NERVOUS SYSTEM OF VERTEBRATES. 373 
Optic lobes, corpora bigemina or quadrigemina (mesen- 
cephalon). 
Crura cerebri. 
Optic ventricle or Iter a tertio ad quartum ventriculum. 
Mid brain. 
Cerebellum (with its ventricle and the pons varolii, form- 
Hind brain. 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, 
extends through the whole length of , 
the spiral 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- 
Fig. 368.—Brain and spinal 
skeleton). cord’ of the frog. A, Prout 
7 above, B, from below. a, ol- 
In the embryos of higher Vertebrates fry, Fajen pees seo 
and in the adult lancelet, hag-fish and hemispheres; ¢, optic lobes ; 
: d, cerebellum in the form of a 
lamprey, the vertebral column is rep- lamella bridging over the 
* * fourth ventricle © 3m, spinal 
resented by a rod-like axis (notochord cord ; i terminal Gand\-—Aitter 
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 
