NERVOUS SYSTEM OF VERTEBRATES. 



373 



Mid Drain. 



Optic lob s, corpora bigemina or quadrigemina (mesen- 



cephalon). 

 Crura cerebri. 

 Optic ventricle or Iter a tertio ad quartum ventriculum. 



(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 (Fig. 368) usually A JV\ \[\ 



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- 



VplprnrA Fig. 368. Brain ana spinal 



11 / cord ofjhe frog. A, from 



In the embryos of higher Vertebrates 



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



' d, cerebellum in the form of a 



lamprey, the vertebral column is rep- 1 a men a bridging over the 



V - - ,., . , * fourth ventricle 00 ; m, epinal 



resented by a rod-like axis (notocnord cord; Mermmai 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 



