VERTEBRATES. 



15 



dal region no evidences can be detected of a division into vertebral segments, unless, 

 indeed, ceitain arches which are attached to its ventral face are to be so regarded. 



To understand the nature of these, it is necessary to anticipate here the description 

 of a feature of the alimentary canal, which is common to all vertebrates. In the 

 aquatic vertebrates the anterior part of the alimentary canal communicates with the 

 outside by a series of gill-clefts on either side, through which the water streams for 

 respiratory purposes. Although these gill-clefts entirely disappear, with the exception 

 of the first, in the adults of air-breathing vertebi-ates, yet they are present in the 

 embryos of all, even the highest. Between the clefts (also known as visceral clefts) 

 are left certain solid pillars, the visceral folds, and these are strengthened by cartilagi- 

 nous supports, the visceral arches, which are only developed to a small extent in the 

 adults of air-breathing forms, but attain the maximum of their development in aquatic 

 species, as the gill or branchial skeleton (Fig. 17.) Many anatomists consider these 

 arches to be homodynamous with the hremal arches and ribs further back, and thus to 

 represent the hremal arches of the notochordal 

 part of the skull, but this must still be con- 

 sidered as not sufficiently made out ; for the 

 side walls of the head, in which they make 

 their appearance, do not participate in the 

 segmentation, which we shall afterwards see 

 is observable in certain organs of the u])per 

 part of the head, although not in the chon- 

 drocranium. 



The greater part of this visceral or branch- 

 ial skeleton may be conveniently discussed 

 apart from the skull, but the whole of the first 

 arch, and the u]iper part of the second ai-cli, 

 become so intimately related to the skull that 

 it is necessary to take them into consideration 

 along with it. Indeed, an actual fusion gen- 

 erally takes place between the upper (palato-quadrate) part of the first arch, which 

 forms the upper jaw, and the chondrocranium, while the lower, or Meckelian, part (the 

 lower jaw) is hinged upon the upper part (Fig. 17.) That we have also to take into 

 consideration the ujiper part of the second arch (hyomandibulo-symplectic tract, km, 

 SI/) is due to the fact that it loses to a great extent its union with the lower part, and 

 takes on in many forms the function of the suspension of the lower jaw, being on this 

 account styled the 'suspensorium.' But in higher forms this same tract loses that 

 function and becomes subservient to the organ of hearing in a way to be afterwards 

 described. 



The stage of development of the salmon's skull represented in Fig. 17 affords a 

 good idea of the cartilaginous substratum in and on which the future osseous struc- 

 ture is erected. To understand how this is effected, it is necessary to recall some 

 points which have been referred to above, as to the develoj)ment of bones in the 

 skin. The sturgeon's skull is completely cartilaginous, but is covered on the outside 

 by a series of bony plates formed in the skin, chiefly by the fusion of the cement 

 plates of originally separate teeth ; these are cement bones, and they afterwards i)lay 

 a very important part in filling up gaps or fontanelles in the chondrocranium, and in 

 strengthening it from the outside. Cement bones are not confined to the free surface 



Fir.. 17. — Skull of young saluiou. menibrano bones 

 removed ; fl, auditory capsule : h, branchial 

 arches ; c, ceratohyal ; /, superior fontanelle ; f/, 

 glossoliyal ; //. hviiohyal : Inn, hymnandihular 

 oartilajie ; i, imerhviil ; /, labial cartilagis ; m, 

 llecl<clian cartii.iKC ; n, liol.ichord : o, optic 

 foramen ; p, paiacliorilal cartilage : pp. palato- 

 pterygiii.l ; (7, quadrate ; n, supraorbital ridge ; 

 SI/, symplectic ; /, trabecular ; tr, tegmen crauii ; 

 r, trigeminal foramen. 



