NERVOUS SYSTEM. 77 



the olfactory bulbs or ganglia. Their roots or peduncles, 

 originating from beneath cerebrum, are of the nature of com- 

 missures. Decussation (chiasm) of the majority of fibres of 

 optic nerves rarely absent, as in Amphioxus. 



The medulla oblongata is intermediate in character to the 

 brain and spinal cord. It is composed of several ganglia, 

 the gray matter being placed internally. With it all the 

 cranial nerves excepting the olfactory and optic have con- 

 nection.* 



Sympathetic system. The differences obtaining between the 

 representatives of this system are not sufficiently defined to 

 require comment. 



Pisces. In Amphioxus the spinal cord terminates at either 

 end without enlargement. The position of the brain can 

 only be recognized by the points of origin of nerves of spe- 

 cial sense. The spinal cord generally is thickest at the middle 

 third. It gives off fifty-five to sixty pairs of nerves, each 

 nerve arising from a single root. 



With all fishes, except Amphioxus, the brain is well defined. 

 The medulla oblongata is slender, not distinctly divisible 

 into pyramids. It is often open from above, exposing cavity 

 of fourth ventricle, and possesses on either side a number of 

 small projections known as the lateral lobes. These are pecu- 

 liar to fishes, and in Trigla (gunard) and Torpedo (electrical tor- 

 pedo) are specially marked. Olivary body wanting. Cere- 

 oellum relatively of large size, generally roundish and smooth, 

 but in active predatory fishes, as Selachia, hollow, presenting 

 transverse folds, and commonly communicates with cavity of 

 fourth ventricle. With the exception of Scomber (mackerel), 

 where they are small, it is entirely devoid of lobes. Optic 

 lobes are of large size, and constitute the most conspicuous 

 and bulky mass of the brain. They are hollow, occasionally 

 communicate, and each contains, in osseous fishes, a rounded 

 nodule. The optic nerves commonly form a chiasm. Cere- 

 bral hemispheres smaller than the preceding, solid, separated 



* It will be included in description under head of 'brain ' 



