If the relation was a reflex, it would have to be classed now as one of those local nerve circuits, which are pretty common among the viscera, a local call and reply as it were, without mediation of the great long distance trunk lines in the spinal cord and the medulla oblongata.

These nerves are governed from a center in the medulla oblongata, a part of the brain (sec. 270).

It is situated in that part of the brain called the medulla oblongata, and is fancifully called the "vital knot" (sec. 270).

The condition of the lungs themselves is made known to the breathing center, by messages sent along the branches of the great pneumogastric nerve (page 276), leading from the lungs to the medulla oblongata.

The medulla oblongata.

On its under surface is a depression which receives the medulla oblongata.

A, frontal lobe of the cerebrum; B, parietal lobe; C, parieto occipital lobe with fissure between this lobe and D, the occipital lobe; E, cerebellum; F, arbor vitæ; H, pons Varolu; K, medulla oblongata; L, portion of lobe on the opposite side of brain.

The Medulla Oblongata.

The medulla oblongata consists, like the spinal cord, of columns of white fibers and masses of gray matter, but differently arranged.

The functions of the medulla oblongata are closely connected with the vital processes.

[Illustration: Fig. 117.Anterior View of the Medulla Oblongata.

A, chiasm of the optic nerves; B, optic tracts; C, motor oculi communis; D, fifth nerve; E, motor oculi externus; F, facial nerve; H, auditory nerve; I, glosso-pharyngeal nerve; K, pneumogastric; L, spinal accessory; M, cervical nerves; N, upper extremity of spinal cord; O, decussation of the anterior pyramids; R, anterior pyramids of the medulla oblongata; S, pons Varolii.

It is a continuation of the medulla oblongata, and is lodged in the canal of the spinal column.

This tiny channel, just visible to the naked eye, is connected with one of the openings of the medulla oblongata, and extends, as do the anterior and posterior fissures, the entire length of the cord.

This, too, is beyond the control of the will, and is brought about by the vaso-motor nerves of the sympathetic system through a reflex arrangement, the center for which is the medulla oblongata.

Medulla Oblongata.

A part of the medulla oblongata, the destruction of which causes instant death.

568 papyracea Paper ditto c.m. 569 pumila Hairy-leaved Dwarf ditto b.m. 570 oblongata Oblong-leaved ditto c.m. 571 laciniata Cut-leaved Alder c.m. 572 incana Glaucous-leaved Alder c.m. 573

The principal seats or centres of these radiations are (1) the several brains of man, viz., the cerebrum, cerebellum, and the medulla oblongata, respectfully; and (2) the several great centres of nerve substance in the human system, called the plexi, such as the solar plexus, etc.