_The spinal cavity_ is formed by the bones of the vertebral column. In this spinal cavity is found the spinal cord. It is cylindrical and usually about seventeen inches in length, and extends from the medulla oblongata, to the lower border of the first lumbar vertebra, where it terminates in a slender filament of gray substance. There originate from the under surface of the brain twelve pairs of nerves, as follows: 1. Olfactory 2. Optic 3. Motor Oculi 4. Trochlear 5. Trigeminal 6. Abducens 7. Facial 8. Auditory 9. Glosso-pharyngeal 10. Pneumogastric 11. Spinal accessory 12. Hypoglossal There originate from the cord thirty-one pairs of nerves, as follows: Cervical region 8 pairs. Thoracic region 12 “ Lumbar region 5 “ Sacral region 5 “ Coccygeal region 1 “ —— 31 “ The circulation of the blood through the brain will be taken up later. CHAPTER VII. ORGANOLOGY.—Continued. =The Thoracic Cavity.=—The thorax, or chest is a bony, cartilaginous cage. It contains and protects the principle organs of respiration and circulation. The thorax is bounded in front by the sternum and costal cartilages, behind by the twelve dorsal vertebrae and the posterior parts of the ribs, on the sides by the ribs, above by the root of the neck and below by the diaphragm. [Illustration: FIG. 16—Front view of the thorax. (Gray)] In the female the thorax differs as follows from the male: Its general capacity is less, the sternum is shorter, and the upper ribs are more movable and so allow a greater enlargement of the upper part of the thorax than the male. The capacity of the cavity of the thorax does not correspond with its apparent size externally, because, (1) the space enclosed by the lower ribs is occupied by some of the abdominal viscera; and (2) the cavity extends above the first rib into the neck. The size of the cavity of the thorax is constantly varying during life, with the movements of the ribs and diaphragm, and with the degree of distention of the abdominal viscera. From the collapsed state of the lungs, as seen when the thorax is opened, in the dead body, it would appear as if the viscera only partly filled the cavity of the thorax, but during life there is no vacant space, that which is seen after death being filled up during life by the expanded lungs. =Larynx.=—The larynx is the organ of voice, placed at the upper part of the air passage. It is situated between the trachea and the base of the tongue, at the upper and forepart of the neck, where it forms a considerable projection in the middle line. It is for this reason that it is of considerable importance to embalmers, for it is just opposite this projection, on either side of the neck, that the common carotid divides into the internal and the external carotid.