Section 63. In all the epithelial tissues that we have considered we have one feature in common: they are cells, each equivalent to the amoeba, that have taken on special duties-- in a word, they are specialists. The amoeba is Jack of all trades and a free lance; the protective epidermal cell, the current-making ciliated cell, the bile or urea-making secretory cell, is master of one trade, and a soldier in a vast and wonderfully organized host. We will now consider our second kind of cell in this organization, the cell of which the especial aim is the building round it of a tissue.

Section 64. The simplest variety in this group is hyaline (i.e. gla.s.sy) cartilage (gristle). In this the formative cells (the cartilage corpuscles) are enjellied in a clear structureless matrix (Figure XII.), consisting entirely of organic compounds acc.u.mulated by their activity. Immediately round the cell lies a capsule of newer material.

Some of the cells have recently divided (1); others have done so less recently, and there has been time for the interpolation of matrix, as at 2. In this way the tissue grows and is repaired. A thin layer of connective tissue (see below), the perichondrium, clothes the cartilaginous structure.

Section 65. Connective tissue (Figure XIII) is a general name for a group of tissues of very variable character. It is usually described as consisting typically in the mammals of three chief elements felted together; of comparatively unmodified corpuscles (c.c.), more or less amoeboid, and of fibres which are elongated, altered, and distorted cells. The fibres are of two kinds: yellow, branching, and highly elastic (y.e.f.), in consequence of which they fall into sinuous lines in a preparation, and white and inelastic ones (w.i.f.), lying in parallel bundles. Where the latter element is entirely dominant, the connective tissue is tendon, found especially at the point of attachment of muscles to the parts they work. Some elastic ligaments are almost purely yellow fibrous tissue. A loose interweaving of the three elements is areolar tissue, the chief fabric of mesentery, membrane, and the dermis (beneath the epidermis). With muscle it is the material of the walls of the alimentary ca.n.a.l and bloodvessels, and generally it enters into, binds together, and holds in place other tissue. The connective tissue of fishes displays the differentiation of fibres in a far less distinct manner.

Section 66. Through connective tissues wander the phagocytes, cells that are difficult to distinguish, if really distinct, from the white blood corpuscles. These cells possess a remarkable freedom; they show an initiative of their own, and seem endowed with a subordinate individuality. They occur in great numbers in a tissue called, botryoidal tissue (Figure XIV.), which occurs especially in ma.s.ses and patches along the course of the alimentary ca.n.a.l, in its walls. The tonsils, swellings on either side of the throat, are such ma.s.ses, and aggregates occur as visible patches, the Peyer's patches, on the ileum. It also const.i.tutes the ma.s.s of the vermiform appendix and the wall of the sacculus rotundus; and in the young animal the "thymus gland," ventral to the heart, and less entirely, the "thyroid gland," ventral to the larynx, are similar structures, which are reduced or disappear as development proceeds. It is evident that in these two latter cases the term "gland" is somewhat of a misnomer. The matrix of botryoidal tissue is a network of stretched and hollowed connective tissue cells-- it is not a secretion, as cartilage matrix appears to be. During digestion, the phagocytes prowl into the intestine, and ingest and devour bacteria, that might otherwise give rise to disease. In inflammation, we may note here, they converge from all directions upon the point wounded or irritated.

They appear to be the active agents in all processes of absorption (see osteoclasts under bone), and for instance, migrate into and devour the tissue of the tadpole's tail, during its metamorphosis to the adult frog.

Section 67. Within the connective tissue cells fat drops may be formed, as in Figure XV. Adipose tissue is simply connective tissue loaded with fat-distended cells. The tissue is, of course, a store form of hydro-carbon (Section 17) provided against the possible misadventure of starvation. With the exception of some hybernating animals, such store forms would seem to be of accidental importance only among animals, whereas among plants they are of invariable and necessary occurrence.

Section 68. We now come to Bone, a tissue confined to the vertebrata, and typically shown only in the higher types. As we descend in the scale from birds and mammals to lizards, amphibia (frogs and toads) and fish, we find cartilage continually more important, and the bony const.i.tuent of the skeleton correspondingly less so. In such a type as the dog-fish, the skeleton is entirely cartilaginous, bone only occurs in connection with the animal's scales; it must have been in connection with scales that bone first appeared in the vertebrate sub-kingdom. In the frog we have a cartilaginous skeleton overlaid by numerous bony scutes (shield-like plates) which, when the student comes to study that type, he will perceive are equivalent to the bony parts of such scales as occur in the dog-fish, sunk inward, and plating over the cartilage; and in the frog the cartilage also is itself, in a few places, replaced by bony tissue. In the adult rabbit these two kinds of bone, the bone overlying what was originally cartilage (membrane bone), and the bone replacing the cartilage (cartilage bone) have, between them, practically superseded the cartilage altogether. The structure of the most characteristic kind of bone will be understood by reference to Figure XVI. It is a simplified diagram of the transverse section of such a bone as the thigh bone. M.C. is the central marrow cavity, H.v., H.v. are cross sections of small bloodvessels, the Haversian vessels running more or less longitudinally through, the bone in ca.n.a.ls, the Haversian ca.n.a.ls. Arranged round these vessels are circles of the formative elements, the bone corpuscles or osteoblasts (b.c.) each embedded in bony matrix in a little bed, the lacuna, and communicating one with another by fine processes through ca.n.a.liculi in the matrix, which processes are only to be seen clearly in decalcified bone (See Section 70). The osteoblasts are arranged in concentric series, and the matrix is therefore in concentric layers, or lamellae (c.l.). Without and within the zone of Haversian systems are (o.l. and i.l.), the outer and inner lamellae. The bone is surrounded by connective tissue, the periosteum. In addition to this compact bone, there is a lighter and looser variety in which spicules and bars of bony tissue are loosely interwoven. Many flat bones, the bones of the skull, for instance, consist of this spongy bone, plated (as an electro spoon is plated) with compact bone.

Section 69. Among the bony bars and spicules of spongy bone occurs the red marrow-- which must not be confused with the yellow marrow, the fatty substance in the central cavity of long bones. In this red marrow are numerous large colourless cells, which appear to form within their substance and then liberate red blood corpuscles.

This occurs especially in the spongy bone within the ribs.

Section 70. The matrix of bone differs from that of cartilage or of most other tissues in consisting chiefly of inorganic salts. The chief of these is calcium phosphate, with which much smaller quant.i.ties of calcium carbonate, and magnesium phosphate and carbonate occur. These inorganic salts can be removed by immersion of the bone in weak hydrochloric acid, and a flexible network of connecting tissue, Haversian vessels, bone corpuscles, and their processes remains. This is decalcified bone alluded to above.

Section 71. In the very young rabbit, the limb bones, vertebral column, and many of the skull bones are simply plates and bars of cartilage; the future membrane bones, however are planned out in connective tissue. The development of the latter is simple, the connective tissue corpuscles functioning by a simple change of product as osteoblast. The development of the cartilage bones, however, is more complicated. Figure XVII., represents, in a diagrammatic way, the stages in the conversion of a cartilaginous bar to bone. To begin with, the previously sporadically-arranged (scattered anyhow) corpuscles (u.c.c.) are gathered into groups in single file, or in other words, into "columnar" groups (as at c.c.). The matrix becomes clouded with inorganic salts of lime, and it is then said to be calcified. This calcified cartilage then undergoes absorption-- it must not be imagined for a moment that bone is calcified cartilage. Simultaneous with the formation of the cavities (s.) due to this absorption, connective tissue (p.c.i.) from the surrounding perichondrium (p.c.) grows into the ossifying* bar. It is from this connective tissue that the osteoblasts (o.b.) arise, and bone is built up. Throughout life a bone is continually being absorbed and reformed by the activity of the osteoblasts. An osteoblast engaged in the absorption instead of the formation of bone is called an osteoclast.

* The formation of bone is called ossification. To ossify is to become bony.

Section 72. The great thing to notice about this is that cartilage does not become bone, but is eaten into and ousted by it; the osteoblasts and osteoclasts replace entirely the cartilage corpuscles, and are not derived from them.

Section 73. We may mention here the structure of the spleen (Figure 1, Sheet 1). It consists of a connective tissue and muscular coating, with an internal soft matrix much resembling botryoidal tissue, traversed by fibrous trabeculne (= beams, planks) containing blood-vessels, and the whole organ is gorged with blood, particularly after meals. The consideration of its function the student may conveniently defer for the present.

Section 74. Here also, we may notice the lymphatics, a series of small vessels which return the overflow of the blood serum from the capillaries, in the nutrition of the tissues in all parts of the body, to the thoracic duct (see Section 36), and the general circulation. At intervals their course is interrupted by gland-like dilatations, the lymphatic glands, in which ma.s.ses of rapidly dividing and growing (proliferating) cells occur, of which, doubtless, many are detached and become, first "lymph corpuscles," and, when they reach the veins, white blood corpuscles.

5. _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 subst.i.tute 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 ma.s.s, 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 cla.s.s. 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 ca.n.a.l, 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 s.p.a.ce 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 propcesses 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, "p.r.o.ne"); 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 h.o.m.ology; the two things correspond with each other to the extent of imperfect reduplication. "h.o.m.ology" 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 h.o.m.ologous 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 p.u.b.es 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 pa.s.sage, the nasal pa.s.sage (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 pa.s.sages by a middle part.i.tion, 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 pa.s.ses 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 pa.s.sage, 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 pa.s.sages. 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 aquamosal (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 pa.s.sage, and it is completed above by a pair of splints, the nasals. Along the floor of the nasal pa.s.sage, 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.