RANA. 347 



The second and third unite to form a brachial plexus to 

 the fore-limb ; the fourth, fifth and sixth pass to the body- 

 muscles ; the seventh, eighth and ninth unite to form the 

 sciatic plexus continued into the hind-limb. The tenth is a 

 small spinal beside the urostyle. On either side of the aorta 

 is a thin pigmented nerve-chain with ganglia, called the sym- 

 pathetic system. From each ganglion a connection passes 

 to each spinal nerve. Forward, the sympathetic chain ter- 

 minates in the Gasserian ganglion of the fifth cranial nerve. 



The ten cranial nerves are essentially like those of the 

 skate, though smaller and more difficult to follow, and the 

 fifth, seventh and tenth nerves are much simpler. The fifth 

 has three main branches — the ophthalmicus^ the maxillary 

 and mandibular. The seventh has only two main branches 

 — the palatine and the hyomandibular. The vagus has no 

 branchial branches, but supplies the larynx, lungs, heart 

 and stomach. 



At the sides of the vertebrae are a number of masses of calcareous 

 matter called calcareous bodies. They have a curious developmental 

 connection with the ear. 



The brain may be seen by removing the dorsal bones of 

 the cranium. It is small and has a very small cerebellum. 

 The various parts are in one horizontal axis and do not 

 overlap each other. 



The spinal cord passes down the vertebral column, as in 

 the skate, and terminates in the urostyle. 



The frog has no exoskeleton. The endoskeleton can, 



as in the skate, be divided into axial and peripheral parts. 



Sk 1 t 1 "^^^ 2,YA2\ is composed of a skull and vertebral 



column. The skull is composed of the cranium 



and the first two visceral arches, mostly joined together. 



The first important difference from the skull of the skate 

 is the presence of bones in addition to the cartilaginous por- 

 tion. Some of these bones are formed in dermal membrane 

 and sink on to the cranium ; these are called membrane- 

 bones. The others are formed in the cartilage, or rather 

 they replace the cartilage w^hich is destroyed as they grow. 

 These are termed cartilage-bones. The cartilage may be seen 

 extending between the bones, or the membrane-bones may 

 be removed, in which case the true extent of the cartilaginous 

 cranium can be clearly seen. The actual cranium is small 



