THE STERNUM. aL 
first cervical, lower sacral, and all the coccygeal vertebree, and display characteristic differences 
in the cervical, thoracic, and lumbar regions, which have been already described. The articular 
processes (zygapophyses) are secondary developments, and display great diversity of form, deter- 
mined by their functional requirements. It is noteworthy that, in the case of the upper two 
cervical vertebre, they are so disposed as to lie in front of the foramina of exit of the upper 
two spinal nerves, and by this arrangement the weight of the head is transmitted to the solid 
column formed by the vertebral bodies, and not on to the series of neural arches. It is in 
regard to the homology of the transverse processes, so called, that most difficulty arises. In the 
thoracic region they can best be studied in their simplest form; here the ribs—which Gegenbauer 
regards as a differentiation from the inferior or heemal arches, in opposition to the view advanced 
by others that they are a secondary development from the fibrous intermuscular septa—articulate 
with the transverse processes and bodies of the thoracic vertebre through the agency of the 
tubercular (diapophysis) and capitular (parapophysis) processes respectively, the latter being 
placed, strictly speaking, on the neural arch behind the line of the neuro-central synchondrosis. 
An interval is thus left between the neck of the rib and the front of the transverse process ; 
this forms an arterial passage which corresponds to the vertebrarterial canal in the transverse 
processes of the cervical vertebra, the anterior bar of which is homologous with the head and 
tubercle of the thoracic rib, whilst the posterior part lies in series with the thoracic transverse 
process. These homologies are further emphasised by the fact that in the case of the seventh 
cervical vertebra the anterior limb of the so-called transverse process is developed from an 
independent ossific centre, which occasionally persists in an independent form as a cervical rib. 
In the lumbar region the external or transverse process is serially homologous with the 
thoracic ribs, though here, owing to the coalescence of the contiguous parts, there is no arterial 
channel between the rib element and the true transverse process, which is represented by the 
accessory processes (anapophysis), placed posteriorly at the root of the so-called transverse 
process of human anatomy. Support is given to this view by the presence of a distinct costal 
element in connexion with the transverse process of the first lumbar vertebra, which accounts 
for the occasional formation of a supernumerary rib in this region. 
In the sacrum the lateral mass of the bone is made up of combined transverse and costal 
elements, with no intervening arterial channel. In the case of the upper three sacral segments 
the costal elements are largely developed, assist in supporting the ilia, and are called the true 
sacral vertebree ; whilst the lower sacral segments, which are not in contact with the ilia, are 
referred to as the pseudo-sacral vertebree. 
The anterior arch of the atlas vertebra is, according to Froriep, developed from a hypochordal 
strip of cartilage (hypochordal spange). 
THE STERNUM. 
The sternum or breast bone occupies the middle of the upper part of the 
thoracic wall anteriorly. It is connected laterally with the cartilages of the first 
seven ribs, and supports, superiorly, the clavicles. It consists of three parts, named 
respectively the manubrium or presternum; the body (corpus sterni), gladiolus or 
mesosternum ; and the ensiform or xiphoid cartilage (processus xiphoideus) or the 
metasternum. Of these the body is formed by the fusion in early life of four 
segments or sternebre. 
The manubrium, usually separate throughout life from the rest of the bone, 
though occasionally fused with it, is of a flattened triangular form. The anterior 
surface, slightly saddle-shaped, affords attachment to the fibres of the pectoralis 
major and sterno-mastoid muscles. _ It is bounded above by a thick border, the lateral 
parts of which are hollowed out obliquely to form the facets (incisure claviculares) 
for the sternal ends of the clavicles; around the facets, which have an upward, 
outward, and slightly backward direction, the bone is faintly lpped. In the 
‘interval between these two facets there is a sheht notch (incisura jugularis) which 
forms the floor of the characteristic hollow seen at the root of the neck anteriorly 
—the suprasternal notch, or pit of the neck. The lateral borders are excavated 
immediately below the clavicular facets for the reception of the cartilages of the 
first ribs. Below this, the margin of the bone slopes inwards, and is sharp, except 
inferiorly, where it presents a facet which supports a part of the second costal 
cartilage. Around this the bone is usually lipped anteriorly. The upper angles 
correspond to the ridge separating the clavicular facets from the first costal facets ; 
whilst the lower angle, which may be regarded as cut across transversely, forms 
the surface which is united by cartilage to the body of the sternum. The anterior 
edge of this surface is usually prominent. The posterior aspect of the manubrium 
is smoother than the anterior, is pierced by numerous foramina, and is slightly 
| concave from side to side and above downwards. Here are attached some of the 
_ fibres of the sterno-hyoid and sterno-thyroid muscles. 
