Thomas Henry Huxley; A Sketch Of His Life And Work

by Sir P. Chalmers Mitchell

[CHAPTER VIII]

VERTEBRATE ANATOMY

The Theory of the Vertebrate Skull—Goethe, Oken, Cuvier, and Owen—Huxley Defends Goethe—His Own Contributions to the Theory—The Classification of Birds—Huxley Treats them as "Extinct Animals"—Geographical Distribution—Sclater's Regions—Huxley's Suggestions.

We have seen that some of the most important of the contributions made by Huxley to zoölogical knowledge were in the field of the lower animals, especially of those marine forms for the study of which he had so great opportunities on the Rattlesnake. A great bulk of his zoölogical work, however, related to the group of back-boned animals. These, by their natural affinities and anatomical structure, are more closely related to man, and, as Huxley began his scientific work as a medical student, the groundwork of all his knowledge was study of the anatomy and physiology of man. Moreover, throughout the greater part of his working life, he had more to do with the extinct forms of life. The vertebrate animals, from the great facility for preservation which their hard skeleton presents, as well as from the extremely important anatomical characters of the skeleton, bulk more largely in the study of palæontology than does any other group. In each of the great groups of vertebrate animals, in fishes, amphibia, reptiles, birds, and mammals, Huxley did important work. Much of this is embodied in his treatise on Vertebrate Anatomy, but to some particular parts of it special attention may now be directed, as much because these serve as excellent examples of his method of work as because of their intrinsic importance.

The skull is the most striking feature in the skeleton of vertebrate animals, and to the theory and structure of the vertebrate skull Huxley paid special attention, and his views and summary of the views of others form the basis of our modern knowledge. This work was put before the public in the course of a series of lectures on Comparative Anatomy given in 1863, while Huxley was Hunterian Professor at the Royal College of Surgeons, and the beginnings of it were contained in a Croonian lecture to the Royal Society in 1858.

The theory of the skull which held the field was known as the vertebral theory. The great bulk of the nervous system of vertebrate animals consists of a mass of tissue lying along the dorsal line of the body and enclosed in a cartilaginous or bony sheath. The nerve tissue is the brain and spinal cord; the sheath is the skull in front and the vertebral column along the greater part of the length of the animal. The brain may be taken simply as an anterior portion of the nerve mass, corresponding in a general way to an expansion of the spinal cord in the region of the anterior limbs and an expansion in the region of the hind limbs, the latter indeed having recently been shown in some extinct creatures to surpass the brain in size. In a similar simple fashion the skull may be taken as an expanded anterior part of the vertebral column, serving as an expanded box for the brain, just as in the regions of the pectoral and pelvic expansions of the cord there are similar expansions of the surrounding bony case. We know now, from greater knowledge of its embryological development, that the brain contains structures quite peculiar to itself, and differs from the spinal cord in kind as well as in size; but, at the same time, when the vertebral theory of the skull was inaugurated, embryological knowledge and the importance of its relation to anatomical structure were less considered. What Huxley did was to show that the skull, in its mode of origin and real nature, was not merely an expanded portion of the vertebral column, but that it differed from it in kind.

The hypothesis of the vertebral structure of the skull was due both to Goethe, the great German poet, and Oken, a most able but somewhat mystic German anatomist. An attempt had been made by a well-known English anatomist to cast on Goethe the stigma of having tried to rob Oken of the credit for this theory. Huxley set that matter finally at rest, disproving and repelling with indignation the unworthy suggestion. Oken gave out his theory in 1807, and described how it had been first suggested to his mind by the accident of picking up a dried and battered sheep's skull, in which the apparent vertebral structure was very obvious, as, indeed, anyone may see at a glance. It was in 1820, long after the theory had been made current, that the poet first publicly narrated that in a similar way he had long before come to the same conclusion; but Huxley was able to show that, although announcing it later, Goethe had in reality anticipated the anatomist. A passage occurs in a letter to a friend, of a date in 1790, which admits of no doubt. "By the oddest happy chance, my servant picked up a bit of an animal's skull in the Jews' cemetery at Venice, and, by way of a joke, held it out to me as if he were offering me a Jew's skull. I have made a great step in the formation of animals." It is an interesting trait in Huxley's character, to find him zealous in defence of the reputation of a great man, even although that man had been dead more than half a century; but it may be added that his just zeal was at least stimulated by the fact that the maligner of Goethe was Owen, the conduct of whom, with regard to Darwin and Huxley, Huxley had had just reason for resenting.

The theory, then, which had dropped stillborn from Goethe, but which Oken developed, was simply that the skull consisted of a series of expanded vertebræ. Each vertebra consists of a basal piece or centrum, the anterior and posterior faces of which are closely applied to the face of an adjoining vertebra, and of a bony arch or ring which encloses and protects the nervous cord. Oken supposed that there were four such vertebræ in the skull, the centra being firmly fused and the arches expanded to form the dome of the skull. Quite correctly, he divided the skull into four regions, corresponding to what he called an ear vertebra, at the back, through which the auditory nerves passed; a jaw vertebra, in the sphenoidal region, through which the nerves to the jaws passed; an eye vertebra in front, pierced by the optic nerves, and again in front a nose vertebra, the existence of which he doubted at first. Quite rightly, he discriminated between the ordinary bones of the skull and the special structures surrounding the inner ear which he declared to be additions derived from another source. So far it cannot be doubted that the vertebral theory made a distinct advance in our knowledge of the skull. It was to a certain extent, however, thrown into disrepute by various fantastic theories with which Oken surrounded it. Later on, Cuvier removed from it these wilder excrescences, and amplified the basis of observation upon which the underlying theory of the unity of type of the skull throughout the vertebrates was based. Cuvier, however, came to reject the theory, except so far as it applied to the posterior or occipital segment of the skull. Later on, Owen resuscitated the theory, first throwing doubt on the merit of Goethe, and then suggesting that Oken, instead of relying on the observed facts, had deduced the whole theory from his own imagination. Owen, although he made no new contribution to fact or theory in this matter, practically claimed the whole credit of it as a scientific hypothesis.

When Huxley took up the subject, the position was that the vertebral theory was in full possession of the field, under the auspices of Owen. Huxley began afresh from observed facts. The first object of his investigation was to settle once for all the question as to whether the skulls of all vertebrates were essentially modifications of the same type. He took in succession the skulls of man, sheep, bird, turtle, and carp, and showed that in all these there were to be distinguished the same four basi-cranial regions: the basi-occipital, basi-sphenoid, pre-sphenoid, and ethmoid. These were essentially identical with the centra of the four vertebræ of Oken. Similarly, he showed the composition of the lateral and dorsal walls, proving the essential identity of the structures involved and of their relations to the nerve exits in the great types he had chosen. In the series of lectures delivered before the College of Surgeons, he extended his observations to a much larger series of vertebrates, and substantially laid down the main lines of our knowledge of the skull. In two important respects his statements were not merely a codification of existing knowledge, but an important extension of it. He distinguished the different modes in which the jaws may be suspended to the skull, and established for these different kinds of suspensoria the names which have ever since been employed. He proved clearly what had been suggested by Oken, that the region of the ear is a lateral addition to the skull, and he distinguished in it three bones, his names for which have since become the common property of anatomists. Finally, he made it plain beyond any possible doubt that the skulls of all vertebrates were built upon a common plan.