My experiments upon the brain of man show that the pons on each side of the median line is the commanding head of the respiratory impulse, and in marking the organ of respiration on my busts, it is located around the mouth from the nose to the chin. When this region (especially its lower portion) is prominent it indicates active respiration and a forcible voice. Hence there is a great contrast in the vocal power of two such heads as are shown in the adjoining figure. This discovery has been verified by the pathological researches of Dr. J. B. Coste, published at Paris, 1857. [Illustration] Following the line of the ascending fibres, after passing through the pons they continue expanding and plunge into the thalamus and corpus striatum. Their first appearance above the pons (marked in the engraving by the word _Pedunc._) is usually called the _crura_ or thighs of the brain. The right crus, running through the thalamus, expands by successive additions into the right hemisphere, and the left crus into the left hemisphere, of the cerebrum, and the two hemispheres unite together on the median line by the corpus callosum. There is very little space for the crura (plural of crus) between the pons and the thalamus, but if we look at the posterior surface of the ascending fibres or crura we see a larger surface, on which we find a quadruple elevation called the _corpora quadrigemina_ (the four twins). This is an important intermediate structure between the cerebrum and the cerebellum, and in fishes is the largest part of the brain, but in man is the smallest portion, as will be explained hereafter, and is the origin of the optic nerve, as well as a commanding head for the spinal system, from which convulsions may be produced. The quadrigemina are distinguished also as the location of the pineal gland, which rests upon them, to which we may ascribe important psychic functions. The engraving shows the fibres connecting the quadrigemina with the cerebellum, and a channel under them (aqueduct of Sylvius) connecting the ventricles of the cerebrum with those of the spinal cord. What is called the fourth ventricle is the small space between the medulla oblongata and the cerebellum. At this spot the posterior surface of the medulla oblongata, as it gives origin to the pneumogastric nerve, which conveys the sensations of the lungs, becomes the immediate source of the respiratory impulse on which breathing depends, and hence is of the greatest importance to life. A very slight injury at this spot with a lancet or point of a knife would be fatal. It is recognized by converging fibres which look like a pen, and are therefore called the _calamus scriptorius_, or writer's pen.