muscle and binding down the popliteus muscle. By its inferior border this fascia is inserted into the popliteal line of the tibia.

The fascia dorsales pedis are three in number, superficial, middle and deep. The fascia dorsalis pedis is thin and covers the dorsum of the foot, being conrinuous at the ankle with the cruciform ligament, and attached at either side to the borders of the foot, where it becomes united with the plantar fascia. A second or middle layer of fascia covers in the anterior part of the extensor brevis digitorum with its tendons; and a third or deep layer is found beneath the latter, and covering in the interossei muscles. On the interossei muscles the three layers are united, and the extensor tendons are consequently separated from each other in their course to the toes.

The PLANTAR FASCIA consists of three portions, middle and two lateral.

The middle portion (fascia plantaris media) is thick and dense, and composed of strong aponeurotic fibres, closely interwoven with each other. It is attached posteriorly to the inferior surface of the tuberosities of the os calcis, and terminates under the heads of the metatarsal bones in five fasciculi. Each of these fasciculi bifurcates to embrace the base of the corresponding toe, and is attached in the middle to the sheath of the flexor tendons, and at either side to the head of the metatarsal bone. The point of division of the fascia into fasciculi is strengthened by transverse bands, which preserve the solidity of the fascia at its broadest part. The intervals between the toes give passage to the digital arteries and nerves and lumbricales muscles.

The lateral portions (fascia interna et externa) are thin, and cover the sides of the sole of the foot; they are continuous behind with the internal and external annular ligament; and on the mesial side with the central portion. The inner portion is continuous along the border of the foot with the fascia of the dorsum; the outer portion is attached to the os calcis and base of the metatarsal bone of the little toe, forming between these points a thick band, which gives rigin to part of the abductor minimi digiti muscle.

Besides constituting a strong layer of investment and defence to the soft parts situated in the sole of the foot, these three portions of fascia send processes inwards, which form sheaths for the different muscles. A strong septum is given off from each side of the middle portion of the plantar fascia, which is attached to the tarsal bones, and divides the muscles into three groups, middle and two lateral; and transverse septa are stretched between these to separate the layers. The superficial layer of muscles derive part of their origin from the plantar fascia.

CHAPTER VI.

OF THE ARTERIES.

THE arteries are the cylindrical tubes which convey the blood from the ventricles of the heart to every part of the body. They are dense in structure, and preserve for the most part the cylindrical form when emptied of their blood, which is their condition after death: hence they were considered by the ancients as the vessels for the transmission of the vital spirits,' and were therefore named arteries (arp Trpeiv, to contain air).

The artery proceeding from the left ventricle of the heart contains the pure or arterial blood, which is distributed throughout the entire system, and constitutes with its returning veins the greater or systemic circulation. That which emaTo Galen is due the honor of having discovered that arteries contained blood, and net

nates from the right ventricle conveys the impure blood to the lungs; and with its corresponding veins establishes the lesser or pulmonary circulation.

The whole of the arteries of the systemic circulation proceed from a single trunk, named the aorta, from which they are given off as branches, and divide and subdivide to their ultimate ramifications, constituting the great arterial tree which pervades by its minute subdivisions every part of the animal frame. The mode in which the division into branches takes place is deserving of remark. From the aorta the branches, for the most part, pass off at right angles, as if for the purpose of checking the impetus with which the blood would otherwise rush along the cylinders from the main trunk; but in the limbs a very different arrangement is adopted; the branches are given off from the principal artery at an acute angle, so that no impediment may be offered to the free circulation of the vital fluid. The division of arteries is usually dichotomous, as of the aorta into the two common iliacs, common carotid into the external and internal, &c. ; but in some few instances a short trunk divides suddenly into several branches which proceed in different directions; this mode of division is termed an axis, as the thyroid and coeliac axis.

In the division of an artery into two branches, it is observed that the combined area of the two branches is somewhat greater than that of the single trunk; and if the combined area of all the branches at the periphery of the body were compared with that of the aorta, it would be seen that the blood, in passing from the aorta into the numerous distributing branches, was flowing through a conical space, of which the apex might be represented by the aorta, and the base by the surface of the body. The advantage of this provision in facilitating the circulation is sufficiently obvious; for the increased channel which is thus provided for the current of the blood, serves to compensate for the retarding influence of friction, resulting from the distance of the heart and the division of the vessels.

Communications between arteries are free and numerous, and increase in frequency with the diminution in size of the branches; so that, through the medium of the minute ramifications, the entire body may be considered as one uninterrupted circle of inosculations or anastomoses (avà, between, oroua, mouth). This increase in the frequency of anastomosis in the smaller branches is a provision for counteracting the greater liability to impediment existing in them than in the larger branches. Where freedom of circulation is of vital importance, this communication of the arteries is very remarkable, as in the circle of Willis in the cranium, or in the distribution of the arteries of the heart. It is also strikingly seen in situations where obstruction is most likely to occur, as in the distribution to the alimentary canal, around joints, or in the hand and foot. Upon this free communication existing everywhere between arterial branches is founded the principle of cure by the ligature of large arteries; the ramifications of the branches given off from the artery above the ligature inosculate with those which proceed from the trunk of the vessel below the ligature; these anastomosing branches enlarge and constitute a collateral circulation, in which, as is shown in the preparations made by Sir Astley Cooper, several large branches perform the office of the single obliterated trunk.1

2

The arteries do not terminate directly in veins; but in an intermediate system of vessels, which, from their minute size (about 3 of an inch in diameter,) are termed capillaries (capillus, a hair). The capillaries constitute a microscopie network, which is distributed through every part of the body, so as to render it impossible to introduce the smallest needle-point beneath the skin without wounding several of these fine vessels. It is through the medium of the capillaries, that all the phenomena of nutrition and secretion are performed. They are remark

I have a preparation, showing the collateral circulation in a dog, in which I tied the abdominal aorta; the animal died from over-feeding nearly two years after the operation. The only organs in which capillaries are wanting, are the corpora cavernosa penis, and uterine placenta.

able for their uniformity of diameter, and for the constant divisions and communications which take place between them, without any alteration of size. They inosculate on the one hand with the terminal ramusculi of the arteries; and on the other with the minute radicles of the veins.

ARTERIES are composed of three coats, external, middle, and internal.

The external coat [tunica adventitia,] is firm and strong, and formed of tissues which take a longitudinal or oblique direction; it is thin in the large arteries, but thicker than the middle coat in arteries of small and medium size, and composed of areolar and elastic tissue. The areolar tissue consists of fine and coarse fasciculi and separate fibrils, among which are scattered numerous elongated nuclei; in arteries of the smallest size the areolar tissue is more nucleated and loses its fibrous character, and ultimately in the minutest vessel is changed into a delicate homogeneous membrane, in which the nuclei are gradually lost. The elastic tissue exists in small quantity in the large arteries, occupies the deep layer of the external coat, and is not distinctly separable from the middle coat. In arteries of medium size it is more abundant, consists of larger fibres, and is laminated, the laminæ approaching in character to elastic membrane. In small arteries the elastic tissue exists as a fine network in the deep layer of the external coat; in vessels still smaller it presents itself in a developmental form as fusiforın cells, and in the smallest disappears altogether.

FIG. 204.

b

2

The middle or contractile coat [tunica media,] is thick and laminated, and composed of structures which take a transverse or circular direction; hence it is brittle and easily cut through when a ligature is applied. In the small arteries it is wholly composed of smooth muscular fibres; in arteries of medium size, as the external and internal carotid, brachial, crural, and popliteal, of a mixture of smooth muscular and elastic tissue; and in the large arteries, as the aorta, common carotids and iliacs, chiefly of elastic tissue. The smooth muscular tissue is the only component structure of the middle coat of small arteries; in which there are two or three laminæ, the fibres about 3 of an inch in diameter being arranged in rings; in smaller arteries there is but one muscular lamina, the elements of the fibres being shorter and in proportion as the arteries are more and more minute, becoming reduced to the earliest developmental form, namely, short elliptic cells with oblong nuclei; ultimately they are completely lost. In arteries of medium size, elastic fibres are superadded to the muscular fibres, the latter predominating for awhile, and in the large arteries the smooth muscular fibres are reduced to their cellular form and gradually diminished in number and importance, until they exist as mere rudimentary elements scattered among the layers of elastic tissue, of which the middle coat of large arteries is principally composed. The elastic tissue is absent altogether in the middle coat of small arteries, but makes its appearance in arteries of medium size as fine fibres disposed in a network of wide meshes. In the larger kind of medium-sized arteries, the elastic network is mingled with areolar tissue, and a tendency to the alternation of these tissues begins to be apparent; furthermore, in some of the largest of the medium arteries the elastic tissue is found, in parts of the vessel, to occupy the whole thickness of the

SMOOTH MUSCULAR FIBRE CELLS FROM THE MIDDLE COAT OF THE AR

TERY: 1, from the popliteal; 2, from a twig of the anterior tibial, half a line in diameter, a, a, elongated or rod-like nucleus. The middle cell (b) has been rendered transparent by immersion in vinegar. Magnified 350 times; after Kölliker.

"

middle coat, thus leading upwards to the large vessels in which the elastic tissue is the chief constituent. While thus encroaching as it were on the structure of the middle coat of arteries, the elastic tissue presents a corresponding series of transitional forms; at first it exists as fine fibres disposed singly or in a fine network with open meshes; then the fibres become larger and the meshes closer; and interlaced so as to form a fibrous membrane with narrow meshes; next by the increase of breadth of their fibres and their intimate union or fusion, a homogeneous membrane is formed, in which the meshes appear as simple perforations (fenestrated membrane). In medium-sized arteries the elastic tissue forms a single layer, lying exteriorly to the muscular fibres in the largest arteries of this class; it is mingled with areolar tissue, and exhibits a tendency to become laminated, the lamina alternating with similar layers of areolar tissue. In the large arteries this is the common character of the middle coat; the elastic tissue is laminated, and between the lamina are strata of areolar and muscular tissue. The presence of elastic tissue as a chief constituent of the middle coat of arteries, gives them a yellow color, while those arteries in which the muscular tissue is abundant have a reddish tint. In the aorta, the lamina of the middle coat are forty or fifty in number.

FIG. 207.

The internal coat [tunica intima,] is the thinnest of the three, and easily broken in the transverse direction; it is composed of two layers, an internal layer or epithelium, and an external elastic membrane, which is either homogeneous or made up of elastic elements, disposed in a longitudinal direction. In the middle-sized and large arteries there is a third and intermediate layer, consisting of an imperfect areolar tissue provided with a network of elastic tissue, and presenting in certain situations a peculiar structure which has received the name of striped lamella. The epithelium is composed of slender fusiform cells with elongated nuclei, lying parallel with the axis of the vessel; they are more lengthened in the small than in the large arteries, and in those of the smallest calibre are only distinguishable by their nuclei. The elastic layer is a thin, homogeneous membrane of the fenestrated kind, perforated with minute oblong openings having a direction parallel with the axis of the vessel. It is folded into longitudinal rug when the vessel is empty, and becomes extremely delicate and is finally lost in the minute vessels. Exteriorly

EPITHELIAL CELLS OF BLOOD VESSELS; a, b, from a vein; c, from an artery. Magnified 350 times; after Kölliker.

this layer assumes a reticulated character, and is made up of a longitudinal network of elastic fibres. The intermediate layer of the internal coat, found only in the larger arteries, is lamellated and composed of elements which have a longitudinal direction. The striped lamellæ are situated in this layer, and consist of clear lamellæ, sometimes striped, sometimes nucleated, and sometimes homogeneous. They lie immediately beneath the epithelium, and when nucleated bear so close a resemblance to that layer, as to have been regarded as a metamorphosed epithelium; when homogeneous their nearest analogue, is elastic membrane, to which tissue they appear to belong. In the internal coat of certain arteries, as of the axillary and popliteal, Kölliker has found smooth muscular fibres. In taking a summary survey of the constituent tissues of the three coats of arteries in their order of succession from without inwards, it will be seen that the external coat consists of areolar and elastic tissue; the middle coat of smooth muscular fibre, areolar and elastic tissue, and the internal coat of elastic tissue and epithelium.

The in

FIG. 208.

CAPILLARIES or capillary vessels have only one coat, which is thin, transparent, and structureless; resembling, in this respect, cell-membrane and the sarcolemma of muscular fibre; in approaching the arteries and veins this coat becomes somewhat thicker, and is finally lost in the elastic membrane of the internal coat of the arteries, and in that of the middle coat of the veins. creasing thickness of the coat of capillaries causes a distinction of these vessels into fine and coarse, and the latter gradually augmenting in size and complexity of structure, become the transitional vessels. On the inner surface of capillaries, and dispersed at short distances, are oblong nuclei; the nuclei become placed nearer together in the coarse capillaries and transitional vessels, and in the arteries and veins constitute the epithelial layer of the internal coat. In distribution, capillary vessels form plexuses by their frequent communication with each other; and these plexuses present considerable variety in the closeness of their ineshes and the number and size of the vessels. They are most abundant and largest in the lungs, liver, kidneys, and other secreting glands, also in the skin and mucous membrane, where a secreting function has to be performed; and least abundant and smallest in the muscles, nerves, organs of sense, serous membranes and tendons, where nutrition only has to be accomplished. In the bones the capillaries are large, but the plexuses scanty.

TRANSITION OF A MINUTE ARTERY INTO CAPILLARY VESSELS; from the brain: after Kölliker. 1. Minute artery. 2. Transitional capillary. 3, 3. Coarse capillaries with thick coat, represented by a double contour line. 4, 4. Fine capillaries, with single contour. The nuclei are seen widely scattered in 4 and 3: more closely congregated in 2; and still more so in 1; where they form an epithethelium. a. The transverse

The arteries, in their distribution through the body are included in a loose areolo-fibrous investment, which separates them from surrounding tissues, and is called a sheath. Around the principal vessels the sheath is an important structure; it is composed of areolar tissue, intermingled with aponeurotic fibres, and is continuous with the fascia of the region in which the arteries are situated, as with the thoracic and cervical fascia in the neck, transversalis and iliac fascia, fascia lata in the thigh, &c. The sheath of the arteries contains also their accompanying veins, and sometimes a nerve. The coats of arteries are supplied with blood like other organs of the body, their vessels proceed from the neighboring small branches, and are named vasa

elongated nuclei of muscular cells; the incipient muscular coat of the artery.