416 



VERTEBRATE LIFE AND ORGANIZATION 



I Olfactory n. 

 Olfactory bulb 



tuzmiBphjire ' 

 E Optic nr— 



Diencephalon 



Optic lob^0 

 Pituitary ^land- 



M^ulla. 

 oblongata - 



Ciioroid pkxus 

 Spixial <2ord~— 



Optic tra.<| 



Mtmdilji 



1 



1 



-IIan4X« 



■First ^pi 



Figure 21.18. A, A dorsal and B, a ventral view of the brain of the frog. (Modified 

 after Gaupp.) 



cord, and a peripheral nervous system, which includes the nerves con- 

 necting the brain and cord with the receptors and effectors of the body. 

 Both the brain and the spinal cord, which lie respectively within the 

 cranium and the neural canal of the vertebral column, are hollow. A 

 single, dorsal, tubular nerve cord, you will remember, is a diagnostic 

 characteristic of chordates. Within the brain, parts of the central cavity 

 are expanded to form large chambers known as ventricles. All parts of 

 the nervous system are composed largely of specialized, elongate cells, 

 the neurons, described earlier (Chapter 3). 



The structure of the brain is shown in Figure 21.18. It can be 

 divided into five major regions: (1) An anterior telencephalon bears 

 the paired olfactory bulbs and rather small cerebral hemispheres, the 

 latter containing the first and second ventricles. (2) An indented region, 

 the diencephalon, lies posterior to the cerebral hemispheres. Its lateral 

 walls constitute the thalamus. The pituitary gland is attached to a 

 part of the floor of the diencephalon known as the infundibulum. An 

 inconspicuous pineal body extends from the roof of the diencephalon 

 to the brow spot. Most of the roof of the diencephalon is thin and 

 vascularized, forming a choroid plexus which dips into the third ven- 

 tricle. This region is followed by (3) the mesencephalon bearing the 

 paired optic lobes containing optic ventricles; (4) the metencephalon 

 with a small, dorsal, transverse ridge known as the cerebellum; and 

 (5) the myelencephalon consisting of the medulla oblongata. The 



