2101. In addition to the series of vertebræ on the back, a vertebral column is formed moreover along the ventral surface, and without doubt there only where the air-organ, the branchia or lung, is situated. This inferior vertebral column is the breast-bone or _sternum_. 2102. All the systems, even those that are of a subordinate character in the animal, follow the direction of the main vertebral column. The intestine, as well as the vessels, are placed in accordance with it. Thus the principal trunks of the vessels take their rise along the vertebral column, the other vessels being given off from them in this situation, like the lymphatics from the intestine. 2103. The vascular ramules which surround the intestine and the skin, run out from a main stem, and are directed in a symmetric manner downwards and upwards (in the horizontal body of animals), or towards the belly and back. 2104. If new bony rings originate, they must also take these directions. They accompany the vessels that run in a circle, as the vertebral column accompanies the vascular trunks. These annularly-disposed bony twigs or apophyses constitute in the direction downwards the ribs, in that upwards the vertebral arches. Anterior and posterior to the vertebral column there consequently originates a long canal formed by the bony rings. In the anterior canal lie the galvanic or vegetative organs, in the posterior or upper, the organs of light must be situated "par excellence." The former of these canals is called the thoracic and abdominal cavity, the latter the vertebral canal. The vertebral canal is not the bony or medullary cavity itself, but it has been formed by several bony cysts in the same manner as the thoracic cavity. It consists of the body and the two arches. These are thus posterior (superior) ribs. The vertebral canal has the same signification as the thoracic cavity has; it is only a posterior thoracic cavity. It therefore contains, like the anterior canal, viscera dissimilar in kind to bone; the one including the spinal cord, the other vessels, intestine, and lung. 2105. The bony cysts that have originated through constriction do not all harden into calcareous matter, but they remain as alternating membranous cysts. Between the rings there are permanent cysts. The membranous cysts form the _joint_, or articular capsule. An articular capsule is a bone which has remained soft. 2106. This change in the ossific process takes place through the attachment of muscles, concerning which we shall treat in the sequel. 2107. The whole osseous system is consequently a symmetrical arrangement of several polar cysts and rings. 2108. The vertebra is not a single ring, but is at once a tolerably compound osseous system. The whole osseous system is nothing but a vertebra repeated.