56 A MANUAL OF ANATOMY 



The skeletogenous (mesencliymatous) tissue between the cartilaginous 

 bodies of successive vertebrae forms the intervertebral discs, and the skeletog- 

 enous tissue between the successive cartilaginous neural arches forms the 

 ligamenta subflava. 



Osseous Stage. — For the stage of ossification see Ossification Of the Verte- 

 brae. 



Development of the Ribs. 



The ribs are developed from the mesenchyme of the costal processes of the 

 lateral extremities of the vertebral bows in the thoracic region, which pro- 

 cesses invade the intersegmental septa or ligamenta intermuscularia between 

 contiguous muscle-segments. They are at first curved membranous rods, 

 which subsequently undergo chondrification and form the cartilaginous ribs, 

 these in turn undergoing ossification, and giving rise to the osseous ribs. 

 They are intersegmental in position, and are confined to the thoracic region 

 of the trunk. In the other regions their development is arrested, but rudi- 

 ments of them persist. Thus, as stated, in the cervical region they are repre- 

 sented by the costal processes of cervical vertebraj; in the lumbar region, by 

 the transverse processes (costal elements) of lumbar vertebrae; and in the 

 sacral region, by the costal elements of the upper three sacral vertebrae. 



Development of the Sternum. 



The ventral extremities of the upper nine cartilaginous ribs, on either side, 

 become expanded, and these portions unite. In this manner an elongated 

 strip of cartilage is formed on either side, each of which bears nine cartila- 

 ginous ribs. These strips are known as the hemisternal cartilages, and by 

 their union a single sternal cartilage is formed. This cartilaginous sternum 

 undergoes ossification in the manner described in connection with the sternum 

 as a bone. " 



The eighth and ninth cartilaginous ribs, on either side, lose their connec- 

 tion with the hemisternal cartilages, and the portions of these hemisternal 

 cartilages, which are contributed by the expanded ventral or anterior ends 

 of the eighth and ninth cartilaginous ribs, give rise, by their union, to the 

 cartilaginous metasternum (ensiform process). 



In exceptional cases ossification may take place in each hemisternal 

 cartilage independently, and under these circumstances the hemisterna may 

 remain permanently separate, thus giving rise to the extremely rare con- 

 dition of sternal fissure. 



The two halves of the cartilaginous metasternum usually unite along their 

 entire extent. Their distal ends, however, may remain permanently separate, 

 and so give rise to a bifurcated metasternum. Again, the two halves may unite 

 in such a manner as to leave a permanent metasternal foramen. The presence 

 of a sternal foramen is explained under Varieties of the Sternum. 



n— THE BONES OF THE HEAD. 



The head or skull is supported on the upper end of the vertebral 

 column, and is divisible into the cranium and face. The cranium 

 or brain-case is composed of eight bones, namely, the occipital, 

 two parietals, frontal, two temporals, sphenoid, and ethmoid. 

 The face, which protects organs of special sense, such as the eyes, 

 the olfactory mucous membrane, and the tongue, is composed of 

 the following fourteen bones, the majority of which are arranged 

 in pairs : the two superior maxillae, two malars, two nasals, two 

 lachrymals, two inferior turbinates, two palate bones, the vomer, 

 and the inferior maxilla. All the bones of the skull, except the 

 inferior maxilla, are immovably united by sutures. 



