervical rib in that situation (fig. 23, 4). 



In the lumbar vertebras the transverse processes are elongated laterally, and at their root 

 two other processes become apparent, viz., the mamillary or mctapopliysis directed preaxially, 

 and the accessory or anapophysis, directed postaxially. Several circumstances in the anatomy 

 of the bones and muscles 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 

 capitular and tubercular processes, and the part lying between them, as to leave no arterial 

 passage. This view receives confirmation from the presence of a costal element in connection 

 with the transverse process of the first lumbar vertebra in the foetus, and its occasional 

 development to form a supernumerary rib (p. 13 ; fig. 23, 8). 



In the sacral part of the column still greater departure from the form of the transverse 

 process of the upper vertebras takes place by the large development and ossific union of the 

 lateral parts. Throughout the whole five vertebras recognised as sacral in man, this may be 

 looked upon as occurring to some extent in portions of the bones which are serially 

 homologous with the combined capitular and tubercular processes ; but in the upper sacral 

 vertebras, another element is interposed between the transverse process and the iliac surface 

 of articulation, constituting the greater port'on of the lateral mass of the sacrum. This 

 element is generally regarded as serially homologous with part of a rib, and those vertebras 

 which are thus connected to the ilium by means of a costal element are distinguished as true 

 sacral, the remaining anchylosed vertebras being called pseudo-sacral. 



With the exception of the anterior arch of the atlas already referred to, there are no parts 

 developed in the huma,n skeleton corresponding to the hypapopkyses which occur in connec- 

 tion with the vertebral column of animals, such as the ''chevron" bones below the caudal 

 vertebras of cetacea, and the hasmal arches enclosing the main artery of fishes. 



II. THE THORAX. 



The skeleton of the thorax consists of the dorsal vertebrae 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 with the rest of the trunk by the cartilages of the first seven ribs on 

 each side ; and at the upper end it gives attachment to the clavicles. It consists 

 originally of six segments. The first of these usually remains separate from the 

 rest, and is called the manubrium : the succeeding four are united into one in the 

 adult, and form the body : the sixth generally remains cartilaginous for some years 

 after birth, and often partially so even to advanced age, constituting the ensiform 

 process ; in middle life it is most frequently united by bone to the body. 



The sternum is flattened from before backwards, and presents a slight longitudinal 

 curve with the convexity in front. It is of unequal width, being broad at the upper 

 part of the manubrium, considerably narrower at the lower end of that portion and 



