The blood is collected from the foot into two sets of veins which may be distinguished as the deep and the superficial sets. The veins of the deep set are those which accompany the branches of the popliteal and anterior tibial arteries. Those of the superficial set are larger and carry most of the blood. On the dorsum of the foot blood is collected from the third, fourth, and fifth digits by two or three superficial metatarsal veins, which unite opposite the proximal end of the metatarsus into a single vein. This receives a similar branch coming from the dorsum of the second digit, and has numerous branches in the ankle region which form here a network, communicating with the vena saphena parva ([Fig. 163], k), and with branches from the plantar side of the foot. From this network two or three main veins pass proximad on the medial side of the leg, along with the branches of the arteria saphena, and unite with a similar vein from the plantar side of the foot to form the vena saphena magna ([Fig. 127], k, [page 310]), which accompanies the arteria saphena to join the femoral vein (a).
On the plantar side of the foot small veins pass from the digits to a superficial venous plantar arch, which lies just proximad of the fibrous pad on the sole of the foot. At its medial side this arch extends proximad and dorsad to join that branch of the vena saphena magna which comes from the dorsum of the second digit. On the lateral side it extends proximad along the lateral margin of the foot, sends a communicating branch dorsad to the branches of the V. saphena magna, and continues along the lateral border of the foot to the ankle. It forms the beginning of the V. saphena parva ([Fig. 163], k, [page 401]). This receives branches from the lateral surface of the ankle, which anastomose with those of the saphena magna. The saphena parva then passes proximad along the lateral surface of the leg, close to its ventral border, to the popliteal space. At the distal end of the popliteal space it divides into two branches. One passes inward through the fat of the popliteal space, receiving branches from the lymphatic gland situated here, and joins the popliteal vein. The other (k′) remains superficial and continues proximad over the lateral surface of the biceps to the proximal end of that muscle. Here it passes between the biceps and caudofemoralis to join the inferior gluteal vein.
IV. THE LYMPHATIC SYSTEM. SYSTEMA LYMPHATICUM.
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.