DEVELOPMENT OF THE OSSEOUS SYSTEM. 985 



teriorly, the number of cranial nerves that correspond to spinal nerves, and the 

 number of visceral arches, concluded that there are at least nine cranial vertebrae 

 through which the chorda passes. To this vertebral portion of the head is sub- 

 joined a pre vertebral or an evertebral portion, comprising the ethmoid and anterior 

 orbital region. Froriep showed that the hypoglossal nerve represents the fusion 

 of several spinal nerves. In mammals the cranium up to the vagus results 

 from the fusion of four rudimentary vertebrae, while the anterior portion of 

 the skull, through the cranial nerves, situated further forward, permits the recog- 

 nition of a systematic arrangement of the vertebrae. 



The development of the bones of the face is intimately related to the trans- 

 formation of the visceral arches and clefts. Toward the large buccal opening 

 there projects from each side the medial extremity of the first visceral arch, \vith 

 two processes: the superior maxillary process (Fig. 382, ^,3), which grows more 

 toward the side of the buccal opening: and the inferior maxillary process (u), 

 which extends along the lower border of the mouth. From above downward 

 there now grows the frontal process (/) as a prolongation of the base of the skull, 

 a thick process, provided at its lower outer angle with a spine (i, the internal 

 nasal process). The frontal process and the superior maxillary process (3) unite 

 in such a manner that the former (/) insinuates itself between the latter on each 

 side. At the same time a small external nasal process (2), a continuation of the 

 lateral portion of the cranium, situated above the superior maxillary process, unites 

 with the latter. Between the superior maxillary process and the external nasal 

 process there is a cleft leading to the eye (a), which grows together to form the 

 lacrimal duct (B, O). The buccal opening is thus separated from the nasal open- 

 ing above it. The division, however, extends also in the depth of the mouth; 

 the superior maxillary process produces the hard palate, the frontal process the 

 intermaxillary bone (Fig. 382, B, Z), which occurs also in man, and later unites 

 with the upper jaw. In many animals the inter- 

 maxillary bone persists as a separate bone (os 

 incisivum) , and bears the incisor teeth. The hard 

 palate is closed in the ninth week; and upon it 

 the nasal septum, which is derived from the fron- 

 tal process, is supported at right angles. From 

 the inferior maxillary process there develops the 

 lower jaw (B, U), At the margins of the buccal 

 cavity the lips and alveolar border are formed. 

 The tongue (z) develops behind the junction of the 

 second and third pairs of visceral arches, according 

 to Born from an intercalated piece between the 

 inferior maxillary processes; its root from the 

 second visceral arch. 



These formations may suffer interruption. FIG. 381. Left-sided Hare-lip. 



1. Hare-lip (oronasal fissure, Fig. 382, Q 

 results from nonunion of the internal nasal 



process on the one hand and of the superior maxillary and external nasal 

 processes on the other hand. The cleft runs into the nasal orifice. As a 

 rule, it passes between the incisor teeth, although rarely also in front of the canine 

 tooth. In the presence of maxillary fissure there are often supernumerary incisor 

 teeth. The intermaxillary bone has two centers of ossification, one in the internal 

 nasal process, the other in the region of the superior maxillary process. From 

 the external nasal process, which does not extend all the way down, no especial 

 bone results. The nose and the mouth may be united either only through 

 the soft parts (hare-lip, Fig. 381), or entirely, also through the hard palate (wolf's 

 throat) ; both malformations may be unilateral or -bilateral. The formation 

 of cleft palate may be due to the circumstance either that the superior maxillary 

 and the frontal process wholly or in part remain too short, so that they do not 

 come in contact; or the frontal process grows too far forward like a snout, and often 

 also is diminished in size; so that the superior maxillary process cannot reach it. 



2. A failure of union between the internal and external nasal process on the 

 one side and the superior maxillary process of the other side results in the oblique 

 facial cleft (oro-orbital cleft, Fig. 382, D) ; the nasal orifice is not slit. 



3. The oral cleft (macro stomia} is an abnormally large lateral cleft between 

 the superior and inferior maxillary processes, which may extend as far as the ear 

 (Fig. 382, B, ni). 



4. The occurrence of a fistula of the lower lip is extremely rare; it is regarded 



