THE CEREBRO SPINAL AXIS.
The mass of nervous matter included under this heading is commonly referred to as the brain and spinal cord. The brain is that portion of it which is contained within th:, cranial cavity, and the spinal cord is the part suspended within the spinal canal. In this description we will view the axis as an isolated object, and the divisions of it made for the convenience of description, will be marked by distinctions upon the axis itself, without any, reference to the relations of its parts to those of any other part of the body.
After giving a general description of the structure of any part, we will direct the attention to the natural order of dis section from above downwards, as represented by the castings before referred to, which are intended to accompany and illustrate these pages.
When the anatomy of the spinal cord has been reached in the order of description and studied, its various columns will be traced upwards to their associations above, by which a complete review of the whole will be brought before the mind and its parts associated into the forniation of a structure.
A very proper and convenient division of the cerebro spinal axis is the cerebrum, cerebellum and spinal cord with their respective conections, and in this description we will follow the divisions commonly in use though without strict adherence to old boundaries. We will, for instance, say that the cere
brum is all that portion of the brain above the pons Varolii, including the hemispheres their peduncles and commissures.
The cerebellum and pots will Le described together, on ac count of the close association of their parts and the necessity of maintaining the analogy between this structure and that of tbe cerebrum. The medulla oblongata will be outlined, as that portion of the spinal cord which extends from the lower border of the pons Varalii to the lower part of the decussation of the anterior pyramids of the medulla, below which point is the spinal cord proper.
Viewed as a whole the cerebro spinal axis presents two lateral 'halves, united by commissures and divided in the mid dle before and behind by a contimmus but modified longi tudinal fissure, which in front is called the anterior and behind the posterior median fissure.