appearing to be composed of extremely minute disks of a partially opaque substance, imbedded at regular intervals in a more transparent matter. A more faint striation, separating these disks into longitudinal series, is also observable. When the sarcolemma is torn, the contractile substance of dead muscle may, under some circumstances, be either divided into disks (Fig. 99, C), but it may be

A, a muscular fibre, devoid of sarcolemma, and breaking up at one end into its fibrilla; B, separate fibrilla; C, a muscular fibre breaking up into disks; D, a muscular fibre, the contractile substance of which (a) is torn, while the sarcolemma (6) has not given way. (Magnified about 350 diameters.) more readily broken up into minute fibrilla (Fig. 99, A, B), each of which, viewed by transmitted light, presents dark and light parts, which alternate at intervals corresponding with the distances of the transverse striæ in the entire fibre. Nuclei are observed here and there in the contractile substance within the sarcolemma.

In the heart, the muscular fibres are striated, and have the same essential structure as that just described, but they possess no sarcolemma.

A. Seen longitudinally. The width of the meshes corresponds to that of an ultimate fibre. a. small artery; b, small vein.

B. Transverse section of striated muscle. a, the cut ends of the ultimate fibres; b, capillaries filled with injection material; c, parts where the capillaries are absent or not filled.

Smooth muscle consists of elongated band-like fibres, devoid of striation, each of which bears a rod-like nucleus. These fibres do not break up into fibrillæ, and have no sarcolemma (Fig. 101).

16. Nervous tissue contains two elements, nerve-fibres and ganglionic corpuscles. Ordinary nerve-fibres, such

as constitute the essential constituents of all the cerebrospinal nerves except the olfactory, are during life, or when perfectly fresh, subcylindrical filaments of a clear, somewhat oily, look. But shortly after death, a sort of coagulation sets up within the fibre, and it is then found to be composed of a very delicate, structureless, outer membrane (which is not to be confounded with the neuri

lemma), forming a tube, through the centre of which runs the axis cylinder, which is probably composed of an aggregation of very fine filaments. Between the axis cylinder and the tube is a fluid, rich in fatty matters, from which a solid strongly refracting substance has been thrown down and lines the tube.

a

FIG. 101. Smooth or non-stri..

ated muscular fibres of a small artery; the middle one having

from the middle coat

been treated with acetic acid, shows

Such is the structure of all the larger nerve fibres, which lie, side by side, in the trunks of the nerves, bound together by delicate connective tissue, and enclosed in a sheath of the same substance, called the nzurilemma. In the trunks of the nerves, the fibres remain perfectly distinct from one another, and rarely, if ever, divide. But when the nerves enter the central organs, and when they approach their peripheral terminations, the nerve-fibres frequently divide into branches. In any case they become gradually finer and finer; until at length, axis-cylinder, sheath, and contents are no longer separable, and the nerve fibre is reduced to a delicate filament, the ultimate termination of which, in the sensory organs and in the muscles, is not yet thoroughly made out.

17. In Lesson VIII. mention is made of peculiar bodies called tactile corpuscles, which are oval masses of specially modified connective tissue in relation with the ends of the nerves in the papillæ of the skin. In Fig. 103 four such papillæ, which have been rendered transparent and stripped of their epidermis, are seen, and the largest contains a tactile corpuscle (e). mode, in which nerves not connected with tactile corpuscles end in the skin, is not definitely known.

more distinctly the nucleus a. (Magnified about 350 diameters.)

This

In muscles, the nerve-fibre seems to pierce the sarcolemma and to end inside the ultimate muscular fibre in a peculiar knob or plate.

In the brain and spinal cord, on the other hand, it is

certain that, in many cases, the ends of the nerve-fibres are continued into the processes of the ganglionic corpuscles. 18. The olfactory nerves are composed of pale, flat fibres

A, a nerve-fibre in its fresh and unaltered condition; B, a nerve-fibre in which the greater part of the sheath and coagulated contents (a b) have been stripped off from the axis cylinder (cc); C, a nerve-fibre, the upper part of which retains its sheath and coagulated contents, while the axis cylinder (a a) projects; D, a ganglionic corpuscle-a, its nucleus and nucleolus. (Magnified about 350 diameters.)

without any distinction into axis-cylinder and contents, but with nuclei set at intervals along their length.

Similar fibres are found in the sympathetic nerves, mingled with fibres of the same structure as those of the spinal nerves.

19. Ganglionic corpuscles are chiefly found in the cerebro-spinal axis; in the ganglia of the posterior nerve roots, and in those of the sympathetic; but they occur also elsewhere, notably in some of the sensory organs (see Lesson IX.).

They are spheroidal bodies, consisting of a soft semisolid cell substance in the midst of which is a large

FIG. 103.-PAPILLE OF THE SKIN OF THE FINGER.

a, a large papilla containing a tactile corpuscle (e) with its nerve (d): b, other papillæ, without corpuscles, but containing loops of vessels, c. (Magnified about 300 diameters.)

clear and transparent area usually termed the nucleus. Within the nucleus again is generally a smaller body commonly termed the nucleolus (Fig. 102, D, a). Each ganglionic corpuscle sends off one, two, or more prolongations, which may divide and subdivide; and which, in some cases, unite with the prolongations of other ganglionic corpuscles, while, in others, they are continued into nerve-fibres.