Part 20

_Relations._--Outer surface with the medial head of the gastrocnemius (Fig. 91, _k_) and with the superficial fascia. Inner surface with the tibialis posterior (Fig. 91, _o_). Medial border at the proximal end with the popliteus (Fig. 92, _k_), lateral border with the tibialis posterior (Fig. 91, _o_), and flexor longus hallucis (Fig. 91, _m_).

_Action._--Flexor of the phalanges.

=M. tibialis posterior= (Fig. 91, _o_).--A slender flat fusiform muscle beneath the flexor longus digitorum and between it and the flexor longus hallucis.

_Origin_ by fleshy fibres from nearly the whole medial surface of the head of the fibula, from the inner surface of the aponeurosis between it and the flexor longus digitorum (_n_), and by a few fleshy fibres from the ventral surface of the tibia between the oblique ridges. Some fibres may also arise from the outer surface of the aponeurosis covering the flexor longus hallucis (_m_). The muscle ends at about the middle of the tibia in a slender flat tendon which passes parallel to the tendon of the flexor longus digitorum (_n_) through the dorsal groove on the medial surface of the distal end of the tibia. This groove is converted into a canal by a transverse ligament. Beyond the canal the tendon turns onto the plantar surface of the foot and passes through a groove on the ventral surface of the scaphoid bone. It then divides.

_Insertion_ into the outer tuberosity on the surface of the scaphoid, and onto the proximal end of the ventral surface of the medial cuneiform.

_Relations._--Outer and medial surface with the flexor longus digitorum (_n_). Lateral surface with the flexor longus hallucis (_m_).

_Action._--Extensor of the foot.

B. MUSCLES ON THE DORSAL AND LATERAL SURFACES OF THE LOWER LEG.--=M. peroneus longus= (Fig. 90, _q_).--A slender, fusiform muscle lying superficially on the lateral side of the leg, dorsad of the soleus.

_Origin_ by tendon-fibres from the lateral surface of the head of the fibula and from the proximal half of the lateral surface of its shaft.

At the junction of the middle and distal thirds of the lower leg the muscle ends in a slender tendon which passes through the groove on the lateral surface of the lateral malleolus. This groove is converted into a canal by a transverse ligament. The tendon passes through the groove on the peroneal tubercle of the calcaneus, then turns onto the sole of the foot and passes through the peroneal groove on the cuboid bone. It then turns mediad and passes through the groove between the ventral processes of the lateral cuneiform and the metatarsal until it reaches the first metatarsal. The entire groove is converted into a canal by the overlying ligaments.

_Insertion_ into (1) the base of the fifth metatarsal, and (2) the outer side of the base of the first metatarsal, and (3) by slender branches into the bases of the other metatarsals.

_Relations._--Outer (lateral) surface with the superficial fascia and the tendon of the biceps femoris. Inner surface with the peroneus tertius (_r_) and peroneus brevis (_s_), and with the extensor longus digitorum (_p_). At the proximal end the dorsal border touches the tibialis anterior (_n_).

_Action._--Flexor of the foot.

=M. peroneus tertius= (Fig. 90, _r_).--A slender fusiform muscle beneath the peroneus longus (_q_).

_Origin_ by fleshy fibres from about the second quarter of the lateral surface of the fibula. The muscle ends in a slender tendon (_r′_), which passes with that of the peroneus brevis (_s_) through the groove on the ventral border of the lateral malleolus. The groove is converted into a canal by a transverse ligament. Emerging from it, the tendon turns and passes along the outer margin of the foot. In passing the sesamoid at the base of the first phalanx of the fifth digit it is united to it by a band which passes from the sesamoid to the tendon.

The _insertion_ is finally into the lateral border of the extensor tendon of the fifth digit as it passes from the first to the second phalanx.

_Relations._--Outer surface with the peroneus longus (_q_), the soleus (_o_), and the superficial fascia. Ventral border with the soleus (_o_) and flexor longus hallucis (Fig. 91, _m_). Inner surface with the peroneus brevis (Fig. 90, _s_).

_Action._--Extensor and abductor of the fifth digit and flexor of the foot.

=M. peroneus brevis= (Fig. 90, _s_) lies beneath the other peronei.

_Origin_ by fleshy fibres from the distal half of the surface of the fibula, which it embraces. At the lateral malleolus the muscle ends in a thick tendon which passes through the canal on the ventral surface of the malleolus. In the canal it is enlarged and surrounded by a synovial bursa. It then turns onto the foot and passes over the dorsal surface of the calcaneus on its lateral side and has its

_Insertion_ into the tubercle on the lateral side of the base of the fifth metatarsal.

_Relations._--Outer surface with the peronei longus (_q_) and tertius (_r_). Inner surface with the bone.

_Action._--Extensor of the foot.

=M. extensor digitorum longus= (Fig. 90, _p_).--A fusiform muscle which lies beneath the tibialis anterior (_n_), against the interosseous membrane, and between the tibialis anterior and the peronei.

_Origin_ by a thin flat tendon from the lateral surface of the lateral epicondyle of the femur just dorsad of the origin of the popliteus. The tendon becomes narrower and thicker and passes through the capsule of the knee-joint and over a slight groove on the tibia just dorsad of the head of the fibula. The belly of the muscle extends the entire length of the tibia, passing beneath the transverse ligament (5) along with the tendon of the tibialis anterior (_n_). At the ankle it runs through a fibrous loop which is attached to the dorsal surface of the calcaneus (the annular ligament of the calcaneus); it then turns and passes onto the dorsum of the foot. The muscle ends in four slender tendons, the two middle of which begin at the transverse ligament, while the others begin at the fibrous loop. The tendons diverge to the four digits. Each tendon is connected with the fibrous sheath surrounding the base of the first phalanx, and each has a synovial bursa beneath it at this point. Near the distal end of the first phalanx the tendon is united on the lateral side to the conjoined tendon of the extensor brevis digitorum and an interosseus muscle, and on the medial side to the tendon of the interosseus. The lateral side of the most lateral tendon is not thus united, but is joined on the outer side by the tendon of the peroneus tertius.

_Insertion._--The tendon thus formed on the dorsum of each phalanx by the junction of the two or three tendons above mentioned is firmly attached to the base of the second phalanx at its proximal end in the middle of its dorsal surface. In crossing the joint between the first and second phalanges the tendon forms a fibrous pad which protects the dorsum of the joint and glides over it. It then continues to be inserted into the base of the terminal phalanx.

_Relations._--Outer surface with the tibialis anterior (_n_) and the superficial fascia. Inner surface with a part of the tibialis anterior, with the bones, the interosseous membrane, and the peroneus brevis (_s_).

_Action._--Extensor of the phalanges.

=M. tibialis anterior= (Fig. 90, _n_).--This is the superficial muscle covering the lateral side of the tibia.

_Origin_ by fleshy fibres from the proximal one-sixth of the lateral surface of the shaft of the tibia, from the proximal third of the medial border of the shaft and head of the fibula, and from the intervening interosseous ligament. The triangular body of the muscle covers the lateral surface of the tibia and the outer surface of the extensor longus digitorum (_p_). Near the malleolus it ends in a strong tendon which passes beneath the transverse ligament along with the tendon of the extensor longus (_p_). The tendon crosses the dorsal surface of the foot obliquely toward its medial side.

_Insertion_ into the outer surface of the first metatarsal.

_Relations._--Outer surface with the superficial fascia and the tendon of the biceps femoris. Inner surface with the extensor longus (_p_), the tibia, and the interosseous membrane.

_Action._--Flexor of the foot.

4. Muscles of the Foot.

A. MUSCLES ON THE DORSUM OF THE FOOT.--=M. extensor brevis digitorum= (Fig. 90, _u_).--A broad thin muscle covering the tarsus and part of the metatarsus on the lateral side of the dorsum of the foot. It may be divided into three slips.

_Origin_ from the distal border of nearly the whole calcaneal annular ligament (5), and from the proximal end of the dorsal surface of the three lateral metatarsals. At the middle of the metatarsals the muscle ends in three flat tendons which pass into the three interspaces between the four tendons of the extensor longus. Each tendon is divided into two branches.

_Insertion._--The lateral branch of each tendon is inserted into the cartilaginous plate which lies in the metatarsophalangeal articulation of the digit on the outer side. The medial branch joins the lateral side of the extensor longus tendon on the dorsum of the first phalanx.

_Action._--Extensor of the digits.

B. MUSCLES OF THE SOLE OF THE FOOT.--=M. flexor brevis digitorum= (=pedis perforatus=).--This is the direct continuation of the tendon of the plantaris. It lies immediately beneath the superficial plantar fascia. The flat belly of the muscle has its medial border attached by an oblique tendinous band to the medial surface of the scaphoid and the medial cuneiform. It divides into four slips which diverge to the four toes, each ending in a flat tendon. The slips decrease in size from the lateral to the medial side. The three lateral ones overlap one another proximally.

_Insertion._--Each tendon expands at the distal end of the metatarsal and is wrapped about the tendon of the deep flexor which perforates it. The two halves unite beneath the perforating tendon and pass together with the perforating tendon through fibrous rings, the annular ligaments. One of these is attached to the sesamoids at the base of the phalanx. The other is attached to the head of the first phalanx. The tendons are finally inserted into the bases of the second phalanges.

The two annular ligaments are connected by fascia so as to form a continuous canal for the tendons. This canal is lined by a synovial membrane. Covering this canal is a more superficial layer of fascia. Before passing into the proximal annular ligament the two middle tendons unite each with the tendon of the corresponding lumbrical muscle. Each also gives off a branch, the lateral one on its lateral side and the medial one on its medial side. These branches, like those of the two middle ones, are inserted into the common phalangeal fascia.

_Action._--Flexor of the second phalanges of the digits.

=M. quadratus plantæ= (called also the plantar head of the flexor longus digitorum) is a thin flat muscle which takes _origin_ from the dorsal part of the lateral surface of the calcaneus and the cuboid. Its fibres converge to a flat tendon which passes transversely across the flexor longus digitorum and beneath the flexor brevis digitorum and calcaneometatarsal.

_Insertion._--Into the medial part of the outer surface of the tendon of the flexor longus digitorum.

_Action._--It holds the flexor longus tendon in place.

=Mm. lumbricales.=--These are six.

_Origin._--The three larger ones have origin from the outer surface of the expanded portion of the flexor longus digitorum on its distal half. Each ends in a slender tendon.

_Insertions._--The tendons unite with the divisions of the tendon of the flexor brevis which pass to the three lateral digits, at their entrance to the first annular ligament. The one to the fifth digit is sometimes absent.

The other three have _origin_ from the tendon of the flexor longus digitorum at the point where it divides. Each occupies one of the three intervals between the four divisions, and its fibres originate from the proximal ends of the two tendons which bound its interval.

_Insertion_ by a very slender tendon into the medial side of the first phalanges of the third, fourth, and fifth digits near their bases.

_Action._--Move the third, fourth, and fifth digits toward the medial side.

=Mm. interossei.=--The interosseus of the second digit forms five portions, which cover the ventral surface of the digit.

_Origin._--(1) By two tendons from the ventral process of the lateral cuneiform. The muscle divides into four parts, three of which pass to the medial side of the digit and act as abductors. They are called the long, short, and middle abductors (abductor longus, brevis, and intermedius digiti secundi).

_Insertions._--The short head into the medial sesamoid of the metatarsophalangeal joint, the middle into the medial side of the base of the first phalanx, and the long into the extensor communis tendon near the distal end of the first phalanx. The names long, short, and middle refer to the lengths of the tendons as determined by their points of insertion.

The _fourth_ portion which comes from the lateral cuneiform has its _insertion_ into the lateral side of the extensor tendon near the distal end of the first phalanx. It is therefore an adductor longus digiti secundi. In some cases there is an adductor brevis from this same head.

(2) The fifth part of the interosseus of the second digit has _origin_ from the middle of the ligament covering the peroneal canal, along with the middle adductor of the fifth digit. The two muscles diverge. _Insertion_ into the outer side of the base of the first phalanx of the second digit. It is an adductor medius digiti secundi.

The interossei of the third and fourth digits are alike.

_Origin_ from the ventral surface of the base of the metatarsal. The muscles cover the ventral surface of the shaft. At the head each divides and passes onto the sides of the metatarsal.

_Insertion._--Each half of the muscle shows a tendency to divide into two parts. One of these, the more superficial and ventral, ends in a tendon which joins the extensor tendon near the distal end of the first phalanx. The other part is inserted by short tendon- and muscle-fibres into the side of the base of the first phalanx; one of the parts is therefore a middle, and the other a long, adductor or abductor, the adductores and abductores digiti tertii and quarti.

The fifth digit has five short muscles.

=M. abductor medius digiti quinti.=

_Origin_ from the ventral surface of the calcaneus and from the fifth metatarsal.

_Insertion_ by a slender tendon into the lateral side of the base of the first phalanx of the fifth digit.

=M. adductor medius digiti quinti.=

_Origin_ with the adductor medius digiti secundi.

_Insertion_ on the inner side of the base of the phalanx of the fifth digit.

=M. opponens digiti quinti.=

_Origin_ by a flat tendon from the middle of the ligament which covers the peroneal canal. It passes obliquely outward parallel to the last.

_Insertion_ on the inner side of the shaft of the fifth metatarsal.

The two remaining muscles of the fifth digit take _origin_ from the outer part of the ligament covering the peroneal canal. They cover the ventral surface of the shaft.

_Insertion._--The lateral one is inserted into the lateral sesamoid and is therefore an abductor brevis. The medial one is inserted into the extensor tendon and is therefore an adductor longus.

C. MUSCLES OF THE TARSUS.--=M. calcaneometatarsalis= (part of M. adductor minimi digiti?).--A weak muscle made up largely of tendon-fibres, some of which pass directly from origin to insertion. It may be regarded as a ligament.

_Origin_, the lateral and ventral surface of the calcaneus near the proximal end. It passes distad and slightly laterad outside of the quadratus plantæ.

_Insertion_, the lateral side of the base of the fifth metatarsal, and the adjacent surfaces of the cuboid and calcaneus.

_Action._--Probably causes slight motion of the cuboid on the head of the calcaneus.

=M. scaphocuneiformis.=--A small muscle lying on the sole of the foot in the depression between the lateral cuneiform, medial cuneiform, and the lateral tubercle of the scaphoid. It is hidden by the overlying ligaments.

_Origin_, the lateral tubercle of the scaphoid bone. It passes distad and laterad.

_Insertion_, the lateral surface of the medial cuneiform.

_Action._--Rotates the medial cuneiform on the scaphoid and would thus act as an opponens of the great toe if the great toe were present.

THE VISCERA.

I. THE BODY CAVITY.

The greater part of the viscera are situated in the body cavity or cœlom. This is divided by the diaphragm into two parts, the =thoracic cavity= and the =abdominal cavity=. Each is lined by a serous membrane, in which the part covering the outer wall of the cavity is distinguished as the =parietal layer= from the part covering the viscera, which is known as the =visceral layer=.

The thoracic cavity is bounded by the thoracic vertebræ, the ribs, the sternum, and the diaphragm. The cranial opening of the cavity is filled by the trachea and œsophagus as they enter from the neck region. The thoracic cavity is lined by two thin layers of tissue, the outer one of which is the =fascia endothoracica=, while the inner is the =pleura=. The fascia endothoracica is a sheet of connective tissue which lines the entire inner surface of the thoracic cavity, descending from the dorsal median line to the heart, and passing into the fibrous layer of the pericardium. The pleura is a thin membrane covering the fascia endothoracica and corresponding to the peritoneum of the abdominal cavity. It forms two sacs, the =pleuræ=, lining respectively the right and left halves of the thoracic cavity. Each of these two sacs is closed, the viscera being suspended within them by folds of the membrane, so that the cavity is everywhere separated from the viscera by a sheet of the pleura. That portion of the pleura which lines the thoracic wall is known as the parietal layer; it may be divided into that covering the ribs (=costal= pleura), and that covering the diaphragm. That portion which covers the viscera is the visceral layer, or, since it covers chiefly the lungs, it may be distinguished as the =pulmonary= pleura. The medial walls of the two pleural sacs come in contact in the median plane, forming a median vertical partition passing lengthwise of the thoracic cavity. This partition is known as the =mediastinal septum=. The space between the two layers which make up the mediastinal septum is known as the mediastinum, or mediastinal cavity; it contains numerous organs of the thorax. Three parts are usually distinguished in this cavity: a =ventral= mediastinal cavity, containing chiefly blood-vessels and the thymus gland; a =middle= mediastinal cavity, enclosing the heart and the anterior and posterior venæ cavæ; and a =dorsal= mediastinal cavity, containing the trachea, the œsophagus, and the aorta.

The abdominal cavity lies caudad of the diaphragm; in it are sometimes distinguished the =abdominal cavity proper=, extending as far caudad as the cranial edge of the pubis, and the =pelvic= cavity, lying caudad of this, in the region surrounded by the innominate bones and the sacrum. The two cavities are not distinctly marked off, so that it is convenient to consider the abdominal cavity as undivided. Both parts are lined by the =peritoneum=.

The =peritoneum= is a thin transparent sheet of connective tissue supporting on its surface a layer of flattened epithelial cells, the peritoneal epithelium. It forms a sac which lines the entire abdominal cavity. This sac is closed in the male; in the female, however, it communicates with the exterior through the uterine (or Fallopian) tubes and uteri. All the organs of the abdominal cavity are outside the sac. In the course of their development these organs have encroached on the peritoneal sac. Each has grown against the outer wall of the sac to a greater or less extent and has forced a part of this wall ahead of it into the cavity. In some cases the encroachment has gone so far that the organ in question lies apparently within the peritoneal cavity, suspended from the wall of the sac by a fold of that wall. The wall may thus be divided into three portions. One of these, the =parietal= layer, lines the wall of the body cavity. The second (the =mesentery= in case of the alimentary canal, or a =ligament= in the case of another organ) suspends the organ from the body wall. Between the layers of each mesentery or ligament blood-vessels may pass to the organs. The third portion or =visceral= layer covers the organ in question, forming its =serous= covering.

The reflections of the peritoneum to form the mesenteries and ligaments may be thus described:

Caudally the peritoneum covers the whole surface of the bladder and is reflected from its ventral wall to the linea alba as the =suspensory ligament= of the bladder. Farther craniad the peritoneum suspends the rectum and colon from the middorsal line, forming the =mesorectum= and =mesocolon=. The mesocolon continues craniad to the level of the caudal end of the right kidney and is broadest at its cranial end. At this end the mesocolon passes into the =mesentery= proper, which suspends the small intestine and is very broad and much folded. Its dorsal border is attached to the median line opposite the caudal end of the kidneys and is very short compared to its very long ventral or intestinal border. Toward the cranial end the mesentery of the jejunum passes gradually into the very much shorter duodenal mesentery. This is drawn out at the caudal end of the duodenum into a fold, the =duodenorenal= ligament which attaches the duodenum to the kidney.

The =mesogastrium= or peritoneal fold for the stomach passes from that part of the median dorsal line lying between the kidneys and the diaphragm, to the greater curvature of the stomach. It does not pass directly to the stomach, but passes first ventrad of the small intestine as far as the pelvis. Thence it turns craniad to reach the greater curvature of the stomach. The fold thus formed is called the =great omentum=. It forms the dorsal and ventral walls of a sac, the =omental sac=, the cavity of which is called the =lesser peritoneal cavity=. The descending limb of the fold forms the dorsal wall of the sac, and its ascending limb forms the ventral wall. Each of these walls is double like a mesentery, so that the great omentum consists of four sheets of peritoneum. Between the two sheets forming the descending limb lies the left half of the pancreas, which passes thence to the right into the duodenal mesentery. A transverse fold passes from the descending limb of the omentum along the cranial border of the pancreas to the duodenal mesentery. Farther to the right the descending limb of the omentum, which is here shorter, encloses the spleen and holds it in position parallel to the greater curvature of the stomach and about one centimeter from it. This part of the great omentum is sometimes called the =gastrosplenic= or =gastrolienal= omentum.

The great omental sac communicates with the peritoneal sac by an opening, the =foramen epiploicum= or =foramen of Winslow=. This opening lies caudad and dextrad of the caudate lobe of the liver. This lobe itself lies within the sac. Along the ventral border of the foramen epiploicum pass the common bile-duct from the liver and the portal vein to the liver.