XII.] 



THE CRANIAL SKELETON. 



I2 5 



Fishes. These branchial arches extend on each side of the 

 throat upwards towards the spine-, and support the gills on 

 their inner sides. Other cartilaginous arches which were 

 spoken of in the Second Lesson as existing in the Lamprey 

 and some Sharks, may support the outer sides of the gills. 

 The branchial arches become successively smaller posteriorly, 

 or as we recede from the head. Thus we see what numerous 

 large and important parts of the lowest Vertebrates are rudi- 

 mentarily represented by the human hyoidean cornua. 



FIG. 113. Diagram of the Changes undergone by the Hyoid in a Frog in passing 

 from the Tadpole stage to the adult condition. 



( Constructed from Parker s Memoir). 



Upperjiiost left-hand figure, the youngest condition ; Imvest right-hand figin-e, 

 the adult. 



h, the hyoidean arch, ultimately the corniculum ; b 1 b 4 , the four branchial 

 arches which become gradually atrophied, the cornua (or thyro-hyal), th being 

 their representatrve in the adult ; b , another branchial rudiment ; M, the 

 body of the hyoid. 



The cornicula represent only the so-called hyoidean arch 

 namely, that arch in Fishes which comes behind the lower 

 jaw and in front of the branchial arches. 



We have seen that in man's own class this arch may con- 

 sist of several distinct bones, that is to say, of a tynipano- 

 hyal, a stylo-hyal, an epi-hyal, and a cerato-hyal, though in 

 man himself the epi-hyal is only represented by ligament. 

 It is, therefore, less surprising that in the lowest vertebrate 

 class this arch should be large and complex with bony 



