THE CRANIUM, 117 



Similar changes occur in the portion of the matrix which surrounds the spinal 

 cord. Opposite each vertebral body chondrification takes place, producing the 

 cartilaginous vertebral arches, the intervening tissue becoming transformed into 

 the ligaments which extend between the arches, chiefly the interspinous liga- 

 ments and the ligamenta subflava. Below each subflavan ligament an openino 1 is 

 left, through which the spinal nerves make their exit from the spinal canal, the 

 nerves, like the provertebne, alternating in position with the vertebral centra. 

 (7) The notochord contained in the centre of this chondrifying mass does not con- 

 tinue to grow, but becomes in the human subject relatively smaller, so as, at last, 

 to form a mere slender thread, except opposite the secondary segmentations ; 

 that is to say, corresponding to the intervals between the bodies of the perma- 

 nent vertebra?. Here it presents thickenings and forms an irregular network, 

 the remains of which are to be found at all periods of life in the central pulp of 

 the intervertebral disks. 



Development of the Ribs and Sternum. The ribs are formed by extensions of 

 the blastema of the vertebrae in the mesoblastic layer of the blastodermic mem- 

 brane. These speedily undergo chondrification, and appear as cartilaginous bars, 

 and become separated from the vertebrae at their posterior extremities. At their 

 anterior ends the costal bars, which are to form the nine upper ribs, turn upward 

 and fuse together so as to form a cartilaginous strip bounding a central median 

 fissure. The strips on either side then join in the middle line from before back- 

 ward, and so give rise to a longitudinal piece of cartilage, which represents the 

 manubrium and gladiolus of the sternum. In the process of development the 

 sternal attachment of the eighth rib disappears, while that of the ninth sub- 

 divides, one portion remaining attached to the inferior extremity of the cartilag- 

 inous sternum and becoming developed into the ensiform cartilage ; the other por- 

 tion receding from the sternum and becoming attached to the rib above. 



The further development of the vertebrae, ribs, and sternum, and the ossifica- 

 tion of their cartilaginous structure, are described in the body of the work. 



Development of the Cranium in general, and the Face. We have seen that the 

 first trace of the embryo consists in the formation of a longitudinal fold of the 

 epiblast on either side of a median groove, and that 

 these folds or ridges grow backward and meet in the 

 median line, thus forming a canal. This canal, at the 

 cephalic extremity of the embryo, is dilated and forms 

 a bulbous enlargement. The bulbous enlargement 

 soon expands into three vesicular dilatations, which 

 are known as the three primary cerebral vesicles, from 

 which all the different parts of the encephalon are 

 presently to be developed. The most anterior of the 



three forms the thalamencephalon, whilst a hollow 



projection from it forms the cerebral hemispheres ; the jj 



middle one forms the mesencephalon ; the posterior FlG - 90 - Longitudinal section 



i ii iii i t i mi f tne head of an embrvo four 



the metencepnalon and the myelencepnalon. The weeks old, seen from the inside 

 primary cerebral vesicles are at' this time, of course, LSSd SflAff^JSTZ 

 hollow, and their cavities freelv communicate with i^ ?rme dia ^' ^ bra L n- 5 - , Mi , d J? le 



11 . . _ Drain. t>. riinoer Drain. 7. Alter 



each other at the points of constriction. As the braj n- & Anterior portion of the 



i ,1 L i , -11 i temonum cerebelli. 9. Its lateral 



embrvo grows, the cerebral vesicles become twice bent portion intervening between NOS. 

 forward on their own axis (Figs. 90, 91, A and B). b^t^J^BZ^ CT ?S 

 The upper or posterior curvature is called the cerebral, audltor y vesicle. 

 the lower or anterior, the frontal protuberance. 



Thus, we have a triple cavity (see Fig. 91, A, where the three cavities are 

 marked c. me, and mo) lined by epiblast and covered by the same structure. 

 Between these two layers of epiblast. a layer of mesoblast, derived from the pro- 

 tovertebral plates of the trunk, is prolonged and spreads over the whole surface 

 of the cerebral vesicles. From these structures the cranium and its contents are 

 developed. The external layer of the epiblast forms the superficial epithelium 



