The Frog 



65 



genous substances of the body. It has 

 tion of this substance takes place to a 1 



Fig. 17. The Central Nervous System and Principal 

 Nerves of a Frog, Seen From Below. 



/., Olfactory lobes; II., 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; VII-hd., VII -pal., 

 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. 



been shown that the forma- 

 arge 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. 



So far as we know at 

 this time, practically all 

 excreted substances of the 

 kidney pass through the 

 glomeruli. The exact func- 

 tion of the glomeruli is not 

 known, though there are 

 many theories regarding it. 

 The bladder arises as an 

 outpushing of the ventral 

 wall of the cloaca. It is 

 regarded as homologous 

 ( • ) with 



the allantois (Fig. 363) of 

 the embryo of higher verte- 

 brates. 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 

 cloacal opening so as to 

 make it possible for urine to 

 collect in the bladder. 



THE NERVOUS SYSTEM 



One of the necessary 

 conditions of life is what is 

 commonly called irritability, 

 which means that the organ- 

 ism can, when properly 

 stimulated, perform certain 

 movements. In the higher 



