Chap. 34 AMPHIBIANS 705 



ventricles (Fig. 34.22). The outer part of the tube wall contains long processes 

 of nerve cells (white matter), whose fatty sheaths cause the whiteness. The 

 inner part, like a letter H surrounding the central canal, contains the bodies 

 of nerve cells and looks pearly gray (gray matter). The central canal is a 

 remnant of the open groove which was present in the brain and cord during 

 the early development of the nervous system (Chap. 19). The cord extends 

 backward from the opening in the cranium (foramen magnum) to the seventh 

 vertebra where it tapers into a fine thread of non-nervous tissue, the filum 

 terminale (Fig. 34.21). Like the nerve chain of the bee, the frog's nerve cord 

 is in an evolutionary process of shortening. At the levels of the front and hind 

 legs, it is enlarged by the large number of nerve cells and nerve cell fibers in- 

 volved in the movement of the legs. There are similar arrangments in other 

 animals — the ganglia near the bases of the wings and legs of the grasshopper 

 are also extra large because of the many nerve cells involved with movement. 



Brain. During its development, the brain (encephalon) at first forms three 

 and then five enlargements with constrictions between them. These five divi- 

 sions are found in all vertebrates. The divisions and the structures they contain 

 are as follows: 



TELENCEPHALON. This is composcd of the olfactory and cerebral lobes, 

 chiefly the latter (Fig. 34.22). To the former, nerves pass from the sensory 

 epithelium of the nostrils. Each cerebral lobe contains a cavity (first and second 

 or lateral ventricles). These are continued forward into the olfactory lobes 



Epiphysis 



Olfactory 

 lobe 



Optic lobe 



Cerebellum 



Spinal canal 



Cerebra 

 hemisphere 



Spinal cord 



Tholomencephalon 



Medullo 

 Cerebellum 



Fig. 34.22. Upper, brain of frog, side view. Lower, diagram of ventricles of the 

 frog's brain — V.l, V.2, V.3, Optic V., and V.4. (Upper, courtesy, Romer: The 

 Vertebrate Body, ed. 2. Philadelphia, W. B. Saunders Co., 1955.) 



