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330 — ANATOMY. —

ent vessel. In addition to this ])riiuaiy current there is a secondary
current always fiowing from the (•a[)illary blood- xessels outward into
the lymph-sinus which carries Avith it immense numbers of lymph-cor-
puscles, which enter the efferent lymphatic vessels.
The thoracic duct is the general trunk of the lymphatic system, into
which most of the lymphatic vessels of the body empty. It is from
eighteen to twenty inches in length, extending from the root of the neck
downward to the second luml)ar vertebra. It measures in diameter about
one-eighth of an inch, though at its inferior extremity, where it expands
into the receptaculum chyli, it is somewhat wider. Its walls have the
same general structure as the walls of the lymphatic vessels, consisting
of three coats—an internal, or endothelial ; a middle, elastic and nnis-
cular ; and an external, or fibrous. The inner surface of the duct is
abundantly supplied with valves. This general duct em])ties into the
venous circulation at the junction of the left internal jugular and sub-
clavian veins.

Lymphatic Vessels of the Head and Neck.
The lymphatic glands and vessels of the head and neck may be
divided into a superficial and a deep set. The superficial set may again
be subdivided, according to their location, into
1. ^1 Fdcial Group, consisting of two or three small glands situated
in front of the ear at the root of the zygoma and upon the outer aspect
of the parotid gland. A small gland is occasionally found near the side
of the root of the nose, though the rest of the facial region is singularly
free of glands, none being found above the line of the mouth (Fig. 1 57).
2. ^1 Post-aural Group, consisting of three or four glands situated
slightly above the insertion of the sterno-cleido-mastoid muscle, and one
other at the base of the occipital bone.
3. A Suhtuaxi/fart/ Group, from eight to ten in number, situated
beneath the base of the inferior maxillary bone. The largest of this
grou]) is in close relationship with the outer surface of the submaxillary
salivary gland. This grou]) lies cpiite superficially, being only covered
in by the skin and superficial fascia.
4. A Gcrvical Group, more numerous than the preceding, which is
arranged along the course of the external jugular vein. At the inferior
boundary of the neck these glands are found in greatest number, espe-
cially in the s]>ace behind tiu» insertion of the clavicular portion of the
sterno-cleido-mastoid muscle. At this ])()int they penetrate the dee])er
region of the chest and become connected with the axillary glands.
The deej) glands may also be divided, according to their location,
into
1. A Far'utl Group, from six to eight in number, situated in the
spheno-maxillary space and alongside of the pharyngeal wall.
2. A Cervical Group, located along the course of the carotid artery
and jugular vein, and extending from the uj)])er limit of the neck down-
ward as far as the thorax.
The lymphatic; vessels of the head and neck may be divided into a