This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(1832). All the above nerves posterior to the optic arise from a chain of ganglia constituting the medulla oblongata; but above these are situated other important masses entering into the composition of the encephalon, from which no nerves take their origin, viz. the cerebral hemispheres and the cerebellum.
(1833). The cerebral hemispheres in all the Vertebrata are undoubtedly the seat of the mental powers; and as this portion of the brain becomes developed and perfected, brutality and stupidity give place to sagacity and intelligence.
* Cuv. et Val. op. cit. p. 347.
(1834). In the higher quadrupeds, and more especially in Man, the proportionate size of the hemispheres of the brain is so enormous that they overlap and conceal all the parts we have been describing; but as we descend to lower forms their relative dimensions become gradually smaller and their structure less complicated, until in fishes, the least intelligent of all the creatures belonging to this great division of the animal kingdom, they are found in such a rudimentary condition that they are frequently far inferior in size even to the olfactory or optic ganglia (fig. 322, c).
(1835). The lobes representing the hemispheres in fishes (fig. 324, b) are quite smooth externally, and within are hollowed into a large ventricle, in the floor of which is seen the upper surface of the optic ganglia (fig. 324, b, d.) They present none of that complication of parts met with in the brains of higher orders: their inner surface is lined with transverse fibres (h), and a simple commissure passes across the anterior part of the ventricle, bringing the two sides into communication with each other; behind the commissure a passage leads to the third ventricle, the infundibulum, and the pituitary gland.
Fig. 324. Brain of the Perch (after Cuvier): a, cerebellum; b, cerebrum; c, olfactory ganglia; i, olfactory nerves; d, optic ganglia; g, supplementary lobe; A, transverse fibres in the walls of the cerebral ventricle; n, commissure of the optic nerves; p, q, r, s, t, u, the third, fourth, fifth, sixth, seventh, and eighth pairs of cerebral nerves.
(1836). The cerebellum (fig. 324, a) is at once recognizable from its position and singleness. In the Perch its form is that of a blunted cone, with the summit directed slightly backward; but the shape and relative dimensions of this part of the brain are extremely variable. It consists, in fishes, only of the central portion (processus vermiformis), so that there are neither lateral lobes nor pons Varolii: its surface is composed of cineritious substance, and in its centre is a ramified medullary axis containing a ventricle.
(1837). One very remarkable feature in the structure of the ence-phalon of fishes is the existence of supplementary lobes (fig. 324, g), placed behind the cerebellum, which are sometimes united by a commissure: occasionally, as in the Trigla, there are as many as five pairs of such supplementary masses; but probably, instead of regarding these as belonging to the brain, it would be more proper to consider them as being merely the first ganglia composing the spinal cord enormously developed in proportion to the importance of the nerves which they give off to the pectoral fins.
(1838). The spinal nerves of fishes arise by double roots from the sides of the medulla spinalis, which generally extends from one end of the canal formed by the superior vertebral arches to the other. The posterior roots are dilated into ganglia soon after their origin, but the ganglia are extremely minute. The spinal cord of the Moon-fish (Orihagoriscus Mola) is, however, an exception to the usual conformation: in this remarkable fish the spinal ganglia are all collected into a stunted mass placed immediately behind the brain; and from this all the spinal nerves are given off, in the same manner as those forming the cauda equina in the human subject.
(1839). The sympathetic system in the creatures we are now examining is of very small size when compared with that met with in the higher Vertebrata; nevertheless it occupies the usual position, and communicates, as in Man, with the commencements of the spinal nerves.
(1840). There are few subjects more calculated to arrest the attention of the physiologist than the progressive development of the generative system in the Vertebrate classes; and it is not a little interesting to watch the gradual appearance of additional organs, both in the male and female, as we advance upwards in the series of animated beings from the cold-blooded and apathetic fishes. In its simplest condition, the whole generative apparatus, even of a vertebrate animal, is in both sexes merely a capacious gland provided with an excretory duct, wherein, in the female, ova are secreted, and in the male a fecundating fluid is elaborated from the blood. The eggs of the female, when mature, are expelled from the nidus in which they were formed, and cast out into the surrounding water. The male, urged apparently rather by the necessity of getting rid of a troublesome burden than by any other feeling, ejects the seminal secretion in the same manner; and the fecundating fluid, becoming diffused through the waves, vivifies the eggs with which it is casually brought into contact.
Such is the whole process of reproduction in the osseous fishes.
(1841). In the females of such fishes, the ovary, or roe, as it is generally called, consists of a wide membranous bag, ordinarily divided into two lobes, but sometimes, as in the Perch, single (fig. 317, q.) This extensive organ, when distended with ova, fills a large proportion of the abdominal cavity; and its lining membrane is folded into broad festoons, wherein the ova are formed, and lodged until sufficiently mature for expulsion. When ripe, the eggs escape into the cavity of the ovary, and are expelled in countless thousands into the surrounding element through the orifice of the ovarian sac (fig. 317, r), which is situated immediately behind the anus (k) and in front of the urinary canal (s).