FROG : NERVOUS SYSTEM AND SENSE ORGANS 71 



It leaves the neural canal between the first and second 

 vertebrae, curves round the throat, turns forward below the 

 mouth, and proceeds to the tongue. The second spinal 

 nerve is a large strand running straight outwards. It re- 

 ceives 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 the hind-limb, the 

 principal being the very large sciatic nerve. 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. 



The brain may be divided into three regions, known 



respectively as the hind, mid, and fore brains. 



The hind-brain consists of the medulla ob- 

 longata 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 mem- 

 brane over the fourth ventricle (Fig. 24). 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 orpedunculi cerebri, a roof consisting of a 

 pair of rounded swellings known as optic lobes, and inter- 

 nally a narrow passage, the aquaductus cerebri, continuous 

 behind with the fourth ventricle and above with cavities in 

 the optic lobes. The fore-brain consists of the thalamen- 



