42 



CLASS XIV. 



Let us now consider shortly those organs in fishes which have 

 reference to the functions of animal life. Here in the first place 

 the nervous system claims attention. Of this the central parts, the 

 spinal marrow and brain, offer commonly a different relation to each 

 other than in the higher vertebrates. The mass of the spinal 

 marrow or cord, in proportion to that of the brain, is very large ; 

 the cord extends, with few exceptions, to the end of the vertebral 

 column. Consequently a cauda equina is only seldom present: 

 a disposition by which the last spinal nerves arising far from the 

 place where they pass outwards from the vertebral column, and so 

 also leaving the cord under a very acute angle, lie close together in 

 a bundle 1 . On the under and upper surface the spinal cord is 

 divided longitudinally by a fissure into two lateral parts. In the 

 interior, through the entire cord, there runs a narrow canal which 

 extends into the brain to the fourth ventricle, into which it expands. 



The brain is small, not only, as we have said, in proportion to 

 the preponderant spinal marrow, but also to the whole body, of the 

 weight of which it commonly forms less than the TWo" tn or 

 even the Q'QQ th part 2 . This small magnitude of the brain may be 

 inferred from the smallness of the cranial cavity, though even this, 

 at least in bony fishes, is still much larger than the brain which it 

 includes. For there remains between the delicate membrane which 

 immediately covers the surface of the brain and the hard membrane 

 that covers the inner surface of the cranial bones, a space which is 

 occupied by a loose cellular tissue, a species of arachnoid, which is 



Salmones (Coregonus palcea, Cuv.) in AGASSIZ Hist, natur. des Poissons d'eau douce, 

 i Livr., Neuchatel, 1842 ; DUVERNOY Sur le dtveloppement de la Poecilie de Surinam, 

 Ann. des sc. natur., 3ime Serie, I. 1844, PP- 3 X 3 S^o. PI. 17. On the development 

 of rays and sharks may be consulted, RATHKE JBeitrage zur Gesch. der Tkierwelt, iv. 

 1827, s. 4 66, and F. S. LEUCKART Untersuchungen iiber die dusseren Kiemen der 

 Embryonen von Rochen u. ffayen, Stuttgart, 1836, Svo. 



1 In the sun-fish (Orthagoriscus), according to ARSAKY, and also in Diodon, accord- 

 ing to OWEN (Lectures on Comp. Anat. II. p. 173), there is a very short, conical spinal 

 cord, with a cauda equina; in Lophius the cord is also short, but extends as far as the 

 twelfth vertebra ; here also there is a cauda equina, which in part covers the spinal cord. 



2 In a pike the cerebral mass was found to be j^Q-g- of the weight of the body ; in 

 a sheat-fish, scheidfisch or sly silurus (silurus glanis), only . * ? . of it. Comp. HALLER 

 Elem. PJiysiol. iv. pp. 5, 6, and CUVIER Lee. d'Anat. comp. u. p. 152, where several 

 examples are adduced. The weight of the brain in a full-grown man may be estimated 

 at about JQ. -^. of the weight of the whole body, although such estimates cannot easily 

 be rigorous, since the weight of the human body is so different in different individuals. 



