98 



GENERAL ANATOMY. 



Fig. 60. 



of the chorda dorsalis. These advance in the form of a membranous capsule, 

 which covers the end of the chorda dorsalis, forming the rudiment of the base 

 of the skull, and moulds itself on the cerebral vesicles, so as to constitute the 

 membrane in which the vault of the skull is developed. The membranous 

 capsule presents at the base of the skull two thickenings (lateral trabeculae of 

 Rathke) directed forwards, and inclosing an opening (pituitary opening) which 

 is partly closed by a thinner membrane the middle trabecula. The upper end 

 of the chorda dorsalis terminates in a pointed extremity, which extends about 

 as far forwards as the body of the sphenoid bone, where it becomes lost about 

 the situation of the pituitary body. The membrane becomes replaced by carti- 

 lage in the part corresponding to the base of the skull and the trabeculae. A 

 portion of this primitive cartilaginous cranium becomes atrophied and disap- 

 pears, a portion persists forming the cartilages of the nose and those of the 

 articulations; the rest forms the cartilaginous nidus of the basilar part of the 

 occipital, the greater part of the sphenoid, the petrous and mastoid portions of 

 the temporal, the ethmoid bone, and the septum nasi. / 



As the cerebral extremity of the foetus grows it becomes twice bent forwards 

 on its own axis (Fig. 61). The upper or posterior curvature is called the cere- 

 bral ; the lower or anterior, the frontal pro- 

 tuberance. From the anterior end of the 

 chorda dorsalis four prolongations proceed 

 on either side, and meet in the middle line 

 (Fig. 60, 4, 7, 8, 9). These are the pharyn- 

 geal arches, and in them, and in the frontal 

 protuberance, certain bones are developed, 

 which are called secondary bones, to distin- 

 guish them from those above enumerated, 

 which are formed from the primitive cra- 

 nium itself. Between the first pharyngeal 

 arch and the frontal protuberance is situated 

 the buccal depression, which afterwards be- 

 comes the cavity of the mouth. The frontal 

 protuberance next gives off' two lateral parts 

 (lateral frontal protuberances), on each of 

 which a depression is formed, the olfactory 

 fossa, bounded on either side by the internal 

 and external nasal processes. There is a 

 groove external to the external nasal pro- 

 cess, which afterwards is transformed into 

 the lachrymal canal, and another groove 

 loading from the olfactory fossa to the buc- 

 cal cavity the nasal groove. 



The first pharyngeal arch divides at its 

 anterior extremity into two parts a superior 

 and inferior maxillary protuberance. The 

 latter unites very early to its fellow of the 

 opposite side to form the lower jaw. The 

 superior maxillary protuberances are displaced outwards and unite to the ex- 

 ternal nasal process; from this part are developed the internal plate of the 

 pterygoid process, the palate bone, the superior maxillary, and the malar. The 

 lateral masses of the ethmoid, the os unguis, and nasal bones are furnished by 

 the internal nasal process. The rest of these processes on either side are united 

 into a single protuberance, the incisive tubercle, from which the intermaxillary 

 bone and the middle of the upper lip are formed, and, according to some, the 

 vomer. 



Besides the lower jaw, the inferior maxillary protuberance furnishes a transi- 

 tory cartilaginous mass the cartilage of Meckel from which the malleus and 



' 



Fiice of an embryo of 25 to 28 dnys. (Mag- 

 niiifd 15 times.) 1. Frontal prominence. 2, 

 3. Right nnd left olfactory fosgw. 4. Infe- 

 rior maxillary tubercles, united in the middle 

 line. 5. Superior maxillary tubercles. 6. 

 Mouth. 7. Second pharyngenl nrch. 8. 

 Third. 9. Fourth. 10. Primitive ocular 

 vesicle. 11. Primitive auditory vesicle. 



