386 THE FROG. PHYLUM CHORDATA 



to the tongue. The second spinal nerve is a large strand running 

 straight outwards. It receives branches from the first and third, 

 forming thus the brachial plexus, and proceeds as the brachial 

 nerve to the arm. The third spinal nerve is small, and beyond the 

 brachial plexus resembles the fourth, fifth, and sixth spinal nerves. 

 All these are small and run backwards to supply the muscles and 

 skin of the belly. The seventh, eighth, ninth, and tenth spinal 

 nerves join to form a sciatic plexus, from which arise several nerves 

 to join the hind limb, the principal being the very large sciatic 

 nerve which runs below the dorsal surface of the thigh 

 between the semimembranosus and biceps femoris muscles. 

 The tenth nerve leaves the vertebral canal by a foramen in the 

 side of the urostyle. The roots of the last four pairs of nerves 

 do not issue from the spinal canal at once, but run backwards 

 for some distance from their origin to reach their point of exit. 

 Thus they form inside the vertebral canal a bundle known as 

 the Cauda equina. 



BRAIN 



The brain (Fig. 303) may be divided into three regions, known 

 respectively as the hind, mid, and fore brain. The hind brain 

 consists of the medulla oblongata and the cerebellum. The medulla 

 oblongata is the hindermost part of the brain. It is continuous 

 behind with the spinal cord, which, as it is traced into the brain, 

 widens, the central canal enlarging into a cavity in the medulla 

 known as the fourth ventricle of the brain, the ventral side 

 thickening, and the dorsal thinning out into a slight membrane 

 over the fourth ventricle (Fig. 303 I). The pia mater above this 

 membrane is very vascular and thrown into folds which project 

 into the ventricle, forming thus a structure known as the posterior 

 choroid plexus. The cerebellum is a narrow band across the roof 

 of the front part of the fourth ventricle. In many other animals 

 it is relatively much larger. The mid-brain is the region in front 

 of the medulla. It has a thick floor formed by two longitudinal 

 columns known as the crura cerebri, a roof consisting of a pair 

 of rounded swellings known as optic lobes, and internally a 

 narrow passage, the aquaeductus Sylvii, continuous behind 

 with the fourth ventricle and above with the cavities in the 

 optic lobes. The fore-brain consists of the thalamencephalon 

 and the cerebral hemispheres. The thalamencephalon lies immedi- 



