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the skin of the glans penis and glans clitoridis. Each consists of a thin connective tissue capsule enclosing a core of homogeneous or nucleated semifluid substance.
As the nerve-fibre pierces the capsule, it loses its medullary sheath, and the axis cylinder is continued into the core of the bulb where it may pursue a somewhat tortuous course, but more frequently divides into minute varicose fibrils which form an intricate plexus. The end bulbs of the glans penis and glans clitoridis are named genital corpuscles and differ from those just described in that they are larger and possess thicker capsules. Similar endings, termed articular bulbs, are found in the synovial membranes of certain joints, e.g. those of the fingers.
A, composed of three cells with two inter-
cylinder of the nerve-cell is observed
In B there is but one
(2) Corpuscles of Grandry (Fig. 741).-These are seen in the skin covering the beaks of aquatic animals, and in the mucous membrane of the duck's palate. Each consists of two or more flattened epithelial cells enclosed within a capsule, and the axis cylinder ends in flattened "tactile discs" which lie between the cells.
(3) Corpuscles of Pacini (Fig. 742).—These are widely distributed and consist of small oval bodies which measure from 2 to 3 mm. in length and about 1 mm. in width. They are found on the cutaneous nerves of the hand and foot, on the infraorbital and intercostal nerves, on the cutaneous nerves of the neck, nipple, and mamma, and on the nerves of the solar plexus. They are present in the parietal peritoneum and on the nerves of the joints, and are very plentiful in the mesentery
of the cat. The capsule of the corpuscle consists of a number of connective tissue tunics arranged concentrically around a central core of more or less clear protoplasm; the deeper tunics are closely applied to each other, but those towards the circumference of the corpuscle are here and there separated by narrow lymphatic spaces. Each corpuscle is attached to a nerve trunk by a narrow pedicle composed of a single medullated nerve-fibre which pierces the capsule and, on reaching the core, loses its medullary sheath. The axis cylinder is continued into the core as
far as its distal end and there terminates in one or more enlargements in which the neuro-fibrillæ form a dense plexus. The corpuscles of Herbst (Fig. 743), which are found in the skin of birds, differ from the Pacinian corpuscles in that their cores consist of nucleated cells, between which the axis cylinder extends as a single or branched process.
(4) Corpuscles of Golgi and Mazzoni.-These are present in the subcutaneous tissue of the pulp of the fingers and also in other parts of the skin. Their capsules are thinner and their cores thicker than those of the Pacinian corpuscles, while their axis cylinders undergo a greater degree of ramification and their terminal filaments end in somewhat flattened expansions.
(5) Tactile Corpuscles of Wagner and Meissner (Fig. 742).-These are plentifully distributed in the papillæ of the corium of the hand, foot, and front of the forearm. They are found also in the skin of the lips, in the mucous membrane of the tip of the tongue, in the palpebral conjunctiva and the skin of the nipple. They are oval in shape, and their length varies from 04 mm. to 15 mm., and their thickness from 03 mm. to 06 mm. One or more nerve-fibres pierce the capsule of the corpuscle, losing, at the same time, their medullary sheaths. The axis cylinders, which are frequently varicose, assume a spiral or convoluted course and end in terminal enlargements. From the deep surface of the capsule imperfect membranous septa are continued inwards between the nerve ramifications.
FIG. 744.-AN ORGAN OF RUFFINI FROM THE SUBCUTANEOUS TISSUE (Ruffini). (From Quain's Anatomy.) a, Entering nerve-fibres; b, d, endings of their axons; e, c, capsule of organ; c', core.
(6) Organs of Ruffini (Fig. 744). These were found by Ruffini in the subcutaneous connective tissue of the fingers. They are of considerable size, and their shape is oval or fusiform. One or more nerve-fibres penetrate the side of the capsule, within which they pursue a curved course and then lose their medullary sheaths.
FIG. 745.-ORGAN OF GOLGI FROM THE HUMAN TENDO-CALCANEUS, CHLORIDE OF GOLD
m, Muscular fibres; t, tendon bundles; G, Golgi's organ; n, two nerve-fibres passing into it.
The axis cylinders break up into a close-meshed network which lies between, or partly encircles, the smaller fasciculi of connective tissue.
(7) Neuro-tendinous Spindles (Fig. 745).-These were first described by Golgi in
1878. They consist of long spindle-shaped bodies, and are usually found near the junctions of the tendons with their muscles. Each is surrounded by a capsule which encloses a number of intrafusal tendon fasciculi. The nerve-fibres pierce the side of the capsule and then lose their medullary sheaths; the axis cylinders subdivide, and their terminal branches ramify between, or partly encircle, the smaller tendon bundles and end in plate-like expansions.
(8) Neuro-muscular Spindles (Fig. 746).These are widely distributed throughout the voluntary muscles but are more numerous in the muscles of the limbs than in those of the trunk, and are plentifully found in the muscles of the hand and foot. They have not yet been seen in the intrinsic muscles of the tongue, and only a few are present in the ocular muscles. They lie in the connective tissue between the muscular bundles, and each consists of a lamellated capsule enclosing a fasciculus of striped muscular fibres (intrafusal fibres), together with minute blood-vessels and three or four medullated nerve-fibres. These intrafusal muscular fibres display many of the characteristics of embryonic muscle; they are smaller both in length and diameter than ordinary muscular fibres; they contain numerous nuclei near the centre of the spindle where their cross striation is less distinct; they also possess more protoplasm than ordinary muscular fibres. The nerve-fibres pierce the side of the capsule, inside which they lose their
medullary sheaths and undergo subdivision; they are then prolonged in a spiral or annular manner around the individual muscular fibres and end in flattened or ovoid enlargements.
Three intrafusal muscle fibres are shown: x, nerve-fibres entering spindle; a, axis cylinders terminating around and between the intrafusal fibres in b, ring-like; c, spiral; and d, irregularly ramified endings.
THE VASCULAR SYSTEM.
BY THE LATE ALFRED H. YOUNG,
Professor of Anatomy, University of Birmingham.
REVISED BY ARTHUR ROBINSON, M.D.,
THE vascular system consists of a series of tubes, called vessels, which run through all parts of the body. Some of the vessels contain a coloured fluid called blood, others are filled with a colourless fluid, called lymph; hence the distinction between the blood-vascular system and the lymph-vascular system. The two systems differ, not only as regards their contents, but also in their relations. to the tissues amongst which they lie; for whilst the vessels of the blood-vascular system, with the possible exception of the splenic vessels, are closed, those of the lymph-vascular system communicate with the serous sacs.
The tubes or vessels of the blood-vascular system vary in size and in the structure of their walls, but all contain blood, which is conveyed, through them, to and from the tissue elements of the body. The blood is propelled along the vessels chiefly by a central propulsive organ-the heart. The outgoing vessels from the heart, along which blood is transmitted to the tissues, are termed arteries; the vessels which return blood from the tissues to the heart are known as veins; whilst the smallest tubes-those which connect the arteries and veins together, constituting at once the terminations of the arteries and the commencements of the veins are called capillaries.
Blood capillaries are very small (hair-like) vessels, with exceedingly thin walls which permit of the easy passage of the nutritive materials outwards from the blood to the tissues, and, of the passage in the opposite direction, of some of the products of tissue changes and of food material absorbed from the alimentary canal. Arteries and veins are simply conducting passages; structurally they differ from capillaries in the greater complexity of their walls. They vary greatly in size, but are always larger than capillaries. The calibres of the arteries and veins increase progressively from the periphery up to the heart, where both sets of vessels reach their greatest size. With the increase in calibre there is a corresponding increase in the thickness and complexity of the walls of the vessels.
Structure of Blood Capillaries.-Blood capillaries measure from 8 to 12.5 μ in diameter, and about 75 mm. in length. Their walls are simple, and, in the smallest capillaries, consist of elongated elastic endothelial cells, with sinuous edges, pointed extremities, and oval nuclei. The cells are cemented to one another, along their margins, by intercellular cement, which readily stains with nitrate of silver. Here and there the cement substance appears to accumulate, forming minute spots indicative of the less perfect apposition of the edges of the cells. Such spots when small, form the so-called stigmata; when larger they are known as stomata.
The larger capillaries are invested by a connective tissue sheath consisting of branched cells which are united together and to the endothelial cells of the capillary wall. This sheath is termed the tunica externa capillaris.
Capillaries are arranged in networks, the nature and character of which differ 56 a
in different tissues. The small arteries which end in them are known as capillary arterioles, and the venous radicles which commence from them are appropriately termed capillary veins.
Structure of Arteries and Veins.-The delicate elastic endothelial membrane which forms the wall of the simplest capillaries extends also, as a continuous lining, throughout the whole of the blood-vascular system. In the arteries the constituent cells are fusiform, narrow, and pointed, whilst in the veins they are somewhat shorter and broader.
The most essential structural difference between capillaries on the one hand and the arteries and veins which they unite together on the other, is the presence, in both the arteries and the veins, of involuntary muscular fibres which are interposed between the endothelial lining and the outer connective tissue sheath. In small vessels, e.g. capillary arterioles, the muscle cells are few in number and more or less scattered. In larger vessels the walls are stronger and thicker, muscular fibres are more numerous and form a continuous layer, whilst yellow elastic and ordinary white connective tissue are present in varying proportions. The walls of the larger vessels are, therefore, complex, and numerous strata may be distinguished; which, for convenience, are regarded as forming three layers, known as the tunica intima and the middle and outer tunics. Superadded to the tunics is the investing fibrous sheath or vagina vasis.
Structure of Arteries. The walls of arteries are stronger and thicker than those of veins of corresponding size, the tunica intima and middle tunic being particularly rich in elastic and muscular elements.
Tunica Intima.-In the tunica intima the endothelial lining is strengthened by the addition of yellow elastic tissue, the fibres of which are arranged in such a manner as to simulate a fenestrated membrane. In arteries of medium size the elastic lamina is separated from the endothelium by a layer of connective tissue consisting of branched cells and numerous fibrils. In the larger arteries the subendothelial connective tissue is considerably increased, and delicate elastic fibres appear which connect it with the more externally situated and fenestrated elastic layer.
Tunica Media. In the capillary arterioles the tunica media consists solely of scattered unstriped muscle fibres; the individual fibres are circularly disposed, but do not entirely surround the vessel. In small arteries the muscle cells are so much increased in number that they form a continuous though thin layer. The larger arteries have two or more layers of muscle cells, and the greater thickness of their walls is mainly due to the increase of the muscular elements of the middle coat. In the larger vessels delicate laminae of elastic tissue alternate with the layers of muscular fibres, and in the aorta and the carotid arteries, as well as in some of the branches of the latter, the elastic elements largely preponderate. In the first part of the aorta, in the pulmonary artery, and in the arteries of the retina, the muscular fibres are entirely replaced by elastic tissue.
Tunica Externa.-The outer tunic of an artery consists almost entirely of fibrillated connective tissue, in which lie many connective tissue corpuscles.
FIG. 747.—STRUCTURE OF BLOOD-VESSELS (diagrammatic). A1, Capillary-with simple endothelial walls. A2, Larger capillary with connective tissue sheath, "adventitia capillaris." B, Capillary arteriole-showing muscle cells of middle coat, few and scattered. C, Artery-muscular elements of the tunica media forming a continuous layer.
FIG. 748.-TRANSVERSE SECTION
A, Tunica intima. B, Tunica media.