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.); 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 (). (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, , 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.) 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.). 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.), 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 ), protecting the middle ear, and from above this there passes an arch, the cheek bone (ju. in ), 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., 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 (). 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 () the bulla hides the periotic bone from without. The student should examine all four figures for these bones before proceeding.) 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.), which is hidden by the maxilla in the side view of the skull.). 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.

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