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LYMPHATIC VESSELS OF THE HEAD AND NECK. 327

staining the surface with a sohition of nitrate of silver. At intervals
l)et\veen these cells are found large free openings which have received
the name of stomata. These openings communicate by means of short
canals with the lymph-capillaries that are found among the fibrous
tissue of \\hicli the diaphragm is composed. Upon the pleural surface
similar openings have also been dcnu)nstrated. The serous cavities
of the body may therefore be regarded as true lymph-spaces, which
communicate primarily with the lymph-capillaries, and secondarily with
the lymphatic trunks. Stomata in all respects similar to those found
on serous membranes have been shown to be present on the surfaces of
mucous membranes, which in all probability are directly coimected with
the lymph-capillaries.
3. Origin in Periv the brain, spinal cord, bone, and other tissues His and Robin have
shown that the capillary blood-vessels are surrounded by a lymph-space
bounded and limited by a cylindrical sheath formed of endothelial cells,
which is in frequent communication with the lymph-capillaries. The
blood-vessel thus floats in the lymph-stream. In addition, the tunics
of the large blood-vessels, both the intima and adventitia, are traversed
by lymph-channels which open very freely into each other. This
arrangement of the blood-vessels permits of a free interchange by
osmosis of the fluid portion of both blood and lymph.
Structure of Lymphatic Vessels.—The lym])hatic trunks have
their origin in the fine plexus of lyniph-capillaries prc\iously described.
In their course toward the centre of the body they pursue generally a
direct route. They anastomose by bifurcation very freely with neigh-
boring vessels, pass through the lymphatic glands, and vary but little
in size from origin to termination. Their walls are so exceedingly trans-
is with difficulty they can be
parent and delicate that when empty it
seen. Their diameter varies from ^ to yV i"<^'l^- ^^^^^' ^'/^ lymphatic
trunks have emerged from the lymph-capillaries they acquire three dis-
tinct coats, which resemble in their structure and arrangement the coats
of the veins.
The internal coat is delicate and elastic, and is composed of a layer
of longitudinal elastic fibres covered with a layer of flattened nucleated
endothelial cells' with wavy or sinuous margins. The middle coat con-
sists of white fibrous tissue, which is arranged longitudinally, and of
unstriped muscular and elastic fibres, which are disposed transversely.
The external coat is composed of identically the same structures, but
the muscular fibres pursue rather a longitudinal than a transverse direc-
tion. These three coats are known respectively as the tunica intima,
tunica media, and tunica adventitia. The walls of the lym])hatic
trunks are abundantly su]>plied with blood-vessels {ram vamnoii), and
it is highly probable* that they are also supplied with nerves {)i('rvce
vasorum), though the latter have not been indisputably demonstrated.
On physiological grounds their existence might be inferred.
Valves.—The lymphatics generally are provided with valves, which
have the same structure and fulfil the same function as the valves with
which the veins are furnished. These valves are very numerous, and
are located at such short intervals along the course of the vessel as to give