(The arrangement of this section has been considerably altered.)

[The lymphatic system of the frog may, for descriptive purposes, be advantageously treated of in four parts:

1. The subcutaneous lymph-sacs.

2. The lymph-hearts.

3. The lymph-spaces of the body, and those lying between the various organs.

4. The lymph-vessels of the separate organs.

The first two parts will be described in this section; the remainder with the organs or parts of the body with which they are more intimately associated. The spleen will be described with the abdominal viscera.

1. The Subcutaneous Lymph-Sacs.

A number of large spaces exist between the skin and the muscles, in which there is no direct attachment between these parts. These spaces are separated from each other by thin connective-tissue septa, stretched between the muscles and the skin, and by attachments of the skin to the underlying structures.

Joh. Müller first demonstrated that these spaces were connected with the lymphatic system, and that the contained fluid was lymph. Although this fact was contradicted by Meyer it received confirmation from Stannius, Leydig, and especially from Recklinghausen, who showed that the fluid of the lymph-sacs was carried away by the lymphatics, and that later it entered the blood-stream.

A. The Lymph-sacs of the Head and Trunk.

a. The septa. The connective-tissue septa themselves contain sinuses which occupy the space between their two surfaces (Fig. 166) these septal sinuses receive the contents of the cutaneous lymph-vessels and forward their contents into lymph-vessels situated in the septa. Such a sinus is shown in transverse section in Fig. 166 which is a section of the ventral septum. The same sinus, in plan, is seen in Fig. 167.

(1) The dorsal septum (Septum dorsale), (Figs. 169 and 171 d) of each side commences at the tympanic membrane; it is then attached to the M. depressor maxillae, M. infraspinatus, and M. longissimus dorsi; and continued backwards over the M. obliquus abdominis externus to the anterior end of the M. glutaeus, whence it extends along the iliac bone, where it is connected with the posterior lymph-heart and the M. pyriformis, to the tip of the urostyle. At the tip of the M. glutaeus it is joined by the iliac septum. The posterior part of the septum is horizontal, and forms the roof of the iliac lymph-sac.

(2) The ventral septum (Septum abdominale), (Figs. 169, 170, and 171 a), commencing at the Symphysis pubis, runs forwards and outwards, on either side, along the outer border of the M. rectus abdominis to the Portio abdominalis of the M. pectoralis (p‴), where it is attached, at a right angle, to the pectoral septum; it then extends forwards, after attachment to the axillary septum, to the outer angles of the maxillary septum. At the root of the anterior extremity it divides to enclose the root of the limb, and so form the axillary septum.

(3) The pectoral septum (Septum pectorale), (Figs. 170 and 171 p) is attached to the superficial surface of the M. pectoralis (Portio sternalis, anterior and posterior); it runs transversely across the body, and is attached at its outer end to the ventral and to the maxillary septa. The septum passes obliquely backwards and downwards to be attached to the skin, and so forms a very acute angle with the M. cutaneus pectoris (cp): a vertical band of connective-tissue, broad behind, narrow in front, extends in the middle line from the septum to the body wall; from it a certain amount of loose connective-tissue extends in all directions, and forms open lymph-spaces between the MM. cutanei pectorales and the vertical septum.

(4) The perineal septum (Septum perineale), (Fig. 170 pe). This median septum extends from the attachment of the dorsal septum to the tip of the urostyle, backwards to the symphysis pubis, where it joins the ventral septa. In its attachment it follows the middle line of the perineum.

(5) The maxillary septum (Septum maxillare), (Figs. 170, 171 m) is attached by either extremity to the dorsal septum, near the tympanic membrane, is continued downwards to unite with the anterior end of the ventral septum, and then runs across the anterior pectoral region, forming a curve with the convexity forwards. It has inserted into it fibres of the M. submaxillaris. The septum is formed of very loose tissue, and frequently contains deposits of fat; it encloses a well-marked septal sinus, the Sinus thoracicus transversus (Ecker).

(6) The iliac septum (Figs. 169, 172 15, 173 S′) is a septum extending from the dorsal septum to the inguinal septum. It is attached to the dorsal septum opposite the anterior end of the M. glutaeus, and extends outwards and downwards to the inguinal septum. In it the M. cutaneus iliacus passes from the trunk to the skin.

(7) The inguinal septum (Lamina inguinalis, Ecker), (Figs. 169 171), the dotted line near 15, Fig. 172, separates the belly from the thigh; on the ventral surface it is attached to the groove which forms the boundary between the belly and thigh; on the dorsal surface it is more posterior, and is attached to the dorsal surface of the muscles of the thigh at some little distance from the trunk. It completely surrounds the root of the hinder limb. It has attached to it the ventral, dorsal, and iliac septa, together with the septa of the thigh.

b. The lymph-sacs.

(1) The dorsal lymph-sac (Saccus cranio-dorsalis), (Figs. 169 and 171 1). This is a large lymph-sac, extending from the tip of the snout to the tip of the urostyle; it is bounded in front by the attachment of the skin to the premaxillary bones; the line of attachment is continued, on either side, along the inner border of the external nares, then forms a pouch towards the jaw and in front of the eye, and runs backwards along the upper border of the orbit, where it is attached to the upper eyelid, to the inner border of the tympanic membrane. In this course the skin is firmly attached to the underlying parts. The lateral boundary of the lymph-sac, behind this point, is formed by the dorsal septum, which separates it from the lateral lymph-sac.

(2) The ventral lymph-sac (Saccus abdominalis), (Figs. 170, 171 2) is triangular in form, with the base forwards at the breast, and the apex behind at the pelvic symphysis; it occupies the space between the skin below and the belly muscles and part of the M. pectoralis above. It is bounded anteriorly by the pectoral septum (p) and laterally by the ventral septa (a).

(3) The lateral lymph-sac (Saccus lateralis), (Figs. 169, 170, and 171 3) exists on either side; below, the sac is bounded by the ventral septum (a), above by the dorsal septum (d), anteriorly by the maxillary septum (m), and posteriorly by the inguinal septum (i) and the wall of the iliac lymph-sac.

(4) The submaxillary lymph-sac (Saccus submaxillaris), (Figs. 170 and 171 4) is a space between the M. submaxillaris and the skin; the sac is bounded behind by the maxillary septum, which separates it from the pectoral lymph-sac, and more laterally from the lateral lymph-sac. In front and at the sides the skin is firmly attached to the margin of the mandible.

(5) The pectoral lymph-sac (Saccus thoracicus), (Figs. 170 and 171 5) lies between the submaxillary and ventral lymph-sacs. The sac is bounded behind by the pectoral septum (p), and in front by the maxillary septum (m).

(6) The iliac lymph-sac (Saccus iliacus), (Figs. 171 and 172 15, 173) does not really belong to the subcutaneous lymph-sacs, as it does not lie directly under the skin except by a very narrow border. It is bounded in front by the iliac septum, above by the dorsal septum, and posteriorly by the inguinal septum. By these boundaries a space is enclosed, which lies under the dorsal septum and dorsal sac, and at the same time partly under the lateral sac. In this sac the M. glutaeus, the anterior ends of the M. vastus externus, M. rectus anterior, and M. cutaneus iliacus, lie free together with the hindmost part of the M. obliquus abdominis externus, and the hinder portion of the posterior lymph-heart. Anteriorly the floor is depressed between the M. obliquus abdominis and the M. glutaeus, the depression leading to a canal, which communicates with the abdominal cavity.

B. The Lymph-sacs of the Anterior Extremity.

a. The septa.

(1) The axillary septum (Septum axillare), (Figs. 170 a′, a″, 171) is practically a portion of the ventral septum, which divides at the root of the arm to enclose it. The dorsal and ventral portions of this circular septum have received special names.

α. The dorsal axillary septum (Septum axillare dorsale) crosses the M. triceps and joins the pectoral septum.

β. The ventral axillary septum (Septum axillare dorsale) passes through the axilla and joins the pectoral septum.

The circular axillary septum has attached to it the following:‍—

(2) The posterior brachial septum (Septum brachiale posticum) (Fig. 169 s); it is attached along the middle line of the extensor surface of the arm (long head of the M. triceps) to the elbow and to the forearm (MM. anconaei), then passes obliquely over the volar surface of the hand to the fourth finger.

(3) The anterior brachio-ulnar septum (Septum brachiale anticum ulnare), (Fig. 170 s″) commences at the same point as the foregoing, passes obliquely over the outer head of the M. triceps, and is continued over the M. flexor antibrachii lateralis, M. supinator longus, and M. extensor digitorum communis, to the dorsal surface of the fourth finger.

(4) The anterior brachio-radial septum (Septum brachiale anticum radiale), (Fig. 170 s‴) passes from the outer angle of the wall of the pectoral lymph-sac, over the inner head of the M. triceps and M. flexor carpi radialis to the thumb.

b. The lymph-sacs are enclosed by these three septa; they are four in number:‍—

(1) The brachio-ulnar lymph-sac (Saccus brachialis ulnaris), (Fig. 170 6) is situated between the first and second septa along the extensor and ulnar surfaces.

(2) The brachio-radial lymph-sac (Saccus brachialis radialis), (Fig. 169 7) is placed along the extensor and radial surfaces.

(3) The anterior brachial lymph-sac (Saccus brachialis anticus), (Fig. 170 8) lies on the flexor surface between the second and third septa; it is a long cavity and interrupted by connective-tissue bands, in which the tendon of the M. sterno-radialis and the nerve and vessels are situated.

(4) The axillary lymph-sac (Saccus axillaris) corresponds to the iliac lymph-sac; it is bounded by the divisions of the ventral septum.

In the hand no large lymph-space exists; the skin is attached by numerous bands to the underlying structures.

C. The Lymph-sacs of the Hinder Extremity.

a. The septa.

(1) The superior femoral septum (Septum femorale superius), (Figs. 169 and 175 f′) is attached above to the inguinal septum near the posterior lymph-heart; and is continued, from this point, over the M. vastus externus and along the M. triceps to the knee.

(2) The inferior femoral septum (Septum femorale inferius), (Figs. 170 and 175 f″) runs from the inguinal septum near the point of insertion of the M. rectus abdominis, along the M. rectus internus minor to the knee.

(3) The intermediate femoral septum (Septum femorale intermedium), (Figs. 169 and 175 f‴) passes from near the posterior lymph-heart over the M. semimembranosus and the M. rectus internus minor to join the inferior femoral septum.

(4) The tibio-femoral septum (Septum femoro-crurale) is a circular band, separating the lymph-sacs of the thigh from the leg sac.

(5) The septa of the foot. At the ankle joint the skin is closely attached all round to the underlying parts. Along the outer border of the foot a septum is attached to the skin externally, and to the underlying parts, especially the M. abductor longus digiti primi, internally. A similar but weaker band is attached along the outer border of the foot, especially to the M. adductor digiti quinti.

b. The lymph-sacs.

(1) The femoral lymph-sac (Saccus femoralis), (Figs. 169, 170, 171, 175 9) covers the lower and outer surfaces of the thigh; it is bounded by the superior femoral and inferior femoral septa, and in front by the inguinal septum.

(2) The suprafemoral lymph-sac (Saccus suprafemoralis), (Figs. 169, 171, 175 10) lies on the upper surface of the thigh, between the superior and intermediate femoral septa; above it is bounded by the inguinal septum.

(3) The interfemoral lymph-sac (Saccus interfemoralis), (Figs. 169, 170, 175 11) is a narrow sac on the inner surface of the thigh, between the inferior femoral septum and the intermediate femoral septum. It covers the M. rectus internus minor, and is interrupted by numerous bands of tissue which pass from the surface of the muscle to the skin. It is, therefore, not a simple sac, but a very wide-meshed trabecular structure of connective-tissue. The sac is triangular in shape, with the base directed forwards at the inguinal septum.

(4) The lymph-sac of the leg is a simple sac enclosing the whole leg, bounded above by the tibio-femoral septum, and below by the attachment of the skin to the ankle.

(5) The lymph-sacs of the foot. On the dorsum of the foot the skin is free, and there is consequently a lymph-sac. On the plantar surface the skin is attached by numerous connective-tissue bands and thread-like tendons, particularly to the flexor tendons. The skin of the dorsal and plantar surfaces meet on the web and enclose a very rich anastomosis of lymph-capillaries.

2. The Lymph-Hearts.

The frog has two pairs of lymph-hearts, one pair anterior, the other posterior.

A. The anterior lymph-hearts (Fig. 176). These organs lie, one on each side, behind the broad transverse processes of the third vertebra (Müller, Panizza, Priestley), in a deep triangular space formed by the separation of the fibres of the M. intertransversarius, between the transverse processes of the third and fourth vertebrae. Each heart is a rounded sac, slightly elongated anteriorly where it is connected with the subscapular vein. [The hearts receive lymph from the anterior part of the body and the surrounding parts, and empty their contents into the vertebral vein. Each of these hearts is supplied by a branch from the second spinal nerve (Volkmann, Eckhard, Schiff, Priestley).]

B. The posterior lymph-hearts (Fig. 177) are situated on either side of the urostyle in the triangular spaces (Müller, Panizza, Priestley), bounded externally and above by the M. glutaeus, internally and above by the M. coccygeo-iliacus, below and externally by the origin of the M. vastus externus, and below by the M. pyramidalis. Each posterior heart is about two lines long and one broad, with its long axis placed antero-posteriorly; the outer surface is uneven and appears to be unequally dilated. The lymph-heart is closely attached to the surrounding parts, especially to the fascia covering the M. ilii-coccygea, and posteriorly to the M. levator ani and the M. pyramidalis. [The posterior lymph-hearts receive lymph from the parts surrounding them, and from the hinder extremities, and forward it into the V. iliaca communicans.

The posterior lymph-hearts are each supplied by a branch from the corresponding coccygeal nerve (Waldeyer) by its dorsal branch. The lymph-hearts are also in close connection with the sympathetic system (Waldeyer).]