Fig. 45.—Brain of Petromyzon. (Enlarged, after Müller.)
I., Upper; II., Lower aspect.
a, Medulla oblongata; ac, nerv. acusticus; b, corpus restiforme or rudimentary cerebellum; d, lobus ventriculi tertii; d’, entrance into the third ventricle; c, hypophysis; fa, nerv. facialis; g, nerv. opticus; h, hemisphere; hy, nerv. hypoglossus (so named by Müller); i, lobus olfactorius; k, sinus rhomboidalis; l, epiphysis; m, nerv. oculo-motorius; q, corpora quadrigemina; tr, nerv. trigeminus; tro, nerv. trochlearis; v, nerv. vagus.
The brain of the Cyclostomes (Figs. [44], [45]) represents a type different from that of other fishes, showing at its upper surface three pairs of protuberances in front of the cerebellum; they are all solid. Their homologies are not yet satisfactorily determined, parts of the Myxinoid brain having received by the same observers determinations very different from those given to the corresponding parts of the brain of the Lampreys. The foremost pair are the large olfactory tubercles, which are exceedingly large in Petromyzon. They are followed by the hemispheres, with a single body wedged in between their posterior half; in Petromyzon, at least, the vascular tissue leading to an epiphysis seems to be connected with this body. Then follows the lobus ventriculi tertii, distinctly paired in Myxinoids, less so in Petromyzon. The last pair are the corpora quadrigemina. According to this interpretation the cerebellum would be absent in Myxinoids, and represented in Petromyzon by a narrow commissure only (Fig. [45], b), stretching over the foremost part of the sinus rhomboidalis. In the Myxinoids the medulla oblongata ends in two divergent swellings, free and obtuse at their extremity, from which most of the cerebral nerves take their origin.
The Nerves which supply the organs of the head are either merely continuations or diverticula of the brain-substance, or proper nerves taking their origin from the brain, or receiving their constituent parts from the foremost part of the spinal chord. The number of these spino-cerebral nerves is always less than in the higher vertebrates, and their arrangement varies considerably.
A. Nerves which are diverticula of the brain (Figs. [41–45]).
The olfactory nerves (first pair) always retain their intimate relation to the hemispheres, the ventricles of which are not rarely continued into the tubercle or even pedicle of the nerves. The different position of the olfactory tubercle has been already described as characteristic of some of the orders of fishes. In those fishes in which the tubercle is remote from the brain, the nerve which has entered the tubercle as a single stem leaves it split up into several or numerous branches, which are distributed in the nasal organ. In the other fishes it breaks up into branchlets spread into a fan-like expansion at the point, where it enters the nasal cavity. The nerve always passes out of the skull through the ethmoid.
The optic nerves (second pair) vary in size, their strength corresponding to the size of the eye; they take their origin from the lobi optici, the development of which again is proportionate to that of the nerves. The mutual relation of the two nerves immediately after their origin is very characteristic of the sub-classes of fishes. In the Cyclostomes they have no further connection with each other, each going to the eye of its own side.[11] In the Teleostei they simply cross each other (decussate), so that the one starting from the right half of the brain goes to the left eye and vice versa. Finally, in Palæichthyes the two nerves are fused together, immediately after their origin, into a chiasma. The nerve is cylindrical for some portion of its course, but in most fishes gradually changes this form into that of a plaited band, which is capable of separation and expansion. It enters the bulbus generally behind and above its axis. The foramen through which it leaves the skull of Teleostei is generally in a membranous portion of its anterior wall, or, where ossification has taken place, in the orbito-sphenoid.
B. Nerves proper taking their origin from the brain
(Figs. [41–45]).
The Nervus oculorum motorius (third pair) takes its origin from the Pedunculus cerebri, close behind the lobi inferiores; it escapes through the orbito-sphenoid, or the membrane replacing it, and is distributed to the musculi rectus superior, rectus internus, obliquus inferior, and rectus inferior. Its size corresponds to the development of the muscles of the eye. Consequently it is absent in the blind Amblyopsis, and the Myxinoids. In Lepidosiren the nerves supplying the muscles of the eye have no independent origin, but are part of the ophthalmic division of the Trigeminus. In Petromyzon these muscles are supplied partly from the Trigeminus, partly by a nerve representing the Oculo-motor and Trochlearis, which are fused into a common trunk.