THE BLOOD, LYMPH, AND HISTOLOGY OF THE VESSELS.
[The parts described in this section will be considered very briefly,
and only from an anatomical point of view.
A. The *blood*[65].
[Footnote 65: Mr. Hurst has noticed a frog (R. temporaria) in which the
blood was perfectly colourless.]
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)[66], 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.]
[Footnote 66: Gulliver, Proc. Zool. Soc. London, 1845, Vol. XIII, p. 93
seq.]
[Footnote 67: Hewson, Phil. Trans. 1773, Vol. LXIII, p. 310 seq.]
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 (Fuchs[68]).]
[Footnote 68: Fuchs, Virchow’s Arch. 1877, Vol. LXXI, p. 78 seq.]
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.
[Footnote 69: His, Virchow’s Arch. 1863, Vol. XXVIII, p. 427.]
[Footnote 70: Arnold, in Stricker’s Handbuch d. Gewebelehre, p. 137.]
*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).]