Part 86

The efferent nerves are those which convey impulses from the nerve centers to the periphery. These impulses may be motor as those going to the muscle cells of the skeletal muscles, the viscera, or the blood-vessels. These motor impulses make movements in these organs possible. In the blood-vessel they result in the control of the caliber of the vessel. These impulses may be of an inhibitory character, as in slowing the heart. They may be secretory impulses stimulating the gland to activity or regulating metabolism.

The ganglia are nerve centers which receive and generate impulses; the nerve trunks are filaments which convey impulses. The gray matter of the cord described above is the ganglionic portion, and the outer white matter is made up of nerve fibers which convey impulses from one part of the cord to another, or to and from the brain.

The nerves that have their roots in the spinal cord superiorly, are sensory; that is, they convey the sensory impulses from the periphery to the cord ganglion and to the brain. They have a ganglion just outside the cord.

The nerves that have their roots in the spinal cord inferiorly; that is, they convey motor impulses from the nerve centers to the periphery. The function of the inferior roots is to supply all the voluntary muscles as well as the oviduct, the intestines, and other hollow viscera, including the blood-vessels with the power of movement. Many of these fibers pass to the sympathetic ganglion and are distributed as sympathetic nerve fibers.

The cord is divided into different tracts, that is, certain groups of fibers convey certain kinds of impulses.

The superior column of the cord conveys to the cerebrum such impressions as temperature, pressure, and muscular tension.

The fibers of the lateral columns carry sensations of pain.

The fibers of the direct cerebellar tract carry impulses which result in the maintenance of the equilibrium of the body.

All the voluntary impulses originate in the cerebrum, pass through the cerebellum and travel direct to the bulb; they then pass over to the opposite side, and travel by the crossed pyramidal tract to the multipolar cells of the inferior horn of the spinal cord, and transmit the impulse through motor fibers that originate at that point in the cord.

All sensory impulses enter the brain on the side opposite their origin, and all motor impulses leave the brain on the side opposite that to which they are distributed. Injury of the motor area of the right side of the brain leads to paralysis of the left side of the body.

An impulse of the vasomotor nerve travels in the lateral column of the cord.

A nerve impulse may originate in the brain and be modified in passing through a ganglion in the spinal cord or in the sympathetic system.

The system of reflex action is as follows: first, an efferent nerve which conveys the impulse from the periphery to a nerve center; second, a ganglion, or nerve center, to receive the impulse and generate other impulses; third, an efferent nerve to convey the impulse from the nerve center to the periphery.

The following is an example of reflex action. The foot of a fowl is pierced with a pin. The sensory impulse is conveyed by sensory nerve fibers to the nerve centers. In this center the ganglionic nerve cells generate a motor impulse which is sent back through the motor nerve fiber to the muscles controlling the part. The result is that the muscle contracts and jerks the foot.

There are many functional centers located in the medulla oblongata. Destruction of this part of the system results in instant death of the bird. In addition to furnishing a path for fibers carrying impulses from the body to the cerebrum, it furnishes a large number of centers for such functions as respiration, swallowing, secretion, temperature, and vasomotor and cardiac activity.

The function of the cerebellum is principally that of coördination. It brings about harmony and rhythm in muscular movements. If the cerebellum be removed, the bird can no longer walk. It has lost, with this removal, all power to coördinate.

The cerebrum of the bird has no convolutions, and the gray, or ganglionic, portion is thin, indicating low power of intelligence. There are certain areas presiding over other functions, as motor areas, sensory areas, and so on.

A careful study of the brain of the fowl shows us that the centers presiding over sight and smell are well developed.

The olfactory bulbs are the centers of the sense of smell.

The optic thalamus is the center of the sense of sight.

The sympathetic nerve system transmits impulses to the involuntary muscular structure of all organs, including those of the intestinal tract, the blood-vessels, and perhaps also the glands.

ESTHESIOLOGY

THE SENSE ORGANS

The five special senses are seeing, smelling, tasting, hearing, and feeling.

THE ORGAN OF SIGHT

The sense of sight in the bird is well developed. The eye (Fig. 26, D, E and F), the organ of sight, is relatively large, round laterally, and rather flattened antero-posteriorly. It is located at the side of the head. The eyeball is only slightly movable. The septum interorbital separates the two eyeballs laterally. As in mammals, the eyeball has three coats, which are from inside to outside, the retina, the choroid, and the sclera. The =sclerotic coat= is completed anteriorly by the cornea with which it forms a union called the corneo-scleral juncture. Around the cornea the sclerotic coat contains a ring of osseous scales varying in number from twelve to twenty. The sclera may become ossified posteriorly, forming an osseous sheath around the optic nerve. The pecten is a vascular comb-like membrane stretching from the nervous opticus to the crystalline lens. The choroid coat is always black. The pupil in the hen is also black and round. The iris contains striated muscular fibers. The membrana nictitans, located at the inner angle is well developed. It is moved by two muscles (Fig. 26, No. B, 7, 8, 10). The lacrimal gland and the gland of Harder are present. There is no meibomian gland.

The tears are secreted by the lacrimal gland and are drained away from the fore part of the eyeball by two small canals which extend into a lacrimal sac. From this sac there extends a tube into the nasal cavity called the lacrimal duct.

The lower lid is the larger and often incloses a small cartilaginous plate. The conjunctiva is a true mucous membrane which covers the anterior portion of the eye cavity attaching to the cornea-scleral juncture.