Microcephaly

“ More delicate injuries produce less monstrous development. It is particularly the partial varnishing or irregular heating of the egg-shell that results in the production of anomalies comparable to microcephaly and cerebral asymmetry. The constancy of the injurious effect of so apparently slight an impression as the partial varnishing of a structure not directly connected with the embryo at all, suggests a most plausible explanation of maternal and other impressions acting on the germ. ”

“ The microcephalism of races, as well as of individuals, of small stature, must not be confounded with the true microcephaly of a dwarfed or imperfectly developed brain, which is invariably accompanied with mental imbecility. ”

“ In regions where dolichocephaly (index less [Pg 11] than 80) abounds, the skulls of criminals show a very low index; if, on the contrary, they are natives of districts where brachycephaly (index 80 or more) prevails, they exhibit a very high index.
SKULL FORMATION
In 15.5% we find trochocephalous or abnormally round heads (index 91) . A very high percentage (nearly double that of normal individuals) have submicrocephalous or small skulls. In other cases the skull is excessively large (macrocephaly) or abnormally small and ill-shaped with a narrow, receding forehead (microcephaly, 0.2%) . ”

“ The epiblast which is involuted to form this pit becomes much thickened, and thereby the opacity, indicated in the figure, is produced.
The muscle-plates have greatly increased in number by the formation of fresh segments in the tail. Thirty-eight of them were present in the embryo figured. The mesoblast at the base of the brain has increased in quantity, and there is still a certain mass of unsegmented mesoblast which forms the tail-swellings. The first rudiment of the heart becomes visible during this stage as a cavity between the mesoblast of the splanchnopleure and the hypoblast (ht.) ”

“ There is considerable variation in size in the cells in different parts of the epiblast. Between the front lobes of the brain the cells are very small, 1/4000th inch, rising to 1/2000th on each side. They are about the latter size over the greater part of the embryo. But over the sinus rhomboidalis they fall again to from 1/3000th to 1/4000th inch. This is probably to be explained by the growth of the medullary fold at this point, which pushes back the primitive groove. At the sides of the head the cells are larger than anywhere else in the epiblast, being here about 1/1000th inch in diameter. ”

“ These two layers are the primary germinal layers. A third layer is subsequently formed between them, by the agency of one or both of them, and is called the mesoblast. From these three simple membranes, which are composed exclusively of cells, are formed all the complex tissues and organs of the adult man. The epiblast develops into the nervous system (brain, spinal cord, and nerves) , and the cuticle, hair, etc. The hypoblast develops into the cellular parts of the digestive canal, the liver, lungs, etc. The mesoblast develops into the muscles, bones, ligaments, blood-vessels, etc. ”

“ CEPHALIC, generally signifies whatever relates to the head. Hence these remedies that are given for disorders of that part, are denominated cephalic medicines. Under this description are comprehended cordials, and whatever tends to promote a free circulation of the blood through the brain: thus cephalic snuff is taken with a view to remove pains in the head, by occasioning the patient to sneeze, and, in this manner, perhaps, giving vent to obstructions in the smaller vessels. Such is the imperfect theory of cephalic medicines ”

“ Those who seek for the source of the arrested or perverted brain development in the reaction of an abnormally growing and ossifying skull on the skull contents are in error. The premature ossification theory does not hold good even for the microcephali; it is to be doubted if it ever had any justification in view of the often open character of the sutures. Taking the well-studied cases of asymmetry, the variability of a single factor shows that caution is needed in referring cerebral anomalies to any single influence. ”

“ For instance, in the case of constitutional conditions of the parents we may have a history of syphilis, a disease which often leaves its traces on the bones of the skull; and in the third case congenital malformation of the brain may be produced by mechanical causes acting on the child in utero, such as an attempt to procure abortion, or deformities of the maternal pelvis rendering labour difficult and instrumental interference necessary. In such cases the bones of the skull may be injured; it is only fair, however, to say that more brains are saved than injured by instrumental interference. ”

“ The layers at the posterior end of the embryo retain the characters of the last stage.
Sixth day after impregnation.—At this stage (Plate 34, fig. 8) the embryo is considerably more advanced than at the last stage. The trunk has decidedly increased in length, and the head forms a relatively smaller portion of the whole. The regions of the brain are more distinct. The optic vesicles (op.) have grown outwards so as to nearly reach the edges of the area which forms the parietal part of the body. The fore-brain projects slightly in front, and the mid-brain is seen as a distinct rounded prominence. ”

“ The pituitary body is in fact an organ derived from the epiblast of the stomodæum. This fact has been demonstrated for Mammalia, Aves, Amphibia and Elasmobranchii, and may be accepted as holding good for all the Craniata [166] . The epiblast in the angle formed by the cranial flexure becomes involuted to form the cavity of the mouth. This cavity is bordered on its posterior surface by the front wall of the alimentary tract, and on its anterior by the base of the fore-brain. ”

“ The layer of cells lining the cavity of the sack is known as the hypoblast, and gives rise chiefly to the cells lining the alimentary canal of the future animal. The outer layer of cells is known as the epiblast, and forms the outer layer of the skin, and, along the future dorsal line, gives rise to the nervous system. The muscles and skeleton and the reproductive cells arise from a set of cells known as the mesoblast, that are formed chiefly from the hypoblast, and that push their way in between the hypoblast and epiblast. ”

“ Secondly, the body-cavity when it appears in the mesoblast plates, does not arise as a single cavity, but as a pair of cavities, one for each plate of mesoblast, and these cavities remain permanently distinct in some parts of the body, and nowhere unite till a comparatively late period. Thirdly, the primitive body-cavity of the embryo is not confined to the region in which a body-cavity exists in the adult, but extends to the summit of the muscle-plates, at first separating parts which become completely fused in the adult to form the great lateral muscles of the body. ”

“ Head form is most conveniently expressed by means of the cephalic index, that is, the ratio of breadth to length. Anthropologists generally speak of any one with an index of 75 (or where the breadth is 75% of the length) and below this as dolichocephalic, or long-headed; from 75 to 80 is the class of the mesocephalic, intermediates; while above 80 is that of the subbrachycephalic and brachycephalic, or round-headed. For the most part, the Old Americans fall into the intermediate class, the average index of males being 78.3 and that of females 79.5. ”

“ There seems, however, reason to believe that many embryos penetrate the intestinal wall completely and reach the body-cavity, in which they can wander freely. When the six-hooked embryo has reached its resting-place, which in addition to the liver may be lung, muscles, brain, connective tissue, or eye, it at once commences its further development, and in a few days becomes visible to the naked eye. ”

“ When ossification sets in, the ear-capsule forms three bones, of which two fuse with the supraoccipital and exoccipitals.
During the formation of the ear-capsule the cranial basis develops from a plate to a trench, for in its hinder section the side parts grow up to form the side walls of the brain, in exactly the same way as the processes of the vertebral rudiments grow up to enclose the spinal column (pp. 122, 192) . The foundations of the skull are now complete, and ossification gradually sets in.
Fig. 11.—Embrionic Cranium of the Adder. Ventral Aspect. (After Rathke.) ”

“ The Cephalic Index at Different Ages of Life.—Another quality that renders the cephalic index of great importance is that it remains constant in the course of growth, since the two maximum diameters, the antero-posterior and the transverse, increase at very nearly the same rate, excepting during the earliest years, at which time the length of the cranium increases slightly more than the width. ”

“ In making a study of the entire embryo we note there has been a rapid growth of the dorsal surface of the head, which has become more bent. This bend, in the region of the midbrain is called the cephalic flexure. The forebrain and part of the midbrain form almost a right angle with the rest of the head. The head is compressed laterally and free from the blastoderm. The dorsal side of the trunk is turned up, and there is a twisting of the axis of the embryo just back of the heart. ”

“ The internal cavity is laid open so as to exhibit an embryo in each half of the Ovum, the one being double the size of the other. The smaller embryo is about as large as a good-sized house-fly. The cord of the largest embryo is nearly an inch and a half long, sacculent, and expanding towards the abdomen so as to form the only covering of that cavity. The cord of the smaller embryo is half an inch in length and filiform. ”

“ In reference to the structure of the head-capsule in the imago, it appears that the clypeus and labrum represent, as already said, an unpaired median outgrowth of the oral piece. According to W. A. Riley (1904) the epicranium or “vertex,” the compound eyes and the front divisions of the genae are formed by the cephalic lobes of the embryo (belonging to the ocular segment) , while the mandibular and maxillary segments form the hinder parts of the genae and the hypopharynx. ”

“ At the time when these spaces are formed, and especially in the hinder two-thirds of the pellucid area, and in the layer of blood-vessels immediately above the hypoblast, a formation takes place which forms in appearance a secondary investment of the capillaries. Dr Klein was the first to give a correct account of this formation. It results from the cells of the mesoblast in the meshes of the capillary system. Certain of these cells become flattened, and send out fine protoplasmic processes. ”

“ The teeth have generally sharp and cutting edges, and the canines are well developed; the incisors are small, and four to six in number. The number of toes is never less than four. There are usually strong and sharp claws. The clavicles are incomplete or absent. In the hand the scaphoid and lunar bones are always united. The brain is well developed, and the hemispheres are well convoluted. The stomach is always simple, while the caecum, if present, is always small. The members of this group have a deciduate and zonary placenta. ”

“ The number of brains of microcephali that have been examined with reference to this point is very small, although their scientific value, after thorough-going observation of the possessors of them during life, is immense. For microcephalous children of some years of age are a substitute for imaginary, because never practicable, vivisectory experiments, concerning the connection of body and mind. ”

“ The cephalic indices show 5 men and 4 women to be dolichocephalic, 3 men and 6 women brachycephalic; the median for the men is 76.9, for the women 79.4. One man is platycephalic, 3 men and 1 woman mesocephalic and the rest hypsicephalic as regards length-height, all are hypsicephalic as regards breadth-height, in each respect the women being markedly more hypsicephalic than the men. The shape is noted as oval in 1 man and 9 women, round oval in 1 of each sex, ellipsoidal in 1 man and pyriform in 4 men. ”

“ Injuries to the mother during pregnancy have been unquestionably the cause of certain malformations, especially of congenital hydrocephalus. The embryo itself and its membranes may become the subject of inflammations, atrophies, hypertrophies, and the like ”

“ As well might we deny the vitality of the blood because it cannot be felt. The muscular tissues, and even the bones to which they are attached, must have some degree of substance before there can be motion, and of course this development depends upon life. Though this foolish notion is now fully exploded in medicine, it still lingers in the popular mind, and doubtless leads to much crime. The life of the fœtus or embryo immediately after conception is just as positive physiologically as at any subsequent period. ”

“ In its formation this involution pushes backwards the segmentation cavity, which finally disappears in the stage C III. The point x remains practically stationary, but by the general growth of the epiblast, mesoblast and hypoblast, becomes further removed from the segmentation cavity in C II than in C I. On the opposite side of the embryo to that at which the involution occurs the epiblast cells as before, grow round the lower layer cells. ”

“ There is a decided [p. 207] tendency to large oval forms, and despite the abundance of microcytes, the average size of the corpuscles is generally strikingly increased. Polychromatophilia and basophilic degeneration are common. Nucleated red cells are always present, although in many instances careful search is required to find them. In the great majority of cases megaloblasts exceed normoblasts in number. This ratio constitutes one of the most important points in diagnosis, since it is practically unknown in other diseases. Blood-plaques are diminished. ”