[The parts described in this section will be considered very briefly, and only from an anatomical point of view.

A. The blood‍65.

The blood varies in colour, according as it is obtained from an artery or from a vein; from an artery it has a bright red or scarlet colour; from a vein a darker shade: in the pulmonary vessels these colours are reversed. The blood consists of a fluid plasma which contains red and white corpuscles: the whole forming a slightly alkaline, opaque, and somewhat sticky fluid.

a. The plasma or liquor sanguinis is a transparent, clear, slightly yellow fluid, faintly alkaline in reaction.

b. The corpuscles are of two kinds, colourless and coloured.

(1) The colourless or white corpuscles are subspherical masses of protoplasm, endowed with the power of spontaneous motion. They are much fewer than the red corpuscles, though the proportion of white to red varies considerably. Three chief varieties of colourless corpuscles can be distinguished, all of which are true cell forms, and behave like other cells with staining or other reagents. They have no cell-wall, but are simply nucleated masses of protoplasm. These corpuscles possess the power of passing through the walls of the blood-vessels, and are then known as migratory cells; such migratory cells can be found in nearly all tissues, but more especially in the connective-tissues.

α. Ordinary large colourless corpuscles are large transparent masses of protoplasm containing one or two nuclei, rarely three or more. The cell contains few granules, and usually has one or more vacuoles or clear spaces containing fluid.

β. Granular corpuscles are less numerous than the foregoing, but larger, and are distinguished by the numerous large dark granules which they contain. The processes such a corpuscle sends out are hyaline.

γ. Smaller corpuscles also occur varying much in form: they may consist of a nucleus with a very small amount of hyaline or granular protoplasm, or of several nuclei with an extremely small amount of protoplasm.

(2) The coloured corpuscles are much more numerous than the colourless corpuscles; each is a bi-concave, oval disc, about 0.0255 mm. in length, and 0.017 mm in breadth (Gulliver)‍66 (R. temporaria, 0.0235 and 0.0145 mm. respectively). Each corpuscle possesses an oval nucleus (Hewson)‍67, which projects into either concave surface of the whole corpuscle. These corpuscles do not possess the power of spontaneous movement; and have no cell-wall. The corpuscles are of a reddish-yellow colour, and give the blood its characteristic hue. The colouring matter is haemoglobin, and when separated it crystallizes in prisms (Preyer). The corpuscles show a well-marked intracellular and intranuclear network.]

B. [The Lymph and Chyle.

The lymph is that portion of the blood which has filtered from the blood-vessels, through the tissues, into the lymphatics. It is a clear, transparent fluid, containing colourless corpuscles, which agree in all particulars with the colourless corpuscles of the blood. During digestion the lymph from the alimentary canal contains particles of fatty matter, and is then known as chyle.] The colourless corpuscles of the lymph frequently contain portions of red corpuscles or granules of pigment. Non-nucleated masses of protoplasm and decolourised red corpuscles (slightly diminished in size) have also been described as occurring frequently in the lymph (Fuchs‍68).]

C. [Histological Remarks on the Vascular System.

a. The heart has already been described (pp. 216–222).

b. The blood-vessels present the same structure as the corresponding vessels in higher animals. The arteries and veins have each the three walls or layers usually described—Tunica adventitia, Tunica media, and Tunica intima. The only characteristic to be mentioned is that the Tunica adventitia contains branched, pigmented corpuscles. The Nervi vasorum, especially of the smaller arteries, are arranged in two plexuses, one in the Tunica adventitia (His)‍69, the second in the muscular coat (J. Arnold)‍70. The fibres of both plexuses are non-medullated, and have the usual beaded appearance; they communicate freely with each other. The capillaries present, as a rule, the usual structure: such special arrangements of the capillaries or peculiarities in structure as occur will be described with the organs in which they are found.

c. The subcutaneous lymph-sacs are lined with a layer of endothelium resembling the endocardium shown in Fig. 140; the endothelial layer possesses stomata (see peritoneum), through which the lymph-sacs communicate with the underlying lymphatics. The endothelium is continued on to the vessels, nerves, etc., which course through the sacs, and excludes these from the cavities. As a rule an artery, vein, and nerve are enclosed in a common sheath.

d. The lymph-hearts are small saccular bodies with thin transparent walls, which contain obscurely striated muscle-fibres. The vessels communicating with the hearts appear to possess valves (Müller). The walls of the hearts are formed of three layers; an external, compared by Waldeyer to an adventitia, a muscular layer, and an endothelial lining. The muscular fibres form a close interlacing mass, the fibres being small, branched (Leydig), and possessed of many nuclei. Among the muscular fibres are pigmented corpuscles, together with medullated and non-medullated nerve-fibres, but no ganglion-cells (Volkmann, Waldeyer, Priestley). The adventitia is a connective-tissue layer with many pigment cells (Leydig, Hyrtl, Waldeyer).]