The Skeleton

Text Book of Biology, Part 1: Vertebrata by H. G. Wells, is part of the HackerNoon Books Series. You can jump to any chapter in this book here. The Skeleton

The Skeleton

Section 75. We are now in a position to study the rabbit's skeleton. We strongly recommend the student to do this with the actual bones at hand-- they may be cleared very easily in a well-boiled rabbit. This recommendation may appear superfluous to some readers, but, as a matter of fact, the marked proclivity of the average schoolmaster for mere book-work has put such a stamp on study, that, in nine cases out of ten, a student, unless he is expressly instructed to the contrary, will go to the tortuous, and possibly inexact, descriptions of a book for a knowledge of things that lie at his very finger-tips. We have not written, this chapter to give a complete knowledge of the skeleton, but simply as an aid in the actual examination of the bones.

Section 76. We may take the skeleton under five headings. There is the central axis, the chain of little bones, the vertebrae, threaded on the spinal cord (see Figure 1 and Section 1); the thorax, the box enclosed by ribs and sternum; the fore-limb and bones connected with it (pectoral girdle and limb), and the hind-limb and bones connected with it (pelvic girdle). Finally there is the skull, but following the London University syllabus, we shall substitute the skull of the dog for of that of the rabbit, as more typically mammalian (Section 4).

Section 77. In Section 3 (which the student should refer to) we have a division of the vertebrae into four varieties. Of these most representative is the thoracic. A thoracic vertebra (Figure 4, Sheet 5, T.V.) consists of a central bony mass, the body or centrum (b.), from which there arises dorsally an arch, the neural arch (n.a.), completed by a keystone, the neural spine (n.s.); and coming off laterally from the arch is the transverse process (tr.p.). Looking at the vertebra sideways, we see that the arch is notched, for the exit of nerves. Jointed to the thoracic vertebrae on either side are the ribs (r.). Each rib has a process, the tuberculum, going up to articulate with the transverse process, and one, the capitulum articulating between the bodies of two contiguous vertebrae. The facets for the articulation of the capitulum are indicated in the side view by shading. At either end of the body of a vertebra of a young rabbit are bony caps, the epiphyses (ep.), separated from the body by a plane of unossified cartilage (indicated, by the dots). These epiphyses to the vertebral bodies occur only among mammals, and are even absent in some cases within the class. In the adult rabbit they have ossified continuously with the rest of the body.

Section 78. A cervical vertebra (C.V.) seems, upon cursory inspection, to have no rib. The transverse processes differ from those of thoracic series in having a perforation, the vertebrarterial canal, through which the vertebral artery runs up the neck. A study of the development of these bones shows that the part marked f.r. ossifies separately from the rest of the transverse process; and the form of the equivalent structures in certain peculiar lower mammals and in reptiles leaves no doubt that f.r. is really an abbreviated rib; fused up with the transverse process and body. The two anterior cervical vertebrae are peculiar. The first (at.) is called the Atlas-- the figure shows the anterior view-- and has great articular faces for the condyles (Section 86) of the skull, and a deficient centrum. The next is the axis, and it is distinguished by an odontoid peg (od.p.), which fits into the space where the body of the atlas is deficient. In development the centrum of the axis ossifies from one centre, and the odontoid, peg from another, which at that time occupies the position of centrum of the atlas. So that it would seem that the atlas is a vertebra minus a centrum, and the axis is a vertebra plus a centrum, added at the expense of the atlas.

Section 79. The lumbar vertebrae (l.v.) are larger, and have cleft transverse processes, each giving rise to an ascending limb, the metapophyses, and a descending one. The latter (generally spoken of as the transverse processes) point steeply downward, and are considerably longer than those of thoracic series. The sacral vertebrae (s.v.) have great flattened transverse processes for articulation with the ilia. The caudal vertebrae (c.v.) are gradually reduced to the mere elongated centra, as we proceed, towards the tip of the tail.

Section 80. All the vertebrae join with their adjacent fellows through the intermediation of certain intervertebral pads, and also articulate by small processes at either end at the upper side of the arch, the zygapophyses. The normals to the polished facets of these point, in the case of the anterior zygapophyses, up and in (mnemonic: ant-up-in), and in the case of the posterior, down and out. The student should make this, and the other features of vertebrae, out upon actual specimens.

Section 81. The thorax is bounded dorsally by the vertebral column, and ventrally by the sternum. The sternum consists of segments, the sternebrae (st.); anteriorly there is a bony manubrium (mb.), posteriorly a thin cartilaginous plate, the xiphisternum (xi.). Seven pairs of ribs articulate by cartilaginous ends (sternal ribs) with the sternum directly, as indicated in the figure; five (false) ribs are joined, to each other and to the seventh, and not to the sternum directly. The last four ribs have no tuberculum (Section 77).

Section 82. The fore-limb (pectoral limb) consists of an upper arm bone, the humerus (hum.) the distal end of which is deeply excavated by the olecranon fossa (o.f.) as indicated by the dotted lines; of two bones, the ulna (u.) and radius (r.) which are firmly bound by ligament in the position of the figure (i.e., with the palm of the hand downward, "prone"); of a number of small bones (carpalia), the carpus (c.); of a series of metacarpals (mc.); and of three digits (= fingers) each, except the first, or pollex, of three small bones-- the phalanges, only the proximal of which appear in the figure. The ulna has a hook-like head, the olecranon (o.) which distinguishes it easily from the distally thickened radius. The limb is attached to the body through the intermediation of the shoulder-blade (scapula, sc.) a flattened bone with a median external ridge with a hook-like termination, the acromion (acr.). There is also a process overhanging the glenoid cavity (g.) wherein the humerus articulates, which process is called coracoid (co.); it is ossified from two separate centres, and represents a very considerable bone in the bird, reptile, and frog. Along the dorsal edge of the scapula of the rabbit is unossified cartilage, which is called the supra-scapula (s.sc.). In man there runs from the acromion to the manubrium of the sternum a bone, the collar-bone or clavicle. This is represented by a very imperfectly ossified rudiment in the rabbit. The scapula and clavicle, the bones of the body connected with the fore-limb, are frequently styled the pectoral girdle, or shoulder-girdle; this name of girdle will appear less of a misnomer when lower vertebrate types are studied.

Section 83. The hind limb and its body bones-- pelvic limb and girdle-- are shown in Figure 2. The limb skeleton corresponds closely with that of the fore-limb. The femur (fe.) answers to the humerus, and is to be distinguished from it by the greater distinctness of its proximal head (hd.) and by the absence of an olecranon fossa from its distal end. The tibia (ti = the radius) is fused for the distal half of its length with the fibula (fb. = ulna). A tarsus (tarsalia) equals the carpus.* Two of the proximal tarsalia may be noted: one working like a pulley under the tibia, is the astragalus (as.); one forming the bony support of the heel, is the calcaneum (ca.). There is a series of metatarsals, and then come four digits of three phalanges each.

* Such a resemblance as exists between one vertebra and another in the rabbit, or between the humerus and the femur, is called serial homology; the two things correspond with each other to the extent of imperfect reduplication. "Homology" simply is commonly used to indicate the resemblance between any two structures in different animals, in origin and position as regards other parts. Thus the heart of the rabbit and of the frog are homologous structures, corresponding in position, and resembling each other much as two memory sketches of one picture might do.

Section 84. The pelvic girdle differs from the pectoral in most land vertebrata in being articulated with the vertebral column. This difference does not exist in fishes. It consist in the rabbit of four bones; the ilium (i.), the ischium (is.), the pubis (pb.), and the small cotyloid bone-- the first two and the latter one meeting in the acetabular fossa (ac.) in which the head of the femur works. The pubes and ischia are fused along the mid-ventral line. Many morphologists regard, the ilium as equivalent to, that is, strictly corresponding in its relation, to the scapula, the pubis to the cartilaginous substratum of the clavicle, and the ischium to the coracoid.

Section 85. These bones will be studied at the greatest advantage when dissected out from a boiled rabbit. Prepared and wired skeletons, disarticulated skeletons, plates of figures, and written descriptions are in succession more tedious and less satisfactory ways to a real comprehension, of this matter. This chapter directs the student's attention to the most important points in the study of the skeleton, but it is in no way intended to mitigate the necessity of practical work. It is a guide simply.

Section 86. The mammalian skull will be better understood after the study of that of some lower vertebrate. We shall describe its main features now, but their meaning will be much clearer after the lower type is read. Our figures are of Canis. In section (Figure VI., Sheet 6), we perceive a brain case (cranium) opening behind by a large aperture, the foramen magnum (F.M.). In front of this is an extensive passage, the nasal passage (E.N. to P.N.) which is divided from the mouth by a bony floor, the palate, and which opens into the pharynx behind at the posterior nares (P.N.) and to the exterior by the anterior or external nares (E.N.). It is divided into right and left passages by a middle partition, the nasal septum. Outside the skull, on its wings, is a flask-like bone, the bulla tympani (b. in Figures 2 and 3), protecting the middle ear, and from above this there passes an arch, the cheek bone (ju. in Figures 1, 2, and 3), to the upper jaw, forming in front the bony lower protection of the cavity containing the eye, the orbit. The cheek arch, nasal passage, and jaws, form collectively the "facial apparatus," as distinguished from the cranium, and the whole skull is sometimes referred to as, the "cranio-facial apparatus." Two eminences for articulation with the atlas vertebra, the condyles (c.), lie one on each side of the lower boundary of the foramen magnum.

Section 87. The floor of the cranium consists of a series of cartilage bones, the basi-occipital (b.o.), basi-sphenoid (b.sp.), pre-sphenoid (p.sp.), and in front, the ethmoid (eth.), which sends down a median plate, not shown, in the figure, to form the nasal septum between right and left nasal passages. Like extended wings on either side of the basi-occipital are the ex-occipital (e.o.) (the bone is marked in Figure 4, but the letters are a little obscured by shading). Similarly the ali-sphenoids (a.s.), are wings to the basi-, and the orbito-sphenoids (o.s.), to the pre-sphenoid bone (p.sp.). Between the ex-occipital and ali-sphenoid there is wedged in a bone, the periotic (p.o.) containing the internal ear (Section 115). Above the foramen magnum the median supra-occipital bone completes what is called the occipital arch. A pair of parietals (pa.) come above the ali-sphenoids, and a pair of frontals (f.) above the orbito-sphenoids. At the side the brain case is still incomplete, and here the squamosal (sq.) enters into its wall. In the external view (Figure 3) the bulla hides the periotic bone from without. The student should examine all four figures for these bones before proceeding.

Section 88. The outer edge of the upper jaw and the cheek arch are made up of three paired bones. First comes the premaxilla (p.m.) (not p.m.1 or p.m.4), containing in the dog, the three incisors of either side. Then comes the maxilla, bearing the rest of the teeth.* The jugal or malar (ju.) reaches over from the maxilla to meet a zygomatic process (= connecting outgrowth) (z.p.) of the squamosal bone.

* In the dog a sabre-like canine (c.), four premolars (p.m.1 and p.m.4) and two molars (m.1 and m.2).

Section 89. In the under view of the skull (Figure 2) it will be seen that the maxilla sends in a plate to form the front part of the hard palate. Behind, the hard palate is completed by the pair of palatine bones (pal.), which conceal much of the pre- and orbito-sphenoid in the ventral view, and which run back as ridges to terminate in two small angular bones, the pterygoids (pt.) which we shall find represent much more important structures in the lower vertebrata.

Section 90. The pre-maxillae and maxillae bound the sides of the nasal passage, and it is completed above by a pair of splints, the nasals. Along the floor of the nasal passage, on the middle line, lies a splint of bone formed by the coalescence of two halves. It embraces in a V-like groove the mesethmoid (nasal septum) above, and lies on the palate.

{Lines from First Edition only.}
-Its position is indicated by a heavy black line in 4, and it is
called, the vomer bone (vo.).-

{Lines from Second Edition only.}
[In the frog it is represented by two laterally situated bones. This is
the vomer bone (vo.).]

The nasal passages are partially blocked by foliated bony outgrowths, from the inner aspect of their walls, which in life are covered with mucous membrane, and increase the surface sensitive to smell. The ethmoid ends in the ethmo-turbinal (e.t.); the nasal, the naso-turbinal (n.t.); and the maxilla, the maxillo-turbinal (m.t.). In the anterior corner of the orbit there is a bone, the lachrymal (lc. Figure 1), which is hidden by the maxilla in the side view of the skull.

Section 91. The lower jaw (mandible) is one continuous bone in the mammal. Three incisors bite against the three of the upper jaw. Then comes a canine, four premolars, and three molars, the first of which is blade-like (sectorial tooth), and bites against the similar sectorial tooth (last premolar) of the upper jaw. The third molar is small. The arrangement of tooth is indicated in the following dental formula:-- I. 3.3/3.3, C. 1.1/1.1, P.M. 4.4/4.4, M. 2.2/3.3

Section 92. Attached just behind the bulla above, and passing round on either side of the throat to meet at the base of the tongue, is the hyoid apparatus (Figure 6). The stylohyal (s.h.), epihyal (e.h.), and ceratohyal (c.h.) form the anterior cornu of the hyoid. The body of the hyoid (b.h.) forms a basis for the tongue. The posterior coruna (t.h.) of the hyoid are also called the thyrohyals.

Section 93. The following table presents these bones in something like their relative positions. A closer approximation to the state of the case will be reached if the student will imagine the maxilla raised up so as to overlie and hide the palatine and presphenoid, the squamosal similarly overlying the periotic bone, and the jugal reaching between them. Membrane bones are distinguished by capital letters.

-Cranium_

-Nasal_ (paired), Ethmoid Bone (median), -Vomer_
-Frontal_ (paired), -Lachrymal_ (paired), Orbito-sphenoid (paired),
Pre-sphenoid (median), Eye
-Parietal_ (Paired), Ali-sphenoid (paired), Basi-sphenoid (median)*,
Periotic Bone (paired)
-Bulla_ (paired)
Supra-occipital (median), Ex-occipital (paired), Basi-occipital (median)

-Upper Jaw_

-Pre-Maxilla_ (paired)
Palatine (paired)
Pterygoid (paired)

-Lower Jaw_

-Maxilla_ (paired)
-Jugal_ (paired)
-Squamosal_ (paired)

*In this table the small bones of the ear are simply indicated by an asterisk.

Section 94. Hidden by the bulla, and just external to the periotic bone, are the auditory ossicles, the incus, malleus, os orbiculare, and stapes. These will be more explicitly treated when we discuss the ear.

Section 95. When we come to the study of the nerves, we shall revert to the skull, and treat of its perforations. The student should not fail, before proceeding, to copy and recopy our figures, and to make himself quite familiar with them, and he should also obtain and handle an actual skull. For all practical purposes the skull of a sheep or cat will be almost as useful as that of the dog.

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