DEVELOPMENT OF THE NERVOUS CENTRES. 



99 



incus are formed. The remains of Meckel's cartilage persist as long as till the 

 end of the seventh or the eighth month of foetal life, in the form of a rod of 

 cartilage lying inside the lower jaw. From the second pharyngeal arch are 

 formed the stapes and stapedius muscle, the pyramid, the styloid process, the 

 stylohyoid ligament, and the small cornu of the hyoid bone. The great cornu 

 and body of the hyoid bone are developed from the third arch, while the fourth 

 pharyngeal arch enters merely into the formation of the soft parts of the neck, 

 and does not give origin to any special organ. The pharyngeal or branchial 

 fissures are four in number, the fourth being situated behind or below the 

 fourth arch ; the first persists, though only in a portion of its extent, forming 

 the Eustachian tube, the meatus auditorius, and the tympanic cavity. The 

 other fissures are wholly closed by the sixth week. 



Development of the Palate. The buccal cavity is at first common to the mouth 

 and nose. Then a lamella is given off from the superior maxillary tuberosity 

 on either side, which is directed horizontally inwards. These two palatine 

 lamellas meet in the median line, in front, about the eighth week, and by the 

 ninth week the septum should be complete. The superior maxillary bones 

 proper, and the soft parts covering them, unite at an early period with the in- 

 cisive bone, and the median portion of the lower lip. The olfactory fossae open 

 into the upper (respiratory) portion of the cavity, forming the nostrils. The 

 student will notice that the various forms of harelip correspond to various 

 interruptions of the process of union ; thus the ordinary single harelip on one 

 side of the median line results from the mere absence of union on that side, 

 between the soft parts which cover the incisive bone and those connected with 

 the proper superior maxillary; if this occurs on both sides, we have the sim- 

 plest form of double harelip ; if besides this the intermaxillary bone remains 

 ununited, it usually is carried forward at the end of the vomer, forming the 

 double harelip, complicated with projection of the intermaxillary bone; if, 

 added to this, the palatine lamellae also remain unu- 

 nited, we have the complete degree of fissured palate Fig. 61. 

 and harelip. Fissure of the soft palate only, or of 

 the soft and a portion of the hard, represent various 

 degrees of non-union of the palatine lamellae. 



Development of the Nervous Centres. The medullary 

 groove above described (page 91 ) presents about 

 the third week three dilatations at its upper part, 

 separated by two constrictions, and at its posterior 

 part another dilatation called the rhomboidal sinus. 

 Soon afterwards the groove becomes a closed canal 

 (medullary canal), and a soft blastema is deposited 

 in it which lines it, corresponding to its dilatations, 

 and, like it, assuming a tubular form. This is the 

 rudiment of the cerebro-spinal axis. As the embryo 

 grows, its cephalic part becomes more curved, and 

 the three dilatations in the anterior end of the primi- 

 tive cerebro-spinal axis become vesicles distinctly 

 separated from each other {Fig. 61). These are the 

 cerebral vesicles anterior, middle, and posterior. 

 The anterior cerebral vesicle (situated at this period 

 quite below the middle vesicle) is the rudiment of 

 the lateral and third ventricles, and of the parts sur- 

 rounding them viz., the cerebral hemispheres, optic thalami, corpora striata, 

 corpus callosum, fornix, and all the parts which form the floor of the third 

 ventricle. The middle vesicle represents the aqueduct of Sylvius, with the 

 corpora quadrigemina, and the crura cerebri. The posterior vesicle is deve- 



Longitudinal section of the head 

 of an embryo four weeks old seen 

 from the inside. 1. Ocular vesi- 

 cle. 2. Optic nerve flattened out. 

 3. Fore brain. 4. Intermediary 

 brain. 5. Middle brain. 6. Hind- 

 er brain. 7. After-brain. 8. An- 

 terior portion of the tentorium 

 cerebelli. 9. Its lateral portion 

 intervening between Nos. 4 and 

 5. 10. The pharyngeal curve, 

 bent into a cul-de-sac. 11. The 

 auditory vesicle. 



