166 FORMATION OF THE VISCERAL SKELETON OF THE HEAD. 



within the head, nor can any parts be distinguished which strictly represent vertebrae, 

 The cartilage of the basis cranii makes its appearance in the form of two longitudinal 

 bars lying on either side of the notochord (parachordal cartilages), and of two other 

 bars (trabeculcv cranii of Rathke) which embrace the pituitary body, and which 

 becoms united together in front and with the parachordals behind to form a con- 

 tinuous mass, which posteriorly completely invests the notochord (fig. 196). The 

 cartilaginous basis cranii may therefore be distinguished into the parachordal and 

 prechordal parts. Of these the first represents the basi-occipital and basi-sphenoid, 

 the second the presphenoid and ethmoid portions. From the basis cranii continuous 

 cartilaginous plates grow on either side over the cerebral vesicles to a greater or 

 less extent in different animals, least in mammals, where only the occipital region 

 becomes thus roofed in by cartilage. Anteriorly the united trabeculse cranii stretch 

 forwards into the fronto-nasal process, where they form the ethmoid cartilage and 

 nasal septum, besides enclosing the nasal pits externally (cartilaginous nasal capsule). 

 From the sides of the presphenoid portion the orbito-sphenoids (lesser wings), 

 containing the optic foramina, are developed, and from the sides of the basi-sphenoid, 



Fig. 197. VIEW PROM BELOW OF THE CARTILAGINOUS 



CRANIUM WITH ITS OSSIFIC CENTRES IN A HUMAN 



F<ETUS OF ABOUT FOUR MONTHS. (After Huxley.) 



The cartilage is clotted to distinguish it from the bone 

 which is shaded with lines. 



b.o, basi-occipital : a.o, lateral occipitals ; f.m, foramen 

 magnum ; o.c, o'.c', bony deposits in the periotic capsule ; 

 p.s, post-sphenoid ; pr.s, pre-sphenoid ; o.s, orbito-sphe- 

 noid ; s.n, septum nasi. 



the greater wings or alisphenoids. A cartilagi- 

 nous capsule, connected with the parachordal 

 portion of the basis cranii, invests the otic vesicle 

 (periotic capsule). Within this, bony centres are 

 eventually formed, which unite to form the 

 petro-mastoid. In the human embryo, chondri- 

 fication begins in the fourth or fifth week in 

 the basilar portion of the skull, and is nearly completed by the eighth 

 week. 



Formation of the visceral skeleton of the head : cartilaginous bars cf 

 the visceral arches. A cartilaginous bar extends from the periotic capsule and 

 basis cranii, within each of the first three visceral arches, and passes forwards to 

 meet its fellow in the middle line. The lar of the mandibular arch is known as 

 MeclceVs cartilage. It is visible in all sections of the foetal jaw up to the seventh 

 month. Its proximal end is attached at first to the basis cranii, afterwards to the 

 periotic capsule; its distal end joins that of its fellow in the middle line of the 

 lower jaw. Only near this conjoined part does Meckel's cartilage take part in the 

 formation of the lower jaw bone, the greater part of this bone "being developed by 

 ossification at several places in the connective tissue around the cartilage. In some 

 animals a short cartilaginous bar is formed in the maxillary process (palato-ptcrygoid 

 lar, fig. 198, A,ppy). Close to it the palatine and pterygoid bones are formed in 

 membrane, but the bar itself entirely disappears. The second or hyoid lar arises 

 from the skull close behind the attachment of Meckel's cartilage, and passes along 

 the second arch. It disappears in part, but in part is converted into the styloid 

 process, stylo-hyoid ligament, and lesser cornu of the hyoiol bone. The body of the 

 hyoid bone (basi-hyal) is an intermediate formation between the second and third 

 arches. The lar of the third arch is known as the thijro-hyoid. Its lower end forms 

 the greater cornu of the hyoid bone ; but its attachment to the skull early disap- 



