THE SKULL. 



55 



forming thus a firm support for it, gives rise to the skeleton of 

 the jaws, as well as, in higher types, to the main part of the facial 

 skeleton. The arches lying posterior to this function primarily as 

 gill-supports. 



FIG. 41. DIAGRAM SHOWING THE PRIMITIVE METAMERIC CONDITION 

 OF THE HEAD. 



E, E, epiblast, which at N is invaginated to form the primitive olfactory pit, the 

 epithelium of which is supplied by the olfactory nerve (Olf) ; M, oral involu- 

 tion ; /, first somite, from which arise the superior, internal, and inferior 

 rectus, and inferior oblique muscles ; II, second somite, from which the 

 superior oblique muscle originates ; HI, third somite, which gives rise to the 

 external rectus ; IV, V, VI, fourth, fifth, and sixth somites : only the sixth 

 gives rise to muscle-rudiments ; VII, VIII, IX, seventh, eighth, and ninth 

 somites, from which the muscles extending from the skull to the pectoral arch 

 arise : the anterior part of the sterno-hyoid is also formed in this region ; a and 

 b indicate the first somites of the trunk ; ///, oculomotor, IV, trochlear, 

 VI, abducent, and XII 1 to XII s , hypoglossal nerves. All the above-named 

 nerves correspond to ventral roots of the nerves belonging to the head-somites, 

 /, II, III, VII, VIII, and IX. The ventral nerves belonging to somites 

 IV, V, and VI are not known : they probably lie in the territory of the tri- 

 geminal. RpV, ramus ophthalmicus profundus of the trigeminal, the dorsal 

 nerve of the first somite; V, the rest of the trigeminal, the dorsal nerve of the 

 second somite, supplying the maxillary and mandibular regions ; VII, VIII, 

 the acustico-facialis, the dorsal nerve of the third and fourth somites, supplying 

 the first primitive gill-cleft (spiracle) (1) ; IX, glossopharyngeal, the dorsal 

 nerve of the fifth somite, supplying the second gill-cleft (2) ; X 1 to X 4 , vagus, the 

 dorsal nerves of the sixth to the ninth somites, supplying the third to the sixth 

 gill-clefts (3 to 6) ; y j , Sv\ ventral, and Sd l , ScP, dorsal, roots of the two first 

 spinal nerves; m, first (mandibular) visceral arch; h, second (hyoiu) arch; 

 6 l to b 5 , the five branchial arches ; E 1 , JK' 2 , first and second ribs. 



On viewing the serial arrangement of the visceral arches, one 

 might be tempted to explain them as being homodynamous with 

 ribs, and to consider this, as well as the corresponding distribu- 

 tion of the branchial nerves as a further support for supposing 



