NERVOUS SYSTEM OF VERTEBRATES. 



373 



Mid brain. 



Optic lobes, corpora blgemlna or quadrigemlna (meseD- 



ceplialon). 

 Crura cerebri. 

 Optic ventricle or Iter a tertio ad quartum ventrlculum. 



Hind brain. 



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

 ing the metencephalon). 

 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 Reptilia. 



The spinal cord (Pig. 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 

 canal. 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 

 in 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- 

 skeleton). 



In the embryos of higher Vertebrates t^^:^\[\Tj'f'°eieVlx 

 and in the adult lancelet, hag-fish and hemispheres ; c, optic lobes ; 



' ° , d, cerebellum m the form of a 



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



■,,, T,., ., ,7, fourth ventricle («) ; m, spinal 



resented by a rod-like axis (notoc/iora cord ; «, 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. The notochord is not 



Fig. 368.— Brain and spinal 

 cord of the frog. A, from 



