Pagina-afbeeldingen
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of the vas deferens is analogous to the vesiculæ seminales of other animals, secreting its own fluid with which the semen is to be mixed before it enters the penis, and the valvulæ conniventes form a surface from which this secretion probably takes place.

It is deserving of observation, that the epididymis and vas deferens were loaded with semen, which there is reason to believe is not the case in quadrupeds, unless immediately previous to the act of copulation; but in fishes, where the connexion between the sexes is less complete, the semen appears to be prepared at a more early period.

The openings from the vasa deferentia are situated on each side, four inches higher up than those of the ureters, rounded in form, and kept closed by the pressure of two oviform ligamentous substances; these openings readily admit of dilatation, so that the hand can pass into them.

The penis has an infundibular form, and terminates by an oblique aperture about three inches in circumference.

The holders correspond with those of the dog-fish, which upon another occasion have been described to the Society; the spur bears a striking resemblance to that of the male ornithorhynchus paradoxus. There is a canal in each holder communicating with a corresponding cavity between the skin and muscles of the abdomen, eleven feet long and two wide. The inner surface of this cavity is smooth, almost polished, and of a beautiful white colour; it contained a white mucus, extremely viscid and tenacious.

Opportunities of examining the brain of a fish of such magnitude are of rare occurrence; I have therefore not only given drawings of the brain, (Pl. XIX, XX.) but also one

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of the brain of a Squalus Acanthias, (Pl. XXI.) that the two may be compared together, with a view to shew the relative size of the parts, one belonging to a shark of thirty feet, the other to one of three feet long.

In the brains of what we consider the animals of greatest intelligence, there is a cerebrum, cerebellum, and medulla oblongata; beneath the cerebrum there are the tubercula quadrigemina. In fishes, the cerebrum is wanting, and there is no part at all analogous to it, unless we consider the enlargements, from which the olfactory nerves arise, to be of that description.

These enlargements are separated from the other parts of the brain by the optic nerves going off in a transverse line between them and the tubercula quadrigemina. In the present specimen, unfortunately, not only the enlargements from which the olfactory nerves go off, were destroyed; but also a portion of each of the anterior tubercula quadrigemina: the cerebellum was, however, entire, and is represented of the natural size,

The brain does not occupy more than one-third of the cranium. The medulla spinalis is large in proportion to the brain. From the termination of what corresponds to the calamus scriptorius in the human brain, a fissure extends on the upper part of the medulla spinalis, dividing it longitudinally into two portions; there is a similar fissure on the anterior surface; into both of these a thin fold of pia mater extends and adheres with firmness to the surfaces with which it comes in contact.

The dura mater is very dense, and adheres firmly to the inner surface of the cranium and theca vertebralis. The pia

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mater nearly resembles that of the human brain ; it becomes thicker where it covers the spinal marrow. The space

between the dura and pia mater is occupied by a cellular membrane of a very fine texture.

As the different parts of the brain are described in the explanation of the drawing, I shall only remark in this place, that the circumstance most deserving of observation respecting it is, that the cerebellum has an increase of size in the Squalus Maximus in a much greater degree beyond that of the Squalus Acanthias than the tubercula quadrigemina. The protuberances from which the olfactory nerves arise were probably large in the same proportion with the tubercula ; at least, in the brain of a shark, preserved in the Hunterian Collection, of a smaller size than that of the Squalus Maximus, but much larger than that of the Squalus Acanthias, that is the case. The eye is small for the size of the fish; the ball has

projections on the sclerotic coat, where the muscles are attached, which make it approach to a quadrangular form; but its internal cavity is circular. The circumference in the widest part is nine inches. The longest diameter three inches, the shortest one inch and three quarters. The sclerotic coat is cartilaginous, one quarter of an inch thick on the posterior part, becoming thinner towards the ciliary processes, where it is only one-sixteenth of an inch.

The cornea is thin, but made up distinctly of three layers, of which the middle one is by much the thinnest. The optic nerve is nearly of the size of the sixth pair, and, where it

perforates the sclerotic coat, projects a little before it gives off the retina, which is extremely thin.

The choroid coat is covered with a tapetum lucidum of the

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colour of an amalgam of silver broken into small portions. There are ciliary processes, not common to fishes in general; they are about one-third of an inch in extent, and slightly projecting; they are lined with a black pigment.

The vitreous humour is unattached to the choroid coat; it is inclosed in strong cells, and the crystalline lens, which is spherical and one inch in diameter, is imbedded in it for twothirds of its substance. In the Tetrodon Mola, which, as well as the Squalus Maximus, in common language has been called the Sun-fish, the vitreous humour has a firm attachment to a groove in the choroid coat one-twelfth of an inch in breadth, extending from the entrance of the optic nerve to the termination of the retina, in the shortest line from the one to the other, and there are no ciliary processes; two such remarkable differences in eyes of nearly the same size, appeared to be deserving of being noticed.

The cartilage upon which the ball of the eye rests is attached to the bottom of the orbit, and is seven inches and a half long; its stem is a flattened cylinder, five-eighths of an inch in diameter; it terminates in a broad concave surface in a transverse direction adapted to the bottom of the ball of the eye, and is connected to the sclerotic coat by a ligament long enough to admit of motion.

There are four straight and two oblique muscles ; the straight very large, much beyond what can be required merely to move so small an eye: the rectus internus and externus are strongest, they are five inches in circumference, while the superior and inferior are only three and a half.

In the structure of the ear, the only remarkable circumstance is the great capacity of the cavities in which the semi

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circular canals are contained, the canals themselves not being much larger than in the skate. They are shewn in the drawing. In the recent subject they were pellucid, containing a transparent fluid.

On the Structure of the Branchial Artery. The muscular structure of the branchial artery of the dogfish, and the direction in which that artery leaves the ventricle, are exactly the same as in the Squalus Maximus, only are seen upon so small a scale, that they do not arrest our attention ; but when magnified, to the size which they acquire in this fish, they make a stronger impression on the mind, and force us irresistibly to enquire into their use.

This direction of the artery appears to be common to fishes in general; but the muscular structure is confined to particular tribes. I find it is common to all the sharks, and there is a similar structure in the sturgeon.

In the wolf fish, the Anarchichus Lupus, the muscular structure of the branchial artery is nearly the same; but the valves are placed close to the opening of the ventricle, and are only two in number.

In the turbot there is no muscular structure in this part of the artery; but the coats are extremely elastic, and admit of being very considerably dilated, particularly at its origin, where three valves are placed, and so contrived that the dilatation of the artery makes them shut more closely.

In the Lophius Piscatorius, there is no appearance of muscularity in the coats of the branchial artery, and no lateral valves as in other fishes; but there is a muscular tube half

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