Introduction

THE conditions to be considered here are all referable to errors in the development of the embryo. They are to be traced to causes acting on the embryo and causing it to deviate in its formation either as a whole or in part from the normal. Malformations may consist merely in slight local deviations from the regular type, as where a muscle or a blood-vessel has abnormal relations, in which case the term Anomaly is frequently used. On the other hand, they may affect the body as a whole and may be such as to produce the most serious deformities, many of them incompatible with life.

In that case the term Monstrosity is frequently used.1 It is with these latter that we are here chiefly concerned. The malformations of individual organs will be referred to in their places, but in this general section we have chiefly to consider those which affect the body as a whole or the more important parts of it. The science of monstrosities or Teratology is a very wide one: it will only be possible here to give a general outline of it.

The congenital malformations form only a part of the congenital diseases (see p. 15). In some of the latter we have simply the ordinary pathological changes such as are met with in extra-uterine life, but they have happened to occur in the foetus. In the case of malformations, on the other hand, the structure of the tissues in themselves is normal, but there is an erroneous arrangement owing to some interference occurring during the period of development.

The malformations are divisible into three great groups. These are malformations by excess, by defect, and by peculiarity of form.

Malformations by excess are divisible according as the size or the number of parts is in excess. There may be an excessive size of the whole body or of some of its parts, but such gigantic formation is uncommon. Much more common is numerical excess, consisting in a doubling or even in a trebling of the body as a whole or of its parts. The group of double monsters thus includes a large number of different forms. Malformations by defect may be either in the form of abnormal smallness of parts, or of a defect in the completion of the development of certain structures. In the latter-case the parts may be of full size, but they represent some period of embryonic life and not that of full development; thus, structures which are formed separately in the embryo in order afterwards to unite and form single structures may fail to do so, and clefts or fissures may be the result. Malformations by peculiarity of form, or Aberrant malformations, are local anomalies in which, without any actual defect, the development has taken a different course from the normal, structures which usually atrophy as development proceeds perhaps continuing to grow, while those which usually develop completely are atrophied. Many malformations of the heart and vessels ' and of the generative organs belong to this class, which also includes many local anomalies. These latter do not fall to be described in this section.

A monstrosity is "a collection of anomalies which are very complex, very grave, rendering impossible or difficult the performance of certain functions, and producing in the individuals so affected a vicious conformation very different from that which their species usually presents." - Geoffroy Saint-Hilaire, Traite de Teratologic, I.

Causation Of Malformations

As the malformations take origin in the embryo at comparatively early stages of its development, it must be generally difficult to trace the exact nature of the cause, but there are at least some indications of the kinds of causative agents.

In the case of the malformations by excess we must suppose an excessive stimulation of the embryo at an early period of development, but the nature of the stimulation is difficult to determine. It may here be pointed out, that in certain of the lower animals which are capable of restoring lost parts there is sometimes a reduplication of the parts reproduced. Thus in the case of the lizard, when the tail is broken off", there may be two or three tails instead of one reproduced. In the salamander also, when the foot or hand is amputated or divided longitudinally, there may be a new formation of one or more supernumerary fingers. In these cases the stimulation of the wound, in which the germinal tissue is forming a new member, results in the reduplication of the member. In a similar fashion the over-stimulation of the embryo may induce a reduplication, which will affect the whole body or parts of it, according to the period and locality of its application. The stimulus may in some cases be related to the activity of the ovum and spermatozoa respectively, but there is reason to believe that external stimulation may in some cases have important influences. Thus Gerlach asserts that by varnishing an egg so as to leave only a certain selected part free for the penetration of air he succeeded in certain cases in producing a doubling of the anterior extremity of the chick. Panum, Dareste, and others have also produced malformations by varnishing, by variations of temperature, and by placing the eggs vertically.

In the case of malformations by defect, the ordinary principles of causation may be taken into account. The causation is to be looked for in external conditions acting on the foetus in utero, or in internal conditions inherent or induced in the foetus itself. In the small and delicate structures which form the early embryo, very slight and simple disturbances may interfere with the completion of parts. Thus, blows on the abdomen may, in their influence, extend through the uterine wall to the foetus. Further, a potent cause of malformations is constriction or adhesion of parts, chiefly by the amnion. Adhesion is mostly the result' of inflammation, and this again may result from injury. Adhesion between the embryo and the amnion at an early period is liable to interfere with the closure of the external structures in the middle line. Such adhesions, being accidental, are apt to produce irregular malformations.

Internal conditions such as accumulations of fluid, or dropsies, are believed to have to do with many forms of defect, more especially those concerning the nervous system. On the other hand, a simple failure of developmental energy has been assigned as a cause of non-completion of the formation of parts.

Inheritance has little influence in the causation of malformations. In the case of the slighter forms or anomalies, such as supernumerary digits, inheritance has a most marked influence, but in the monstrosities proper it has little or no effect.

Sudden frights or shocks are often assigned popularly as the causes of malformations. The sight of a person with harelip or with an amputated limb, by the mother, is supposed to give rise to a like malformation in the foetus, or even a simple fright is stated as the cause. There is, however, little basis for this opinion, and the assigned cause often happens at a period of pregnancy when the malformation must have been already present. Most of the malformations, as we have seen, are referable to the early periods when perhaps the mother is as yet ignorant of the existence of pregnancy.

Classification

The chief malformations form a regular series, and subject themselves to a classification on comparatively simple lines. Most of the systems are based on that laid down by Foerster, which has been already indicated above. Cleland has suggested a classification which follows somewhat on Foerster's lines, taking more account, however, of the causation of the lesions. This classification will be in general followed.

Nomenclature

It is convenient to designate the various malformations by brief descriptive titles, which shall render them readily distinguishable. The names used for the specific forms will be found to follow a definite plan, and to contain, in Greek derivatives, a brief statement of the general features of the malformation.