SEA-FISHERIES LABORATORY. k i47 



veins (V. hep.) which, enter the lower portion of the sinus 

 venosus on its posterior side. 



E.— THE NERVOUS SYSTEM. 



We shall commence our description of the nervous 

 system with the brain and spinal cord, then proceeding to 

 the cranial and spinal nerves, and finally to the 

 sympathetic nervous system. 



1. — The Brain and Spinal Cord.* 



(Figs. 28, 30, 31). 



The brain of the Plaice may be conventionally 

 divided into four regions, including the following 

 structures : — 



A. Hind - Brain- — This comprises the medulla 

 oblongata, which itself includes many structures that can 

 only be regarded as the continuations of corresponding 

 ones in the spinal cord, and the cerebellum. The latter 

 consists of a body and the anterior valvula cerebelli. 



B. Mid -Brain. —Formed by a base (crura cerebri) and 

 side wall, and the tectum opticum or tectum mesencephali 

 (optic lobes). 



C. 'Tween-Brain. — Represented by three parts : (1) 

 the epithalamus (epiphysis generally and the ganglia 

 habenulae) ; (2) the thalamus (optic thalami — thalamence- 

 phalon) ; (3) the hypothalamus (corpus geniculatum, 



* The following works will be found to contain references either to the 

 brain of the Plaice or to allied Pleuronectids : — Cattie, Arch. Biol., iii.. 

 p. 150; le Roux, "Recherch. Syst. Nerveux Teleosteens," Caen, 1887 

 Mayne, "Optic Nerves," Todd's Cyclopsedia, part xxvi. ; Malme, Bihang 

 K. Svens. vet.-akad Handlingar, xvii. ; Mayer, Verhand. K. Leop. -Carol., 

 xxx. ; and Steiner, " Entstehung d. asymmetrischen Baues der Pleuronec- 

 tiden," 1886 ; a recent important work, by J. B. Johnston, on the brain of 

 Acipenser (Zool. Jahrb., Abth. Morph., xv.), may be used as a starting- 

 point in studying the brain of Fishes in detail.^ 



