GENERAL CONSIDERATIONS OF THE BRAIN OR ENCEPHALON Before taking up the special study of the cerebrum the student should notice certain prominent features of the entire brain. To do this the arachnoid and pia mater must be re moved, and great care and patience should be exercised to preserve the integrity of the brain substance and to guard against evulsion of the roots of the cerebral nerves.
The human brain forms the greatly expanded superior ex tremity of the cerebrospinal axis. It is derived from three sac-like dilatations of the epiblastic neural tube, called the anterior, the middle and the posterior brain-vesicles (Fig. 16).
Origin.—The nervous system is derived from the epiblast because that layer is most exposed to environmental stimuli; and, therefore, becomes specialized, by the operation of this obligatory function, for the reception of those stimuli and for the correlation and transmission of impulses adapted to the preservation of the organism. The anlage of the nervous sys tem is a dorso-median thickening of the epiblast, called the neural plate (or medullary plate). This anlage almost surrounds the embryonic mouth, represented by the blastopore. As the most primitive function of an organism is feeding, specializa tion is first required about the mouth in order that the animal may select the proper food for its development. Hence, speciali zation begins about the blastopore very early, in the two-layer stage of the embryo; it differentiates the epiblast into neural plate and cuticle plate. In the neural plate the epiblastic cells assume the long columnar form and rapidly mold the plate into two elongated ridges joined by a transverse arch in front, like a hair pin. The neural groove lies between the neural ridges and at the open, posterior end of the hairpin the blastopore is located. These neural ridges continue to rise up and arch toward the median plane until the neural groove is roofed over; the neural tube is the result. The approximation of the neural ridges is first completed in the cervical region, whence it extends in both directions. For a time, therefore, the tube has an opening at
each end, called the neuropore. The anterior neuropore closes quickly; but the posterior end of the tube for a little longer time remains open, communicating with the exterior through the posterior neuropore and with the archenteron through the blastopore.
The neural tube is well formed by the fifteenth day. The head-foremost movement of the early vertebrate, by the multiplication of stimuli to the head, induces a more rapid growth of the cephalic part of the neural tube. Hence, the anterior part is much larger than the posterior part of the tube; it constitutes the encephalic portion and presents the three primary brain vesicles, the anterior, middle and posterior, out of which the brain is evolved. The slender part of the neural tube, caudal to the brain vesicles, forms the spinal cord.
In the formation of the neural tube, the margin of the neural plate is lifted up into a slight crest on either side of the tube; it is called the neural crest. The neural crest breaks up into the anlagen of the sensory nerve ganglia. It probably furnishes the bipolar cells of the sympathetic ganglia, also; but not the multipolars of those ganglia (Froriep and Kuntz). The latter point needs further investigation. After four weeks of em bryonic life the neural tube presents five brain-vesicles, formed by the subdivision of the first and third primary vesicles into two; they are called the secondary brain-vesicles and are named, from before backward, end-brain, inter-brain, mid-brain, hind brain and after-brain (marrow-brain). A sharp ventral flex ure at the level of the mid-brain, mesencephalic flexure, brings the fore-brain and hind-brain into close approximation; thus, the ventral aspect of the mid-brain is shortened. Later, a dorsal flexure, the Pontine flexure, folds the rudimentary cerebellum backward over the medulla and pushes the pons forward into its very conspicuous position (Fig. 16, D and E).