1. An external investure of areolar tissue.

2. A layer of coarse yellow elastic fibres, with some plain muscular fibres, arranged in a circular or oblique direction. The elastic fibres of this layer branch in a penniform manner, as will be seen when a small portion of the tunic is teazed out and examined with a high power. Between these penniform fibres the smooth muscular fibres are found.

3. A layer of coarse yellow fibres running in a longitudinal direction, and interlacing closely with each other.

4. A thin layer of fine elastic fibres, also taking a longitudinal course. This tunic may be readily stripped off the interior of an artery, by first cutting it across very lightly, and then taking the edge of the cut membrane in the forceps and gently peeling it away. Under a high power the fine elastic fibres will be apparent. Sometimes the membrane is pierced with numerous apertures, constituting the fenestrated membrane of Henle.

5. A layer of oval or fusiform epithelial cells, which should be scraped from the interior of an artery of an animal recently killed, as they are separated soon after death.

The fine elastic layer with the epithelium represents the serous lining membrane of the older authors.

In the illustration, taken from a longitudinal section of the aorta of a horse, the different tunics are shown in their relation to each other.

Veins. The coats of veins are thinner than those of arteries, but correspond to them in structure and arrangement.

Thus we observe an external coat of areolar tissue; next a circular arrangement of course yellow fibres, with plain muscular fibres and white fibrous tissue, corresponding to the circular fibrous coat of arteries, but much less developed: then the two layers of yellow fibres arranged longitudinally, the outer layer being coarse and the inner fine in texture, and having upon its surface the single layer of epithelial cells.

The majority of veins possess semilunar processes, or valves projecting into their interior. These valves are composed of the same tissue as the fine fibrous tunic, and are covered by a layer of epithelium.

Capillaries. Intermediate between arteries and veins, there are exceedingly fine hair-like tubes, termed capillaries, in which arteries terminate, and from which veins arise, excepting in a

very few instances where arteries communicate directly with veins, as in the cavernous structure of the penis.

Capillaries are composed of a fine homogeneous membrane, with here and there oval nuclei. In some of the larger tubes a distinct arrangement of circular fibres may be seen, resembling smooth muscular fibre, but differing from it in the absence of any trace of nuclei.

In some of the organs of the body, the liver for example, it is impossible to distinguish distinct walls of the capillary vessels, in the mass of epithelial cells in which they are imbedded.

The drawing represents some of the capillaries of the human brain.

Fig. 127.

4

d

2. Transition

Finest vessels of the arterial side. 1. Smallest artery.
vessel. 3. Coarser capillaries. 4. Finer capillaries. a. Structureless
membrane, still with some nuclei, representative of the tunica adventitia.
b. Nuclei of the muscular fibre-cells. c. Nuclei within the small artery,
perhaps appertaining to an epithelium. d. Nuclei of the capillaries of
the transition vessels. From the human brain.-Magnified 300 times.

Lymphatic vessels possess still more delicate coats than veins, to which they bear a general resemblance in the arrangement of their elements, and in the existence of valves in their interior.

The external investment is composed of areolar tissue; next there is a layer of circular fibres with fibres like those of unstriped muscle, having elongated nuclei; then a longitudinal coat of fine fibres, like those of connective tissue; and upon this a single layer of epithelial cells.

OF THE NERVOUS SYSTEM.

THE study of the tissues belonging to this system will be much facilitated by a judicious selection of specimens.

In order to obtain a view of an entire nerve, one of the small cutaneous dorsal branches should be taken from the frog, by dividing the skin of the back carefully along the centre of the spinal column, avoiding to cut the roots of the nerves.

On reflecting back the integument the small superficial nerves will be exposed to view, like so many fine threads, extending from the muscles to the skin. A portion, about one-eighth of an inch, of one of the smallest of these is to be cut off with fine scissors, placed in a drop of water on the slide, and gently drawn into a straight line by the aid of the needles; the thin glass cover is then to be applied, and the specimen at once examined with the low power.

The object is an exceedingly beautiful one if well prepared, consisting of a sheath of areolar tissue, with a bundle of nerve fibres in its centre, and frequently a small bloodvessel enclosed in the same sheath, as shown in the illustration.

The addition of a drop of acetic acid will render the sheath much more transparent, and, after a few seconds, bring into view the 'connective tissue corpuscles.'

The nerve will generally appear crooked, as in the drawing; this arises from the contraction of the fibrous sheath, which had previously been stretched, but after being cut shrinks considerably and causes the appearance observed.

Branch of nerve from the dorsal region of a frog. A. Areolar tissue.
B. Bundle of nerve fibres assuming a serpentine course and dividing
into two branches before leaving the common sheath. c. Bloodvessel
also branching.

Examination of Nervous Fibrilla.-After a general observation of the structure of an entire nerve, the student may proceed to examine the fibres of which it is composed. For this purpose it is best to select the largest nerve in the frog's body, the sciatic, which will readily be found as a thick white cord, lying between the muscles at the back of the thigh.

A piece of this nerve, about one-eighth of an inch long, is to be cut off and placed on the slide in a little water, or, what is much better, serum or white of egg, and carefully teazed out in a longitudinal direction, that the minute fibres may be separated from each other without being broken; the covering glass should

then be applied very lightly, and the specimen examined with a high power.

The ultimate fibres of the nerve will be recognised by their peculiar transparency, appearing like fine threads of spun glass. After a few minutes this pellucid aspect is lost, and the fibres are seen to consist of the three parts shown at 3 in the illustration, which represents the nerve fibres of several animals.

Nerve fibres; magnified 350 times. 1. Of the dog and rabbit, in the natural
condition. a. Fine, b. middling thick. c. Thick fibre, from the peri-
pheral nerves. 2. Of the frog, with the addition of serum. a. Drop
forced out by pressure. b. Axis cylinder in the same, continuing into
the tube. 3. Of the fresh spinal cord of man, with the addition of
serum. a. Investment. b. Medullary sheath, with double contours.
c. Axis cylinder. 4. Double contoured fibre of the human fourth ven-
tricle, the axis cylinder (a) projecting and visible in the fibre. 5. Two
isolated axis cylinders from the cord, the one undulated, the other un-
equally thick, with adherent medulla.

In size the fibrillæ vary, even in the same nerve, from 12000