CLASS AMPHIBIA 



501 



The following are a few of the muscles of the other parts of 

 the body: The rectus abdominis (Fig. 421, rct.abd) extends 

 along the ventral side of the trunk; the obliquus externus (obi. 

 exf) covers most of the sides of the trunk; the transversus (obi. 

 int) lies beneath the obliquus externus and serves to contract 

 the body-cavity; the pectoralis major (pet) moves the fore limbs; 

 and the submaxillary (my.hy) raises the floor of the mouth 

 cavity during respiration. 



The Nervous System. - - Three main divisions may be dis- 

 tinguished in the nervous system of the frog: (i) the central, 

 consisting of the brain and spinal cord; (2) the peripheral, con- 

 sisting of the cerebral and spinal nerves; and (3) the sympathetic. 

 It will be sufficient in this place to point out certain selected 

 points concerning the nervous system of the frog, since general 

 accounts of nervous tissue (p. 76), nervous activity (pp. 223-226), 

 and the nervous system of vertebrates (pp. 408-410) have 

 already been given. 



THE BRAIN.- -The brain (Fig. 422) has two large olfactory 

 lobes which are fused together, two large cerebral hemispheres, 

 two large optic lobes, a well-developed mid bra in (ZH), a 

 very small cerebellum, and a medulla oblongata, which is pro- 

 duced by the broadening of the spinal cord. The optic 

 chiasma (Fig. 422, B, Tr.opt), the infundibulum (Jnf), and the 



