20 



NATURAL HISTORY OF VERTEBRATES. 



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

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

 point in morphology, 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 sejjarate 

 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 separate 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 occipital 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 part which lies in front of it — the face. This is perhaps as well 

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

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

 size of the olfactory region of the skull. 



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

 nature and arrangement of its component elements. The peculiarities of each group 

 will be discussed under the special description of each. 



The Visceral Skeleton. 



This term is applied to the series of arches which intervene between the gill-clefts. 

 The first cleft is styled hyomandibulnr, 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 arch. We have already 

 considered the whole of the 

 first arch and the upper 

 part of the second ; there 

 remain for treatment the 

 lower part of the second 

 and the various branchial 

 arches proper. It will be 

 observed that visceral arch 

 is a term applicable to all 

 of these structures, while 

 branchial arch is confined 

 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 must look for the full development of these 

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

 disappear except the first, and this generally is much reduced in size. 



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

 the corresponding segments of the succeeding arches, but a fundamental similarity is 

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



Fig. 24.— Skull and visceral arches of a dog-flsh ; a, auditory capsule ; bb, 

 basibranchial ; c, ceratohyal ; eb, ceratobranchial ; c, epiTjranchial ; e&, 

 extrabrancMal ; 7»«, hyomaudibular ; m, lower jaw ,■ mpf metapterygoid 

 ligament ; n, nasal capsule ; (7, pterygo-quadrate arcade ; pb, pharyngo- 

 l)rancbial ; pn, prenasal cartilage ; s, supraorbital ridge ; sp^ spiracle ; 

 t, palato-trabecular ligament ; v, trigeminal foramen ; 1, 2, 3, 4, 5, labial 

 cartilages. 



