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PREFACE.

Table of Contents

In the following pages the term skeleton is used in its widest sense, so as to include exoskeletal or tegumentary structures, as well as endoskeletal structures. It was thought advisable to include some account of the skeleton of the lowest Chordata—animals which are not strictly vertebrates, but it seemed undesirable to alter the title of the book in consequence.

The plan adopted in the treatment of each group has been to give first an account of the general skeletal characters of the group in question and of its several subdivisions; secondly to describe in detail the skeleton of one or more selected types; and thirdly to treat the skeleton as developed in the group organ by organ.

A beginner is advised to commence, not with the introductory chapter, but with the skeleton of the Dogfish, then to pass to the skeletons of the Newt and Frog, and then to that of the Dog. After that he might pass to the introductory chapter and work straight through the book. I have endeavoured to make the account of each type skeleton complete in itself; this has necessitated a certain amount of repetition—a fault that I have found it equally difficult to avoid in other parts of the book.

Throughout the book generic names are printed in italics; and italics are used in the accounts of the type skeletons for the names of membrane bones. Clarendon type is used to emphasise certain words. In the classificatory table the names of extinct genera only, are printed in italics.

In a book in which an attempt is made to cover to some extent such a vast field, it would be vain to hope to have avoided many errors both of omission and commission, and I owe it to the kindness of several friends that the errors are not much more numerous. I cannot however too emphatically say that for those which remain I alone am responsible. Messrs C.W. Andrews, E. Fawcett, S.F. Harmer, J. Graham Kerr, and B. Rogers have all been kind enough to help me by reading proofs or manuscript, while the assistance that I have received from Dr. Gadow during the earlier stages and from Prof. Lloyd Morgan and Mr. Shipley throughout the whole progress of the work has been very great. To all these gentlemen my best thanks are tendered.

All the figures except 1, 35, 55, and 84 were drawn by Mr. Edwin Wilson, to whose care and skill I am much indebted. The majority are from photographs taken by my sister Miss K.M. Reynolds or by myself in the British Museum and in the Cambridge University Museum of Zoology, and I take this opportunity of thanking Sir W.H. Flower and Mr. S.F. Harmer for the facilities they have afforded and for permission to figure many objects in the museums respectively under their charge. I have also to thank (1) Prof. von Zittel for permission to reproduce figs. 27, 41, 52, 69, 70, 80, 106 A, and 107 C; (2) Sir W.H. Flower and Messrs A. and C. Black for figs. 1 and 84; (3) Prof. O.C. Marsh and Dr. H. Woodward for fig. 35; (4) Dr. C.H. Hurst and Messrs Smith, Elder, and Co. for fig. 55.

A few references are given, but no attempt has been made to give anything like a complete list. The abbreviations of the titles of periodicals are those used in the Zoological Record.

I have always referred freely to the textbooks treating of the subjects dealt with, and in particular I should like to mention that the section devoted to the skeleton of mammals is, as it could hardly fail to be, to a considerable extent based on Sir W.H. Flower's Osteology of the Mammalia.

SIDNEY H. REYNOLDS.

March 10, 1897.

CHAPTER I.
INTRODUCTORY ACCOUNT OF THE SKELETON IN GENERAL.

Table of Contents

By the term skeleton is meant the hard structures whose function is to support or to protect the softer tissues of the animal body.

The skeleton is divisible into

A. The Exoskeleton, which is external;

B. The Endoskeleton, which is as a rule internal; though in some cases, e.g. the antlers of deer, endoskeletal structures become, as development proceeds, external.

In Invertebrates the hard, supporting structures of the body are mainly exoskeletal, in Vertebrates they are mainly endoskeletal; but the endoskeleton includes, especially in the skull, a number of elements, the dermal or membrane bones, which are shown by development to have been originally of external origin. These membrane bones are so intimately related to the true endoskeleton that they will be described with it. The simplest and lowest types of both vertebrate and invertebrate animals have unsegmented skeletons; with the need for flexibility however segmentation arose both in the case of the invertebrate exoskeleton and the vertebrate endoskeleton. The exoskeleton in vertebrates is phylogenetically older than the endoskeleton, as is indicated by both palaeontology and embryology. Palaeontological evidence is afforded by the fact that all the lower groups of vertebrates—Fish, Amphibia, and Reptiles—had in former geological periods a greater proportion of species protected by well-developed dermal armour than is the case at present. Embryological evidence tends the same way, inasmuch as dermal ossifications appear much earlier in the developing animal than do the ossifications in the endoskeleton.

Skeletal structures may be derived from each of the three germinal layers. Thus hairs and feathers are epiblastic in origin, bones are mesoblastic, and the notochord is hypoblastic.

The different types of skeletal structures may now be considered and classified more fully.

A. Exoskeletal structures.

I. Epiblastic (epidermal).

Exoskeletal structures of epiblastic origin may be developed on both the inner and outer surfaces of the Malpighian layer of the epidermis[1] . Those developed on the outer surface include hairs, feathers, scales, nails, beaks and tortoiseshell; and are specially found in vertebrates higher than fishes. Those developed on the inner surface of the Malpighian layer include only the enamel of teeth and some kinds of scales. With the exception of feathers, which are partly formed from the horny layer, all these parts are mainly derived from the Malpighian layer of the epidermis.

Hairs are slender, elongated structures which arise by the proliferation of cells from the Malpighian layer of the epidermis. These cells in the case of each hair form a short papilla, which sinks inwards and becomes imbedded at the bottom of a follicle in the dermis. Each hair is normally composed of an inner cellular pithy portion containing much air, and an outer denser cortical portion of a horny nature. Sometimes, as in Deer, the hair is mainly formed of the pithy portion, and is then easily broken. Sometimes the horny part predominates, as in the bristles of Pigs. A highly vascular dermal papilla projects into the base of the hair.

Feathers, like hairs, arise from epidermal papillae which become imbedded in pits in the dermis. But the feather germ differs from the hair germ, in the fact that it first grows out like a cone on the surface of the epidermis, and that the horny as well as the Malpighian layer takes part in its formation.

Nails, claws, hoofs, and the horns of Oxen are also epidermal, as are such structures as the scales of reptiles, of birds' feet, and of Manis among mammals, the rattle of the rattlesnake, the nasal horns of Rhinoceros, and the baleen of whales. All these structures will be described later.

Nails arise in the interior of the epidermis by the thickening and cornification of the stratum lucidum. The outer border of the nail soon becomes free, and growth takes place by additions to the inner surface and attached end.

When a nail tapers to a sharp point it is called a claw. In many cases the nails more or less surround the ends of the digits by which they are borne.

Horny beaks of epidermal origin occur casing the jaw-bones in several widely distinct groups of animals. Thus among reptiles they are found in Chelonia (tortoises and turtles) as well as in some extinct forms; they occur in all living birds, in Ornithorhynchus among mammals, and in the larvae of many Amphibia.

In a few animals, such as Lampreys and Ornithorhynchus, the jaws bear horny tooth-like structures of epidermal origin.

The enamel of teeth and of placoid scales is also epiblastic in origin[2] , and it may be well at this point to give some account of the structure of teeth, though they are partly mesoblastic in origin. The simplest teeth are those met with in sharks and dogfish, where they are merely the slightly modified scales developed in the integument of the mouth. They pass by quite insensible gradations into normal placoid scales, such as cover the general surface of the body. A placoid scale[3] is developed on a papilla of the dermis which projects outwards and backwards, and is covered by the columnar Malpighian layer of the epidermis. The outer layer of the dermal papilla then gradually becomes converted into dentine and bone, while enamel is developed on the inner side of the Malpighian layer, forming a cap to the scale. The Malpighian and horny layers of the epidermis get rubbed off the enamel cap, so that it comes to project freely on the surface of the body.

As regards their attachment teeth may be (1) attached to the fibrous integument of the mouth, or (2) fixed to the jaws or other bones of the mouth, or (3) planted in grooves, or (4) in definite sockets in the jaw-bones (see p. 107).

Teeth in general consist of three tissues, enamel, dentine and cement, enclosing a central pulp-cavity containing blood-vessels and nerves. Enamel is, however, often absent, as in all living Edentates.

Enamel generally forms the outermost layer of the crown or visible part of the tooth; it is the hardest tissue occurring in the animal body and consists of prismatic fibres arranged at right angles to the surface of the tooth. It is characterised by its bluish-white translucent appearance.

II. Mesoblastic (mesodermal).

Dentine or ivory generally forms the main mass of a tooth. It is a hard, white substance allied to bone. When examined microscopically dentine is seen to be traversed by great numbers of nearly parallel branching tubules which radiate outwards from the pulp-cavity. In fishes as a rule, and sometimes in other animals, a variety of dentine containing blood-vessels occurs, this is called vasodentine.

Cement or crusta petrosa forms the outermost layer of the root of the tooth. In composition and structure it is practically identical with bone. In the more complicated mammalian teeth, besides enveloping the root, it fills up the spaces between the folds of the enamel.

The hard parts of a tooth commonly enclose a central pulp-cavity into which projects the pulp, a papilla of the dermis including blood-vessels and nerves. As long as growth continues the outer layers of this pulp become successively calcified and added to the substance of the dentine. In young growing teeth the pulp-cavity remains widely open, but in mammals the general rule is that as a tooth gets older and the crown becomes fully formed, the remainder of the pulp becomes converted into one or more tapering roots which are imbedded in the alveolar cavities of the jaws. The opening of the pulp-cavity is then reduced to a minute perforation at the base of each root. A tooth of this kind is called a rooted tooth.

But it is not only in young teeth that the pulp-cavity sometimes remains widely open; for some teeth, such as the tusks of Elephants and the incisor teeth of Rodents, form no roots and continue to grow throughout the animal's life. Such teeth are said to be rootless or to have persistent pulps.

An intermediate condition is seen in some teeth, such as the grinding teeth of Horses. These teeth grow for a very long time, their crowns wearing away as fast as their bases are produced; finally however definite roots are formed and growth ceases.