432 STRUCTURE OF VERTEBRATA. 



In the Elasmobranch fishes, the mandibular and hyoid arches do not 

 form any direct part of the cartilaginous brain-case, but in the Teleo- 

 steans and thence onwards, the cartilages or bones arising in connection 

 with the mandibular and upper part of the hyoid arches contribute 

 directly to the formation of the skull. The hyoid proper, or lower 

 part of the hyoid arch, forms the skeleton supporting the tongue. 

 Cartilages arising in the lower part of the third visceral arch assist in 

 the formation of the hyoid bones of the higher Vertebrates, and parts of 

 the two other arches appear to help in forming the laryngeal skeleton of 

 Mammals. 



The mandibular arch in Elasmobranchs and frogs divides into a lower 

 portion — Meckel's cartilage — which forms the lower jaw or its basis, 

 while from the upper portion a bud grows forward, the palato-pterygo- 

 quadrate cartilage, which forms the upper jaw in shark and skate, and 

 has a closer union with the skull in the frog. In higher Vertebrates 

 the lower portion of the mandibular always forms the basis of the lower 

 jaw, a quadrate element is segmented off from the upper part, but the 

 palato-pterygoid part seems to arise more independently. The hyoid 

 arch also divides into a lower portion, the hyoid proper, and an upper 

 portion, the hyo-mandibular, which may connect the jaws with the skull, 

 or from Amphibians onwards may be more remarkably displaced and 

 modified as a columella or stapes connected with the ear. We adhere 

 to the old interpretation, according to which the mandibular and hyoid 

 form two arches ; but Dohrn believes that they are equivalent to four — 

 palato-pterygo-quadrate, Meckel's cartilage, hyo-mandibular, and hyoid, 

 being in his opinion independent arches. 



Returning now to the brain-box itself, we must notice 

 another complication, — the development of " membrane " 

 bones. If we examine the skull of the skate, we find that 

 the brain lies within a cartilaginous capsule ; but this is not 

 entirely closed, spaces (the fontanelles) being left in the 

 roof, which during life are covered only by the tough skin 

 with its numerous "dermal denticles." In the sturgeon, 

 again, the small skin-teeth are replaced by stout bony plates 

 covering over the cartilaginous capsule. From such super- 

 ficial bony plates it is supposed that the "membrane" 

 bones, or ossifications in membrane, which form so import- 

 ant an element in the skull of the higher Vertebrate, have 

 originated. 



In some bony fishes, notably the salmon, we find the brain enclosed 

 in a double capsule. Inside there is a cartilaginous brain-case in which 

 what are called centres of ossification have appeared, and upon this a 

 layer of membrane bones is placed, which can be readily removed without 

 injury to the cartilage beneath. In general, however, we must recognise 

 that, with the appearance of membrane bones, two changes tend to 

 occur, — first, the cartilaginous cranium tends to be reduced and to 

 exhibit considerable openings ; second, in the . remaining cartilage 



