It will be understood that we have here to do with the more general defects, and that the distinctly local ones fall under their respective special sections. In studying the malformations by defect we have to go back to the developing embryo, and to see, in interferences with the expansion and development of parts, the causes of the defects.

A. Defect In Size, Dwarfing

Defect in size of the body as a whole results in the production of a dwarf. Dwarfs may be well formed although diminutive in size. Generally, however, the head is disproportionately large, and sometimes it is so excessive in comparison with the body as to give the body as a whole a deformed appearance. The body is also sometimes deformed, and the extremities crooked. Dwarfs have frequently good health, and may live to a considerable age. The usual height of dwarfs in the adult state is about 2 1/2 feet, never under 2 feet, and sometimes as much as 3 1/2 or 3 3/4 feet.

Partial dwarfing occurs in various parts of the body, but especially in the extremities. The extremities may be well formed, but defective in size. This may affect all four limbs, micromelus, or the arms, microbrachius, or the legs, micropus. The head is often small in idiot children, Microcephalus, and the brain is correspondingly dwarfed. There may be defect in individual parts of the brain, as the cerebellum. (See under Nervous System.) The face is sometimes at birth dwarfed on one side, Hemiatrophy, this apparently depending on some affection of the brain. The intestine may be abnormally short.

B. Defects In The Formation Of Parts

These lesions consist in the incomplete closure of the arches in front or behind, or in defective formation of the limbs or other parts. It is to be remembered that, in the embryo the original blastoderm gives origin to three layers, the upper and lower of which (ectoderm and endoderm) develop laterally and form arches on either side, which are destined to close in the middle line on the dorsal and ventral aspects. There are thus the dorsal or neural arches, and the ventral or visceral ones, either of which may remain unclosed in the middle line.

Causation

Although the causation is obscure in many cases, yet the constancy of some of the resulting forms shows that common influences have been at work. Of these influences, dropsy of the cerebro-spinal canal, as a cause originating in the foetus, and interference by the amnion having an external origin, seem to be the most potent.