NERVOUS SYSTEM OP VERTEBRATES. 



373 



Mid bram. ■■ 



Optic lob s, corpora bigemina or quadrigemina (mesen- 

 cephalon). 

 Crura cerebri. 

 Optic ventricle or Iter a tertio ad quartum ventriculum. 



Hind brain. 



Cerebellum (with its ventricle and tlie pons varolii, form- 

 ing the metenceplialon). 

 Medulla oblongata and fourth ventricle. 



The accompanying sketches represent the typical nervous 

 system of an amphibian, which also resembles that of many 

 fishes, and even the lower ReptiUa. 



The spinal cord (Fig. 368) usually 

 extends through the whole length of 

 the spinal canal, except in the toads 

 and frogs, birds and many mammals, 

 where it stops short of the end of its 

 «anal. In those Vertebrates with 

 limbs, the cord enlarges where the 

 nerves which supply them are sent off ; 

 these are the cervical or thoracic, and 

 lumbar enlargements, especially large 

 in turtles and birds. The white and 

 gray substance of the brain continues 

 m the cord. 



As the most essential characteristic 

 of Vertebrates is the internal skeleton 

 (endoskeleton) we will enter more into 

 detail in describing it, and afterwards 

 notice the external skeleton (exo- 



sVplpt-nn'i ^'S' 368.— Brain ana spinal 



SKeiecon;. c„jij ^f ^^^^ fjo„ ^^ f^^^ 



In the embryos of higher Vertebrates JSylbesTrcTrebVai 



and in the adult lancelet, hag-fish and hemiepherea ; >, 'optic lobes; 



' o d, cerebellum in the form of a 



lamprey, the vertebral column is rep- lameiia bridging over the 



■^ , . , ..-., . , ,77 fourth ventricle («) ; m, spmal 



resented by a rod-like axis {notOClwrd cord ; t, terminal cord.— After 



■or chorda dorsalis) which is composed ^^™ ""'' 

 of indifferent, or only partly organized cells, the substance 

 of the chord resembling cartilage. These chordal cells secrete 

 a membrane called the chordal sheath. Thenotochord is not 



