PISCES. 169 



a swollen end, running forwards and upwards from the roof of 

 the 3rd ventricle. It corresponds to what appears to have been 

 an unpaired dorsal eye (pineal eye) in ancestral forms (see p. 2). 

 The pituitary body is a problematic organ attached to the inf un- 

 dibulum, and consisting of two thin- walled sacs (sacci vasculosi) 

 and a central tube. 



The prosencephalon ( = cerebral hemispheres of higher verte- 

 brates) is the largest part of the brain. It is a rounded mass 

 presenting indications of divisions into right and left halves, 

 and containing a ventricle connected with the 3rd ventricle. 

 An expanded olfactory lobe abuts on the posterior wall of each 

 olfactory capsule, and is connected behind with the prosen- 

 cephalon by a short stalk. It contains an olfactory ventricle 

 which communicates with the brain-cavity last mentioned. 



The short, thick mid-brain which next succeeds contains a 

 ventricle (iter or Sylvian aqueduct), the roof of which is raised 

 up into two ovoid optic lobes containing optic ventricles. 



The axis of the hind-brain is constituted by a long cylindrical 

 bulb (medulla oblongata) containing a cavity (4th ventricle) with 

 a thin roof. A large hollow body, the cerebellum, of elongated 

 oval shape is attached by the middle of its length to the dorsal 

 side of the bulb, which is also produced on each side into a 

 conspicuous lobe (restiform body). 



The spinal cord is cylindroidal in shape and somewhat 

 flattened from above downwards. It is deeply furrowed by 

 two median fissures one dorsal, the other ventral. 



(2) Cranio-spinal Nerves. These may be divided into cranial 

 nerves and spinal nerves, which respectively take origin from 

 the brain and spinal cord. 



There are ten pairs of cranial nerves, as follows : 



I. Olfactory, each of which consists of two bundles of fibres 

 arising from the corresponding olfactory lobe and supplying 

 the lining of the olfactory sac on the same side. 



II. Optic, running to the eyeballs. They unite to form an 

 X-shaped chiasma on the ventral side of the thalamencephalon. 

 The posterior limbs of the X (optic tracts) end in the optic lobes. 



The III., IV., and VI. nerves are known as the eye-muscle 

 nerves. 



III. Oculomotor. These arise from the ventral side of the 

 mid-brain and supply most of the eye-muscles. 



