Degeneracy: Its Causes, Signs and Results

by Eugene S. Talbot

The cranium or skull is a development in part of the vertebræ or bones forming the backbone, and in part of dermal or membranous bones, which of old in reptiles, as in the alligator to-day, formed the protective armour of the skin of the head. As the head end of the spinal cord of the lancelet developed, the cartilage enclosing it developed to protect it. This was the earlier evolution. Later, another skull developed in connection with this. The cranium or skull therefore has, as Minot remarks,[188] a double origin, or, rather, there are two skulls which were originally distinct. In evolution from the lowest fish to the highest mammal, and in the embryonic development of man, these become united.

The primary skull, as already stated, is practically an extension of the vertebræ, which send side outgrowths to cover the brain as the backbone covers the spinal cord. This primary skull extended in front of the notochord (the spinal cord of the human embryo, and the permanent spinal cord of the lancelet), where it gave off two trabeculæ cranii or front skull plates. Behind, the primary skull or chondrocranium gives off two occipal or rear skull plates. It gives off also two plates midway between the trabeculæ and occipitals, which, as they gradually enclose the primitive hearing apparatus, the otocysts (permanent in fish, and embryonic in man), are called periotic capsules. This primary skull is at first cartilaginous, as in sharks. With the increase in the size of the brain in evolution and in human embryology, this cartilaginous primary skull became insufficient to roof over the brain, and thus resulted gaps in it. The fontanelles, or soft places at the top, sides, and back of the head of the new-born child, are the remains of this failure of the chondrocranium, or primary skull, to cover the gains of the nervous system in the struggle for existence. This deficiency, resultant on advance in evolution, would have been a long-standing serious block to further advance, were it not that the skin of the mammal retained a function inherited from the reptiles and bony fish.

These cavities were filled by dermal bones, which, at first serving merely as armour in the skin of the head, came to be protectors of the nervous system. The following bones represent these dermal bones in the embryonic human skull: The frontals, which form the chief part of the forehead; the sutures, or dovetails, of these normally disappear in the adult, so that the forehead seems to be but one bone. This union may not occur (Fig. [7]), as in the case of the philosopher Kant, who had a frontal suture all his life. The dovetails are replaced by solid bone, through a process called synostosis. In the case of the frontal bone it is normal, and in the line of advance. Elsewhere in the skull it is often an expression of defect which may give rise to various cranial states which are either absolutely degenerate in type or degenerate only when occurring in certain races. The parietals and interparietals are also dermal bones which are united by synostosis to form the parietals or side bones of the normal adult skull. The nasal bones which, together with the vomer, form the nose, are likewise dermal bones, and so are the pterygoids and palatines. The maxillaries and præmaxillaries, which, with the mandibles, form the jaws, are dermal bones. The mandibles, however, are in part derived from the chondrocranium.

FIG. 7.

With rise in evolution, and during the progress of human embryonic development, these bones become fewer through their early gristly union or their synostosis. The openings in the skull resultant on the deficiencies in the chondrocranium are larger in the sauropsida (birds and reptiles) than in the ichthyopsida (amphibious and fish); in the monotremata (egg-laying mammals) than in the sauropsida; in the marsupials (pouched mammals) than in the monotremata, and in the higher mammals than in the marsupials. The development of the brain therefore depends on the growing and expanding power of the secondary skull formed by the dermal bones. These, considered as bones, are degenerate from the high type of the vertebræ, and are a mere reminiscence of that outer skeleton whereby early fish and reptiles emulated the lobster. The influence of any check to development such as produces degeneracy, is exerted first on the development of the bone itself, and finally on the relation to other bones by dovetailing.

In accordance with the general laws governing growth, deficiency in one place is apt to result in increase elsewhere. The brain-protective function of the dermal bones being later in development than their old armour function, is apt to be checked by degeneracy in two ways; in the first the bone does not grow in size or sufficiently to unite with its fellows, or this growth occurs only for the benefit of the bone itself, through Spencer’s law of individuation, so that union with the other bones occurs too early for the benefit of the organism as a whole. To the factors underlying this is due the non-increase in intellect after puberty which occurs in the higher apes, and in some of the lower races of men.

These checks also tend to the nutritional benefit of the older primary skull, whence result the irregularities in development that constitute so many of the stigmata of the degenerate cranium. The sutures sometimes do not form because sufficient gristle is not produced to fill the gaps. (Fig. [8].) These secondary gaps are often filled by new dermal bones called Wormian. Sometimes this deficiency coexists with too early synostosis elsewhere.