Part 82

The somewhat large posterior coraco-brachialis, nerve which springs from one or two middle roots of the plexus and accompanies the pectoral nerve.

The anterior thoracic, a large nerve which springs from two or three of the posterior roots of the plexus and extends to the shoulder cavity where it branches. The anterior branch is distributed to the patagium and the front part of the pectoral muscles. The posterior branch supplies muscles along the side of the thorax and extends into the abdominal muscles.

The anterior coraco-brachialis, a small nerve which springs mostly from the inferior longus brachialis, passes to the distal end of the tuberculum humeralis radii, then passes backward between the front part of the humerus and the biceps, where it supplies the anterior coraco-brachialis.

The cutaneous brachialis et inferior brachialis, a small nerve which springs from the posterior roots of the plexus. It is distributed to the skin of the region and branches are given off to the patagii and the ventral wing surface. A few branches extend as far as the upper arm.

The brachialis longus inferior (Fig. 68, No. 2), a continuation of the main trunk of the inferior brachial nerve, which comes out of all the plexus roots except the first. It gives off some branches to the pectoral region and then enters the shoulder cavity. It gives off the anterior coraco-brachialis, passes down the humerus in an S-shape, and divides into two branches, the ulnar and the median.

The ulnar nerve (Fig. 67, No. 3) divides into numerous branches which are distributed to the ulnar side of the forearm and the hand. The ulnar nerve passes below the skin of the ulnar outside rim of the forearm and gives branches to the carpi ulnaris, finally supplying the flexor digitorum; it then passes with the tendon of this muscle downward to the interosseous dorsalis, the abductor indicis, or flexor minimi brevis, the flexor pollicis, and the abductor pollicis, or extensor proprius pollicis.

The median nerve (Fig. 67, No. 2) extends down the arm and gives branches to the biceps muscle, and to the patagii region. Continuing, the median nerve supplies the pronator muscle and the brachialis inferior, or brachialis anticus, and then divides into two branches. The first and largest branch passes between the two pronator muscles giving branches to the pronator profundus, or pronator longus, and the flexor digitorum profundus muscle. This nerve then passes anteriorly along the tendon to the base of the digit, and, on the dorsal side, joins the other branch of the median nerve. At this point there is often a plexus, and at the point of fusion of the ulnar nerve on the ulnar side there may also be a small plexus. These plexuses mainly supply the skin of these regions. The second branch, passes forward, crosses the extensor carpi radialis and lies just below the pronator profundus, or pronator longus, giving branches to the skin. It passes downward on the radial side of the ulnar and supplies the muscles of that part; it then continues down the hand. Branches are given off to the extensor proprius pollicis, the interosseous dorsalis, the flexor digitorum, and to the skin between the thumb and the forefinger.

The intercostal nerves (Fig. 70, No. 18) are given off from the spinal cord of the dorsal region. The superior branches are small and supply the superior dorsal region. The inferior branches lie one behind each rib, innervate the intercostal muscles, and give a few twigs to the superficial thoracic muscles.

=The Lumbo-sacral, or Crural, Nerve Plexus.=—The crural plexus (Fig. 70, No. 35 and 36) is made up of trunks from the last two lumbar and first four sacral spinal nerves. There are two portions of this plexus separated by a considerable distance.

The anterior portion (Fig. 70, No. 35) consists of the lumbar nerves and a portion of the first sacral nerves. The fusion takes place on the bony ridge that separates the lumbar from the sacral region.

The posterior portion (Fig. 70, No. 36) consists of a part of the first sacral and all of the three succeeding nerves.

The anterior nerve of this plexus is distributed to the abdominal muscles. An anterior branch is given off to the sartorius. A large cutaneous branch enters between the sartorius and the ilio-trochanteric eminence, and supplies the outer and the upper surface of the upper thigh region. Several short branches are given off from the middle, or main, mass of the crural plexus to the ilio-trochanteric, or gluteal, muscle. Another nerve passes over the side of the sartorius adjacent to the femoro-tibialis, or extensor femoris, and supplies the ilio-tibialis, or gluteus primus. The rest of this section of the crural nerves pass in a distal direction over the inner and front side of the cutaneous nerve and enter the ilio-femoralis, or gluteus medius, the ambiens and the femoro-tibialis muscle.

The furcalis nerve comes out between the last two lumbo-sacral vertebræ.

The obturator nerve springs from several roots. The anterior root of this nerve comes from the main trunk of the crural plexus, and its last root from the furcalis nerve. The obturator nerve extends in a ventral direction from the plexus, then horizontally on the inner surface of the abdominal cavity. It gives off twigs to the obturator muscle and then passes through the obturator foramen. After leaving the abdominal cavity it gives branches to the accessorius, the obturator, and, the pubio-femoral, or adductor longus muscle.

The crural plexus gives off another large nerve trunk which extends downward. Shortly after emerging from the plexus it gives off a muscular branch to the ilio-femoralis, or gluteus medius muscle. It then passes between this muscle and the shaft of the femur, to the inner posterior surface of the thigh, and to the inner surface of the knee. It gives off a branch to the structures of the knee. At the knee this nerve terminates in several branches, some of which pass to the inner surface of the tibial head, the internal lateral ligament, the periosteum, the internal condyle, and finally to the upper part of the head of the gastrocnemius muscle.

The main part of this nerve passes downward as the cutaneous nerve, along the inner surface of the lower thigh.

The ischiadic plexus (Fig. 64, No. 41) has five to six roots, which fuse into the ischiadic trunk. This main trunk extends out of the abdominal cavity, through the ischiadic foramen, close behind the anterior trochanter of the ilium (Fig. 65, No. 1). It gives a branch to the external ilio-femoral or gluteus medius, and a branch to the post-acetabular portion of the ilio-tibialis, or gluteus primus muscle. The main trunk (Fig. 69, No. 13) passes downward to the lower portion of the thigh. In this course there are two branches which run parallel to the femoral artery and the femoral vein. It gives off a small muscular branch to the accessorius, and further down gives off a long slender branch to the outside of the knee-joint. It gives off a lateral cutaneous branch (Fig. 66, No. 7) to the posterior outer portion of the lower thigh. It supplies motor fibers to the external head of the gastrocnemius muscle.

At the knee region the ischiadica divides into three parts. The largest branch (Fig. 69, No. 14) passes with the tendon of the ilio-fibularis, or biceps femoris muscle, through a loop, the biceps band, and lies on the upper lateral surface of the fibula. It is covered by the external head of the gastrocnemius muscle. It innervates the three posterior muscles of the lower thigh, and then divides into two branches. One, the superficialis peroneus (Fig. 69, No. 16) passes with the profundus nerve, forming a double trunk and occupying the tibio-fibular groove on the antero-lateral side. It passes over the transverse ligament and the tibio-metatarsal joint, and after sending small branches to the structures of the tibial side of the metatarsus, it ends as a cutaneous nerve on the upper side of the third and the fourth toe.

The other branch is the peroneus profundus, which separates from the peroneus superficialis and passes downward in company with the tendons, under the transverse ligament, and then along the anterior upper surface of the metatarsus, where it innervates the muscles of that region. It gives branches to the malleolus, to the tendons of the third toe, and to the median part of the second toe, and supplies the cutaneous structures of the third and the fourth toe.

The third branch of the ischiadica, a long nerve, is given off just after the ischiadica passes through the loop. It passes downward between the two peroneal nerves. It is covered by a sheath. It passes over the posterior outside rim of the intertarsal joint and innervates the tendon sheath. The main portion of this nerve is located on the anterior surface of the tendon Achillis, and passes down on the plantar side, and innervates the periosteum and all plantar foot muscles. It finally radiates to the plantar surface of the three anterior toes.

The integument of the toes is sparingly supplied with nerves.

The nerve trunks that do not pass through the biceps band, or loop can be divided into a medial (Fig. 69, No. 15) and a lateral (Fig. 69, No. 17) portion. The medial portion (Fig. 69, No. 15) soon divides into numerous branches which supply the muscles of the posterior and the inner portion of the thigh. A rather large, long branch passes downward along the tendon of the plantar muscle, which lies on the posterior median edge of the tibia, and gives off twigs to the median and posterior part of the intertarsal joint, supplying the periosteum and other structures and the adjacent skin. It passes downward along the outer side of the medial metatarsal insertion of the tendon Achillis.

The fifth branch is given off from the second trunk and lies laterally. It is covered by the external head of the gastrocnemius, passes along the vena saphena, and gives off a short main branch to the inner side of the intertarsal joint. The main part passes the tibial side of the joint, becomes subcutaneous, and finally innervates the two plantar muscles. A lateral ascending inner branch innervates the flexor perforatus digitorum, and, in company with an outer branch, the external head of the gastrocnemius, and also the flexor pedis perforatus.

The plexus pedundus is formed from the spinal nerves coming out of the plexus ischiadicus. These fibers emerge caudalward and are directed horizontally. They frequently anastomose with each other, especially on the pubic rim and on the outside of the plexus ischiadicus. These branches are deeply imbedded in the kidney substance and innervate the pubio-coccygeus, or depressor coccygis lateralis, the ilio-coccygeus, the transversalis, the sphincter, and other muscles of this region, and the skin of the anal region.

THE BRAIN

=The Brain Coverings.=—The cerebro-spinal axis of birds is similar to that of mammals. The meninges of the brain are three in number, dura mater, arachnoid, and pia mater. The dura mater is the thickest. It is constructed of white fibrous connective tissue, and lines the cranial cavity. Thus it serves as an internal periosteum. It is continuous with the spinal dura mater at the foramen magnum, and is also prolonged as a sheath of the nerves. In birds of flight where the air-sacs and reservoirs are developed to the highest state, Sappy finds that, “just as the medullary tissue is replaced by air in the bones of birds, so might it be imagined that the subarachnoid fluid is also replaced by air around the spinal cord,” and observations justify the correctness of this statement. The dura mater measures exactly the volume of the marrow in birds; so that there does not exist between the fibers and the nervous surface any space for an accumulation of liquid. This anatomical fact is sufficient to demonstrate the absence of subarachnoid fluid in the bird. In denying the existence of this fluid, it ought to be added that in this class of vertebrates, the spinal prolongation is covered by a triple envelope; and that between the pia mater and the dura mater is a thin transparent membrane, which is lubricated by a serous fluid. This fluid however does not collect; it only moistens the arachnoid membrane.

The arachnoid is located between the dura mater above and the pia mater below. The pia mater adheres closely to the nerve tissue.

The falx cerebri exists in fowls and in turkeys. It has the form of a segment of a circle, and extends from the middle of the interval of the openings for the olfactory nerves to the tentorium cerebelli. The falx cerebelli is absent. The tentorium is small and is sustained by a bony plate, and there are in addition two folds, one on each side, that separate the hemispheres from the tubercula quadrigemina. Owing to the absence of the falx cerebelli, the meninges lie close together. The falx cerebri is ossified in birds.