20 



NATCH A L HISTORY OF VERTEBRATES. 



drum of the ear, and that the other bones in the chain, the incus, orbiculare, and 

 stapes, represent the columella auris of the reptiles. Although this is still a moot 

 point in morj)hology, the former account is certainly that which offers fewest difficul- 

 ties to the evolutionist. Such complex bones as are referred to above are no excep- 

 tion in the higher vertebrates. The tendency to the fusion of originally separate 

 elements is very marked in birds, where, indeed, all the walls of the cranial cavity 

 early form a continuous mass in which it is impossible to detect the sejiarate elements. 

 Only the bones of the face retain a certain amount of independence. Again in mam- 

 mals we frequently find all of the sphenoidal elements united into a sphenoid bone, 

 just as the separate occipital elements, together with the interparietal, form the com- 

 plex occil)ital bone. 



The greatest contrast is to be observed within the different groups, as to the 

 relative size of that part of the skull which comes into relation to the brain (the 

 cranium), and the jiart which lies in front of it — the face. This is perhai)s as well 

 seen in the Mammalia as elsewhere, owing chiefly to the enormous development of tiie 

 turbinal surfaces clothed by the nasal mucous membrane, and the consequent large 

 size of the olfactory region of the skull. 



In tlie above account of the vertebrate skull, the object has been to indicate the 

 nature and arrangement of its com])onent elements. Tlie peculiarities of each group 

 will be discussed under the special doscri]ition of each. 



The Visceral Skeletox. 

 This terra is applied to the sei-ies of arches which intervene between the gill-clefts. 

 The first cleft is styled hyomandibular, because it lies between the hyoid arch 

 behind, and the palato-mandibular arch in front. The second cleft is the first branchial, 



and it separates the hyoid 

 arch from the first branch- 

 ial ai'ch. We have already 

 considered the whole of the 

 first arch and the u]iper 

 jiart of the second ; there 

 remain for treatment the 

 lower ]iart of the second 

 and the various branchial 

 ai'ches proper. It will be 

 , ^ , ,. , ,, observed that visceral arch 



Fig. 24.— Skull ami visceral arches of a dog-fish ; «. auditory capsule ;(■(', ,. , , 



basibranchial ; c, ceratohval ; rh. cer.atobrancliial ; e. epibraiichial ; eb. jg ^ term apilllcabie tO all 



extrahrauchial; Am, hyuiandibular: m, lower jaw; OT/i, inetapterygoid '' 



ligaiii.nt; H. nasal capsule; 17, pterygo-quadrate arcade; /)6, pharyiigo- pf these Structures, Willie 



branchial ; /(", prenasal cartilage ; s, supraorbital ridge ; sp, si.iracle ; c j 



/. palato-trabecularligameul; i-,trigeniiua] foramen; 1,2, 3, 4,0. labial branchial arch IS COllIinea 



'^'"•""'s^'- to the third and succeeding 



visceral arches, even although in some fishes both mandibular and hyoid arch, in spite 

 of their specialization, have not entirely lost their gill-bearing function. 



It is to the aquatic vertebrates that we mfist look for the full development of these 

 structures. With the loss of gills in air-breathing forms, all of the branchial arches 

 disap]iear except the first, and this generally is much reduced in size. 



The lower jiortion of the hyoid arch is generally more subdivided in fishes than 

 the corres]3onding segments of the succeeding arches, but a fundamental similarity is 

 nevertheless observable. Thus certain basal elements or copulje unite the ventral 



