Fig. [29] is a section cut from one of the palate teeth of the ray (Myliobátes).

A rather important element in the structure of animals is the “elastic ligament,” which is found in the back of the neck and other parts of the body, especially about the spine. It is made of a vast number of fibres of variable shape and length, branching and communicating, arranged generally in bundles, and remarkable for containing very few vessels, and no nerves at all. At Fig. [14] may be seen an example of elastic ligament, popularly called “paxwax,” taken from the neck of a sheep.

The white fibrous tissue by which all the parts of the body are bound together is seen at Fig. [10]; and at Fig. [11] is a beautiful example of the “ultimate fibres” of the crystalline lens of a sturgeon’s eye.

The muscles of animals are of two kinds, the one termed the striped, and the other the unstriped. Of these, the latter belongs to organs which work independently of will, such as the stomach, etc., while the former belongs to those portions of the body which are subject to voluntary motion, such as the arm and the leg. The unstriped muscle is very simple, consisting merely of long spindle-shaped cells, but the striped or voluntary muscle is of more complex construction. Every voluntary muscle consists of myriads of tiny fibres, bound together in little bundles, enveloped in a kind of sheath. Fig. [24] is an example of this muscular fibre, taken from beef. When soaked in spirit, it often splits into a number of discs, the edges of which are marked by the transverse lines.

A fibre of nerve is drawn at Fig. [23], and is given for the purpose of showing the manner in which the nerve is contained in and protected by its sheath, just like a telegraph-wire in its coverings. Just above is a transverse section of the same fibre, showing the same arrangement from another point of view, and also illustrating the curious phenomenon, that when nerve-fibres are treated with carmine the centre takes up the colouring matter, while the sheath remains white as before. The best way of studying nerves is to decapitate a frog, and cut off a piece of one of the nerves, which, like fine silk threads, come out between the joints of the spine inside the abdomen. By careful teasing out it is easy to obtain preparations showing all the above points, and, in addition, the folding-in of the internal sheath which correspond to the insulators of a telegraph-line.

The blood of animals is analogous in its office to the sap of plants, but differs greatly from it under the microscope. In sap there seem to be no microscopic characters, except that when a branch is cut, as in the vine, the flowing sap may contain certain substances formed in the wounded cells, such as chlorophyll, starch, and raphides; but the blood is known to be an exceedingly complex substance both in a microscopic and a chemical point of view. When a little fresh blood is placed under the microscope, it is seen to consist of a colourless fluid filled with numerous little bodies, commonly called “blood-globules,” varying very greatly in size and shape, according to the animal from which they were taken. Those of the reptiles are very large, as may be seen at Fig. [4], Plate X., which represents a blood corpuscle of the Proteus. In this curious reptile the globules are so large that they may be distinguished during its life by means of a common pocket lens.

In the vertebrated animals these corpuscles are red, and give to the blood its peculiar tint. They are accompanied by certain colourless corpuscles, spherical in form, which are sometimes, as in man, larger than the red globules, and in others, as in the siren and the newt, considerably smaller. The general view of the red corpuscles has sufficient character to enable the practised observer to name the class of animal from which it was taken, and in some cases they are so distinctive that even the genus can be ascertained with tolerable certainty. In point of size, the reptiles have the largest and the mammalia the smallest, those of the Proteus and the musk-deer being perhaps the most decidedly opposed to each other in this respect.

IX.

FIG.
1.Amœba diffluens21.Head of Snake-headed Zoophyte
2.Arcella22.Bugula avicularia
3.Sun animalcule23.Polyzoön, Eretea
4.Miliolina24. Do.  Notamia
5.Paramœcium25.Zoea, Young of Crab
6.Chilodon subdividing26.Hydra tuba
7.Melicerta ringens27.Medusa, cast off from above
8.Spicula of Sponge, Grantia28.Naked-eyed Medusa, Thaumantias
9.Noctiluca miliaris29.Compound Eye, Medusa
10.Rotifer vulgaris30.Larva, Snake Star
11. Do.  jaws31.Water Flea
12.Sponge animalcule32.Serpula, Pushing Pole
13.Sertularia operculata33.Comatula, early stage of Starfish
14.Sponge, Grantia34.Carbonate of Lime, artificial
15.Sertularia operculata, with ovicells35.Sea Urchin, transverse section of spine
16.Actinia, showing weapons36.Serpula, bundle of spears
17. Do. base of weapon more magnified 37.Sun-star, part of skin
18.Sponge granule, ciliated38.Oyster shell in different stages
19.Anguinaria anguina39.Cilia on mussel
20.Spicules of sponge from Oyster Shell