This may affect the brain as a whole or parts of it.

Micrencephalus signifies a general smallness of the brain, which is usually associated with Microcephalus or smallness of the head, the latter term being frecpaently used as synonymous with the former. The defect usually concerns the cerebrum chiefly, although other parts may also be involved. The brain may be defective in structure as well as in size, the convolutions being small, irregular, or defective. The affection may be due in some cases to a simple defect in formative power in the cerebral matter, but it results sometimes from more definite pathological conditions in the foetus. Thus there are cases in which it co-exists with hydrocephalus. In these the size of the head may not be small, but that of the brain may be seriously diminished. Ahlfeld, again, ascribes many cases of micrencephalus, where there is no existing hydrocephalus, to a dropsy at an earlier period. Premature union (synostosis) of the tranial sutures and synchondrosis of the basal bones have been assigned II causes of microcephalus, the cranium not being able to expand the brain remains small. The observations of Bourneville have shown that it is more than doubtful if this can be accepted as an explanation of the condition. Again, thickenings of the pia mater sometimes co-exist, and this indicates an early inflammation as the probable cause of the defect in the brain. Any considerable degree of micrencephalus is accompanied by idiocy.

The average weight of the entire encephalon in the adult is about 50 1/4 ounces in the male and 44 3/4 in the female, the weight ranging from about 43 to 60 ounces in the male, and 40 to 50 in the female, although there are exceptional cases both below and above that range. The average weight of the cerebellum is 5 1/4 ounces in the male and 4 3/4 ounces in the female; that of the pons and medulla oblongata is about 1 ounce in the male, and the same in the female. (See further in Quain's Anatomy).

Parts of the brain are sometimes congenitally small. The convolutions may be imperfectly formed, or there may be asymmetry of the hemispheres. The cerebellum is also not infrequently imperfect. It may be as small as a walnut, or present various degrees of defect.

In a case reported by Fraser (Glasg. Med. Jour., xiii., 1880) there was a congenital ataxia which was found after death to be due to smallness of the cerebellum (2 1/2 ounces). Similar symptoms existed in a sister, who was presumably the subject of a similar defect.