The lymphatic system of the cat has not been worked out in detail, so
that only the main features of the system are given in the following
account.
The lymphatic system consists of a number of vessels, the =lymphatics=,
containing a colorless fluid called =lymph=, and of =lymphatic glands=,
connected with the lymphatic vessels. The lymphatic vessels are found
throughout the body as slender tubes, frequently united into networks,
and containing many valves. They take origin from the spaces in the
connective tissue, so that they are at first without definite walls. The
fluid in the connective-tissue spaces gradually flows together into
tubes with definite walls, and these tubes, the lymphatic vessels,
finally join the venous system. In their passage through the lymphatic
glands the vessels break up into fine channels which again unite on
leaving the gland. In the gland lymph-cells are added to the fluid, and
probably other changes are produced.
Most or all of the lymphatic vessels of the body finally unite into two
trunks before entering the veins. The largest trunk is the =thoracic
duct= (Fig. 118, 5, page 282), which collects the lymph from the entire
caudal half of the body, and from the left forelimb and the left side of
the thorax, head, and neck. It passes along the dorsosinistral side of
the thoracic aorta, lying against its surface, and enters the left
external jugular vein at its junction with the subclavian. The second
main trunk is the =right lymphatic duct=; this collects the lymph from
the right side of the thorax, the right forelimb, and the right side of
the neck and head; it enters the right external jugular.
1. =Lymphatics of the Head.=--A number of lymphatic vessels arise on the
sides of the face, especially in the upper and lower lips. They form a
superficial network of vessels, lying over the ventral half of the
masseter muscle and an area ventrad of it. These lymphatic vessels all
enter two large lymphatic glands (Fig. 131, 12, page 322) lying at the
ventrocaudal angle of the masseter muscle, covering the union of the
anterior and posterior facial veins. Into these glands pass also a
number of lymph vessels from the back of the head. From these two
lymphatic glands two or three small lymphatic vessels pass caudad, lying
on the surface of the external jugular vein. Near the point of the
shoulder, some distance craniad of the junction of the external and
internal jugular veins, these vessels enter another small lymphatic
gland lying on the dorsal surface of the external jugular vein. Thence
one or two vessels continue caudad, still on the surface of the external
jugular, and finally unite with the deep lymphatics of the head and neck
and those of the arm to enter the caudal end of the external jugular
vein, usually (on the left side) after junction with the =thoracic
duct=, from the thorax.
The deep lymphatics of the head come from the internal parts of the
head,--tongue, pharynx, etc., and enter a large lymphatic gland situated
close to the caudoventral surface of the tympanic bulla, at the side of
the pharynx. Thence a large lymphatic trunk passes caudad at the side of
the trachea, along with the common carotid artery and internal jugular
vein, unites with the superficial vessels from the head, above
described, joins (on the left side) the thoracic duct, and enters the
external jugular vein.
2. =Lymphatics of the Neck.=--Two or three lymph-glands are found in the
fatty mass in the neck beneath the clavotrapezius and levator scapulæ
ventralis muscles. These receive lymphatics from the neck and side of
the thorax. The vessels which arise from them join those from the head
and arm to enter the external jugular vein.
3. =Lymphatics of the Thoracic Limbs.=--The lymphatics of the forelimbs
begin as a number of vessels on the dorsum and palm of the hand. These
all pass toward the radial side, and at about the base of the first
metacarpal the vessels from both sides of the hand unite to form two or
three main trunks. These pass towards the elbow, accompanying the vena
cephalica (Fig. 130). There are usually two of these main trunks, one
lying on each side of the vena cephalica. They accompany this vein over
the shoulder, following that portion which finally joins the external
jugular. In the hollow of the shoulder they enter the =cervical
lymph-glands=, one or two lymph-glands imbedded in the fat lying beneath
the clavotrapezius and levator scapulæ ventralis in this region. From
the cervical lymph-glands a single trunk passes mediocaudad, unites with
the lymphatics from the head, and usually (on the left side) with the
cranial end of the thoracic duct, and enters the external jugular vein
at its junction with the subclavian. The common trunk from the head and
arm may enter the jugular separately, but close to the termination of
the thoracic duct. On the right side the common trunk is joined by one
or more small vessels coming from the lymphatic glands of the thorax and
forming the right lymphatic duct.
There is said to be also a deep system of lymphatics in the arm, in
addition to the superficial system above described; this is said to
accompany the branches of the brachial vein. If this system is present
in the cat, it is much less easily demonstrated than the superficial
system.
4. =Lymphatics of the Thorax and Abdomen.=--In the thorax are a number
of lymphatic glands which are apparently not constant in number and
position. One is commonly found dorsad of the bifurcation of the
superior vena cava, one ventrad of the mammary vein, several small ones
about the bifurcation of the trachea, one or more in the region of the
aortic arch, and a number of small ones scattered in the mediastinum.
The vessels from these glands finally join either the right lymphatic
duct or the thoracic duct; the details have not been worked out and are
probably variable.
In the abdominal cavity a number of =mesenteric= glands, of considerable
size, are found in the mesentery and in the mesocolon. In the mesocolon
these are usually separate, forming a chain of glands following the
colon about one and a half centimeters from it. In the mesentery the
glands are mostly united into a very large one, formerly known as the
=pancreas aselli=; this is the largest lymph-gland in the body. It is a
curved structure, four or five centimeters in length, lying in the
central region of the mesentery.
Into the mesenteric glands pass lymphatic vessels from the viscera of
the abdomen. From the mesenteric glands one or two large lymphatic
vessels pass craniodorsad, at first with the portal vein, then dorsad of
it. This vessel reaches the dorsal side of the aorta near the cranial
end of the kidney, where it enters a large fusiform vessel, the
=receptaculum chyli=, which stretches from the cranial end of the kidney
craniad between the crura of the diaphragm into the thorax. The
receptaculum receives other lymphatics from the various organs of the
abdominal cavity as well as vessels coming from the pelvic region, and
from the =iliac= glands. These lie beside the iliac artery, receive
lymphatics from the hind limbs, and send lymphatics craniad to the
receptaculum chyli. The latter forms the beginning of the =thoracic
duct=.
The thoracic duct (Fig. 118, 5, page 282) enters the thoracic cavity
between the crura of the diaphragm as a continuation of the receptaculum
chyli. It lies on the dorsal side of the aorta (_a_), somewhat to the
left. It passes along the aorta, gradually coming to lie more on its
left side, and finally leaves that vessel near the aortic arch. The duct
continues thence straight craniad along the left side of the œsophagus
till it reaches the external jugular vein at its junction with the
subclavian. In this region the thoracic duct usually unites with the
lymphatics coming from the head and arm, then enters the external
jugular vein, or the innominate at the point of union of external
jugular and subclavian.
In its passage through the thorax the duct frequently divides into two
or three divisions which pass along parallel to each other for a
distance, then reunite. Such division into two or three parts is
especially frequent just before the union with the jugular vein. The
numerous valves in the thoracic duct give it a beaded appearance.
5. =Lymphatics of the Pelvic Limbs.=--A number of lymphatic vessels are
formed on the dorsum and the sole of the foot. These pass to the lateral
side, forming a network on the lateral and dorsal surface of the
ankle-joint, and finally unite into two main trunks which accompany the
vena saphena parva. In the popliteal space these enter the =popliteal
gland= which lies imbedded in the fat of that region. From the popliteal
gland one or two trunks accompany the superficial division of the vena
saphena parva across the lateral surface of the biceps muscle, and
finally thus reach the pelvic cavity. Here they join the lymphatics of
the pelvic region, which all pour their lymph at last into the thoracic
duct. Inguinal glands are either very small or not present in the cat.
The pelvic limbs have perhaps a deep system of lymphatics, accompanying
the deep veins; if so, they are not easily demonstrable.
[Illustration: FIG. 133.--CRANIAL PORTION OF THE SPINAL CORD, WITH THE
CERVICAL ENLARGEMENT. DORSAL VIEW.
1-8, the eight cervical nerves. _I_-_II_, first two thoracic nerves.
_a_, dorsal roots (ventral roots not shown); _b_, spinal ganglia; _c_,
dorsal rami; _d_, ventral rami; _e_, posterior median sulcus; _f_,
posterior lateral sulcus.]