SKULL NERVOUS SYSTEM. 



179 



As in Lepidosiren the mandilmlar arch is firmly connected with 

 the skull. The mandibular susjiensoriiitii and the palato-quadrate 

 are in direct connection with the cartilaginous cranium, and form 

 on either side a wide outstanding infra-orbital arch, the anterior end 

 of which either remains free or fuses with the ethmoid cartilage. 

 The ossification appearing at the end of the suspensorium gives rise 

 to the quadrate, while a membrane bone, almost hammer-shaped and 

 overlying the suspensorial cartilage, is called the squamosal or perhaps 

 more correctly tympanic (Ty). Two membrane bones extend forward 

 along the lower side of the palato-quadrate bar the pt&rygoid (Pi] 

 behind and the palatine (PI) in front. The palatine is transversely 

 placed behind the vomer. The outer arch of the upper jaw, formed 

 l>v the prannaxillary and maxillary bones (Jmx, MX) may by means 

 of a third posterior bone the quadrato-jugal (J) be continued back 

 to the quadrate, but in many Perennibranchiata it is incomplete, the 

 maxillaries being absent. The skeleton of the visceral arches is more 

 or less considerably reduced in correspondence with the retrogression 

 of branchial respiration. In the perennibranehiate Amphibia (Am- 

 phibia with gills throughout life) the visceral arches are more 

 numerous, and present an arrangement similar to that found only 

 transitorily in the larvfe of the other forms. In the Salamandrina, 

 in addition to the hyoid arch, the remains of two branchial arches 

 persist ; while in adult Batrachians only a single pair of arches is 

 retained on the hyoid bone. This branchial rudiment is attached 

 to the posterior edge of the body of the hyoid bone, and serves as 

 a suspensormm for the larynx. 



In the pectoral girdle three parts may be distinguished the 

 scapula, the prcecoracoid, and the coracoid, to which a dorsal cartila- 

 ginous supra-scapula is added. While in the tailed Amphibia ( Urodela), 

 the arch is interrupted below, in the Batrachia the two halves are 

 joined to each other in the middle ventral line, as well as to a 

 posterior plate which has the value of a sternum, and an anterior 

 plate known as the episternum. The pelvic girdle is characterised 

 by the narrow form of the iliac bones, which are attached to the 

 strong transverse processes of a single vertebra, and at their posterior 

 end are fused with the ischiac and pubic bones. 



The nervous system is higher in several respects than that of the 

 fishes. The brain (vol. i., fig. 80) is certainly in all cases small, but 

 the hemispheres are large and the differentiation of the thalamen- 

 cephalon and mesencephalon is further advanced. The optic lobes 

 reach a considerable size, and the medulla oblongata encloses a wide 



