A. Vertebrate central nervous system. _S. Inf. Br._, supra-infundibular brain; _I. Inf. Br._, infra-infundibular brain and cranial segmental nerves; _C.Q._, corpora quadrigemina; _Cb._, cerebellum; _C.C._, crura cerebri; _C.S._, corpus striatum; _Pn._, pineal gland. B. Invertebrate central nervous system. _S. Oes. G._, supra-oesophageal ganglia; _I. Oes. G._, infra-oesophageal ganglia; _Oes. Com._, oesophageal commissures.] 2. The region of the mid-brain, medulla oblongata, and spinal cord; from these arises a series of nerves segmentally arranged, which, as in the invertebrate, gives origin to the nerves governing mastication, respiration, and locomotion. Further, the vertebrate central nervous system possesses the peculiarity, found nowhere else, of being tubular, and the tube is of a striking character. In the spinal region it is a small, simple canal of uniform calibre, which at the front end dilates to form the ventricles of the region of the brain. From that part of this dilated {14}portion, known as the third ventricle, a narrow tube passes to the ventral surface of the brain. This tube is called the _infundibulum_, and, extraordinary to relate, lies just anteriorly to the exits of the third cranial or oculomotor nerves; in other words, it marks the termination of the series of spinal and cranial segmental nerves. Further, on each side of this infundibular tube are lying the two thick masses of the _crura cerebri_, the strands of fibres which connect the higher brain-region proper with the lower region of the medulla oblongata and spinal cord. Not only, then, are the nerve-masses in the two systems exactly comparable, but in the very place where the oesophageal tube is found in the invertebrate, the infundibular tube exists in the vertebrate, so that if the words infundibular and oesophageal are taken to be interchangable, then in every respect the two central nervous systems are comparable. The brain proper of the vertebrate, with its olfactory and optic nerves, becomes the direct descendant of the supra-oesophageal ganglia; the crura cerebri become the oesophageal commissures, and the cranial and spinal segmental nerves are respectively the nerves belonging to the infra-oesophageal and ventral chain of ganglia.