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
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.
If the reader has not fully mastered the intricacy of the brain structure, he will find his difficulties removed by studying two more skilful dissections. The following engraving presents the appearances when we cut through the middle of the brain horizontally and reveal the bottom of the ventricles, in which we see the great ganglion, or optic thalamus and corpus striatum, and the three localities at which the hemispheres are connected by fibres on the median line, called anterior, middle, and posterior commissures. These commissures are of no importance in our study; they assist the corpus callosum in maintaining a close connection between the right and left hemispheres.
Behind the thalami we see the quadrigemina, the posterior pair of which is labelled testes, and resting upon them we have the pineal gland, a centre of spiritual influx. Behind the thalami, the posterior lobes are cut away that we may look down to the cerebellum, and the middle of the cerebellum is also removed so that we may see the back of the medulla oblongata and its fibres, called restiform bodies, which give origin to the cerebellum. The fibres from the cerebellum to the quadrigemina are shown, and the space at the back of the medulla, called the fourth ventricle.
As the fibres of the medulla pass up through the pons to the great inferior ganglion, and the fibres of the corpus striatum pass outward and upward to form the cerebrum, this procession of the fibres is shown in the annexed engraving, in which we see the restiform bodies passing up to form the cerebellum, and the remainder of the medulla fibres passing through the pons, and then, under the name crus cerebri or thigh of the cerebrum, passing through the thalamus and striatum to expand in the left hemisphere of the cerebrum. We see the quadrigemina on the back of the ascending fibres and their connection by fibres with the cerebellum behind, as they connect with the thalami in front. This is as complete a statement of the structure of the brain as is necessary, and further anatomical details would only embarrass the memory.
