NERVOUS SYSTEM. 207 



system of lateral ventral muscles, which, as a rule, is absent in other 

 Fishes, and consists of an oblique and a straight ventral muscle ; by 

 means of these the powerful movements and vermiform deflections of 

 these animals are effected. 



NERVOUS SYSTEM. 



THE Nervous System also of Fishes, especially the Brain, exhib- 

 iting numerous diversities, it will be best to become first ac- 

 quainted with the regular arrangement of these structures as they are 

 found in the majority of Osseous Fishes, and then to describe their 

 varieties in the several genera and families of that sub-class ; lastly, 

 their mode of formation in the remaining orders. 



The ordinary type of structure in the Brain of Osseous Fishes is 

 to be observed in many Acanthopterygians, e. g., Perca fluviatilis, 

 and Malacopterygians, as the Pike (Esox lucius), in both of which 

 it presents a very close conformity ; in the common Carp, however, 

 which, from its frequent occurrence, has been chiefly recommended 

 for the purposes of dissection, we already encounter peculiar cere- 

 bral anomalies ; and other species of that genus, as Cyprinus barbus, 

 are therefore better suited for examination. 



The Brain in general does not nearly fill up the cranial cavity, so 

 that between the dura mater that lines the internal surface of the 

 cranium and the soft membrane which very closely invests the brain 

 itself, we find a free space filled up by a quantity of loose cellular 

 tissue, interrupted throughout by adipose cells. A fluid oil is fre- 

 quently found floating in large drops between the meshes of this tis- 

 sue. The membranes situated between the dura and pia mater may 

 be viewed as the arachnoid ; the analogy being still more obvious 

 where they cover the third ventricle. The dura mater is often of a 

 silvery lustre, or partially coated by black pigment. 



We shall do best to commence the consideration of the several 

 parts of the brain with the medulla oblongata, which, though it 

 differs slightly, is to be distinguished from the rest of the spinal 

 marrow by being broader and flatter. Upon the medulla oblongata 

 may be distinguished four thickened tracts, two superior and two in- 

 ferior ; the first are slightly enlarged, from the corpora restiformia, 

 and recede from each other in the middle line, so that the floor 

 of the fourth ventrical or rhornboidal sinus lies freely exposed to 

 view ; they give off processes that are prolonged, forming its crura, 

 into the cerebellum. Some transverse fibres upon the inferior sur- 



