THE FROG 



65 



Vll.p&l, 



V.x. 



genous substances of the body, and it has been shown that the forma- 

 tion of this substance takes place to a large extent in the liver from 



which it is given to the 

 blood by a process of inter- 

 nal secretion. Beside urea, 

 urine contains various salts 

 in solution such as chlorides, 

 sulphates, phosphates of so- 

 dium, potassium, calcium, 

 and magnesium, as well as 

 other substances. 



As far as we know at 

 this moment, practically all 

 of the excreted substances 

 of the kidney pass through 

 the glomeruli. The exact 

 function of the glomeruli 

 are not known, though there 

 are many theories regarding 

 it. 



The bladder arises as an 

 outpushing of the ventral 

 wall of the cloaca. It is re- 

 garded as homologous 

 ( ) with 



the allantois (Fig. 363) 

 of the embryo of higher ver- 

 tebrates. It is very distensi- 

 ble. There are circular mus- 

 cles at the mouth of the 

 bladder which are able to 

 contract and expand, the 

 contraction closing the clo- 



-a pal nnpnincr tn mab-f it nrQ 



sible for urine to collect in 

 the bladder. 



f.t. 



Fig. 17. The Central Nervous System and Principal 

 Nerves of a Frog, Seen From Below. 



/., Olfactory lobes ; //., Optic chiasma ; I.-X., cranial 

 nerves ; 1-10, spinal nerves ; V.md., V.mx., V.op., 

 mandibular, maxillary, and opthalmic branches of 

 fifth cranial nerve; VI', sixth cranial nerve after 

 leaving the Gasserian ganglion ; VH.hd., VII-pol., 

 hyoidean and palatine branches of seventh cranial 

 nerve; IX'., branch from ninth cranial nerve to seventh; 

 IX"., main branch of ninth cranial nerve ; X.v., tenth 

 cranial nerve passing to viscera ; V.x., a small twig from 

 the undivided main branch of the fifth cranial nerve ; X.x, 

 a branch from the vagus to certain muscles ; an.V., 

 annulus of Vieussens through which the subclavian artery 

 passes ; f.t., filum terminale ; G.g., Gasserian ganglion ; 

 hy.n., hypoglossal (first spinal) nerve; inf., infundi- 

 bulum ; pit., pituitary body ; r.c., ramus communicans ; 

 sci.n., sciatic nerve ; sy.c., longitudinal commissure of 

 sympathetic chain ; sy.g., sympathetic ganglion ; v.g., 

 vagus ganglion. (Redrawn from Borradaile.) 



Compare with Figures 472C, 478, 480, 483. 



THE NERVOUS SYSTEM 



One of the necessary 

 conditions of life is what is 

 commonly called irritability, 



AxrVi^n r r r> r> # r 1 v 



[ c * n > Wnen properly 



cti'rnnla t^rl r^rfr^rm ^Artatn 



stimulated, periorm certain 



