Endoskeleton

“ It is, in fact, true of several groups of Vertebrata which have lived through a considerable range of time, that the endoskeleton (more particularly the spinal column) of the older genera presents a less ossified, and, so far, less differentiated, condition than that of the younger genera. ”

“ GENERAL CHARACTERS—PLEUROPTERYGII—ICHTHYOTOMI—ACANTHODEI—PLAGIOSTOMI—SELACHII—BATOIDEI—HOLOCEPHALI
CLASS II. PISCES.
Sub-Class I. Elasmobranchii.
In both the ancient and the modern Sharks, Dog-Fishes, and Rays, the exoskeleton takes the form of a more or less uniform investment of dermal denticles or "shagreen." The endoskeleton is wholly cartilaginous or partially calcified, and there are neither cartilage- nor membrane-bones. The vertebral column is acentrous or chordacentrous, generally with alternating basi- and inter-dorsal elements, and terminating in a heterocercal tail. ”

“ It preserves the form of the body, protects important organs, and supplies the mechanical devices, or machines, upon which the muscles act in the production of motion. The skeleton is adapted to its purposes through the number and properties of the bones, and through the cartilage and connective tissue associated with the bones. The places where the different bones connect one with another are known as joints, and most of these admit of motion. The preservation of the natural form of the skeleton is necessary, both for its proper action and for the health of the body. ”

“ They exhibit a rough symmetry; they rarely attain any size; and they usually cease growing when the skeleton attains maturity--the conversion of cartilage into bone being then completed. While they originate from the ossifying junctions of the long bones, they tend, as the shaft increases in length, to project from the surface of the bone at some distance from the ossifying junction and to "point" away from it. They may cause symptoms by "locking" the adjacent joint or by pressing upon nerve-trunks or blood vessels. ”

“ The disorder of growth is confined to those elements of the skeleton where a core of bone formed in cartilage comes to be encased in a sheath of bone formed beneath the periosteum. To indicate this abnormality the name _diaphysial aclasis_ has been employed by Arthur Keith at the suggestion of Morley Roberts.
Bones formed entirely in cartilage are exempt, namely, the tarsal and carpal bones, the epiphyses of the long bones, the sternum, and the bodies of the vertebrae. Bones formed entirely in membrane, that is, those of the face and of the cranial vault, are also exempt. ”

“ The skeleton begins as a continuum, and a continuum it remains all life long. The things that link bone with bone, {713} cartilage, ligaments, membranes, are fashioned out of the same primordial tissue, and come into being pari passu, with the bones themselves. The entire fabric has its soft parts and its hard, its rigid and its flexible parts; but until we disrupt and dismember its bony, gristly and fibrous parts, one from another, it exists simply as a “skeleton,” as one integral and individual whole. ”

“ In other cases the end of the bone is protruded out of its socket, by inflammation or enlargement of the cartilages or ligaments of the joint, so that it rests on some part of the edge of the acetabulum, which in time becomes filled up. When the legs are straight, as in standing erect, there is no verticillary motion in the knee-joint; all the motion then in turning out the toes further than nature designed, must be obtained by straining in some degree this head of the thigh-bone, or the acetabulum, or cavity, in which it moves. ”

“ But in the case of the true Placoids that analogy is faint indeed. Their skeletons contain true bone;—the vertebral joints of the Sharks and Rays possess each, as has been shown, an osseous nucleus, which retains, when subjected to the heat of a common fire, the complete form of the joint; and their cranial framework has its surface always covered over with hard osseous points. But though their skeletons possess thus their modicum of bone, unlike those of embryonic birds or mammals, they contain, in what is properly their cartilage, no gelatine. ”

“ 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 constituent 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. ”

“ When you want to make a straw man stand upright you run a pole right through him to give him support. Such a support is the backbone to your own body, keeping the trunk from falling together.
In the abdomen nothing more is wanted than this backbone, the sides and front of the cavity being covered in with skin and muscle only. In the chest{13} the sides are strengthened by the ribs, long thin hoops of bone which are fastened to the backbone behind and meet in front in a firm hard part, partly bone, partly cartilage, called the sternum. ”

“ And indeed the trunk is not a bag with soft yielding sides, but a box with walls which are in part firm and hard. You noticed that when you were cutting through the front of the chest you had to cut through several hard places. These were the ribs (Fig. 1, a) , made either of hard bone or of a softer gristly substance called cartilage. And if you take away all the viscera from the cavity of the trunk and pass your finger along the back of the cavity, you will feel all the way down from the neck to the legs a hard part. This is the backbone or vertebral column. ”

“ There is sometimes a well-developed dermal exoskeleton formed of small granules and plates of bone. The bones are solid, and the vertebral centra flat. The cranium bears a pair of enormous pointed frontal horns, and the parietal is greatly expanded and elevated behind, forming with the squamosals a shield which overhangs the anterior cervical vertebrae. The premaxillae are united, and in front of them is a pointed beak-like bone which bites upon a toothless predentary ossification of the mandible. The teeth have two roots. The anterior limbs are but little shorter than the posterior ones. ”

“ In a work of this limited size the sensibility of the various tissues cannot be fully treated but it should be borne in mind that the integument and the subcutaneous tissue, fat and muscles as well as the tendons, their sheaths, the muscles and periosteum and perichondrium covering the bones and cartilages throughout the body, are all highly sensitive to pain. It is also equally true that the bone substance, the bone marrow, and the cartilages are devoid of any of the four modalities of sensation. ”

“ It consists of two layers, an outer fibrous and an inner cellular layer; the cells, which are called osteoblasts, are continuous with those lining the Haversian canals and the medullary cavity.
The arrangement of the blood vessels determines to some extent the incidence of disease in bone. The nutrient artery, after entering the medullary canal through a special foramen in the cortex, bifurcates, and one main division runs towards each of the extremities, and terminates at the ossifying junction in a series of capillary loops projected against the epiphysial cartilage. ”

“ The exoskeleton is of two types; dermal bones are largely developed in the head region, while the tail and posterior part of the body may be naked or may be covered with overlapping scales. The cranium remains chiefly cartilaginous, the palato-pterygo-quadrate bar is fused with the cranium, and the suspensorium is autostylic. The gill clefts are feebly developed and open into a cavity covered by an operculum. The notochord is persistent and unconstricted, and the limbs are archipterygia. The pelvic fins are without claspers. ”

“ Lining the medullary and cancellous cavities is a delicate connective tissue lining, the endosteum, which contains many bone-forming cells, while on the outside of the bone, except at the articular ends, is the periosteum with its outer protective layer and its inner vascular layer containing osteoblasts or bone-forming cells. The periosteum is essential for the growth of new bone where the old bone has died, and if the periosteum is removed from healthy bone the part beneath is liable to die, as it is by the constant growth of the osteoblasts that the bone grows and is renewed. ”

“ In the recumbent posture, when the weight of the trunk is taken off, and the muscles are relaxed, most of the symptoms of the luxation disappear, and the limb may be shortened or elongated at pleasure. In walking, the body is inclined towards the sound side, and the head of the dislocated bone sinks towards the cotyloid cavity by its own weight. As age advances, the limb becomes shorter, in consequence of the femur ascending higher and higher on the ilium; the obliquity of the pelvis augments; and the power of locomotion, already so much impaired, is completely destroyed. ”

“ In adolescents and adults the limbs seldom become affected; the bones composing the spinal column are the seat of the disease, and, along with the distortion of the spine, the position of the ribs is necessarily altered. Certain rare cases have occurred, in which all the bones of the adult were softened to a very great degree. In one remarkable instance, the patient complained of an annoying sense of tightness in the limb most affected, and, on examination, the softened bone was found greatly depressed at that point, as if a strong ligature had been drawn tightly round it. ”

“ In the wide range of the animal kingdom there are scarce any two pieces of organization that less resemble one another in form than the vertebræ of the placoids resemble their skulls; and the difference is not merely external, but extends to even their internal construction. In both skull and vertebræ we detect an union of bone and cartilage; but the bone of each vertebra forms an internal continuous nucleus, round which the cartilage is arranged, whereas in the skulls it is the cartilage that is internal, and the bone is spread in granular points over it. ”

“ Cut the flesh from the thick portion of the tail.
8. Cut off the head at the first cervical vertebra, and clean the skull as previously described elsewhere, but leave the hyoid bone in its place.
9. Cut the flesh away from the neck vertebræ as well as you can. Be careful not to cut the sternum (or breastbone) , which is soft cartilage, and easily cut; nor the ends of any vertebral processes, nor any soft bones.
10. If the skeleton is a small one, it is apt to get quite bloody during the operation. ”

“ It is rarely necessary to remove more of the bone than merely its articular extremity (when performed for disease of the joint) , and if possible this should be done inside the capsule, i.e. through an incision in the capsule, but without involving its attachment to the neck of the bone. When the glenoid is also diseased, mere gouging or scraping the cartilaginous surface will not suffice, but the neck must be thoroughly exposed,{116} so that the whole cup of the glenoid may be removed by powerful forceps. ”

“ A very tough skin—a sheathe—surrounds the neck with blood vessels, nerves, muscles, bones, ligaments, fascia, glands great and small, throat and trachea. In bones we find a great canal for spinal cord. It is well and powerfully protected by a strong wall of bone, so no outer pressure can obstruct the flow of passing fluids, to keep vitality supplied by brain forces, but with all the guards given to protect the cord, we find that it can be overcome by impact fluids to such degree as to stop blood and other fluids from supplying lungs and all below. ”

“ The skeleton of Petromyzon (Fig. 52) consists of a cranio-facial skeleton composed of a cartilaginous unsegmented cranium, with the basal trabeculæ and parachordals and a series of branchial and visceral cartilaginous bars forming the so-called branchial basket-work; to these must be added auditory and nasal capsules. In contradistinction to this elaborate cranio-facial skeleton, the spinal vertebral skeleton is represented only by segmentally arranged small pieces of cartilage formed in the connective tissue dissepiments between segmented sheets of body-muscles (myotomes) . ”

“ The broken extremities of the bones become intimately united by bony matter passing from one to the other. The mass of new bone on the outside, formed by the ossification of the callus, being no longer wanted, is absorbed; by degrees the whole of it disappears, and the bone is left having the same dimensions which it had before the occurrence of the accident.
The process of union is completed in young persons sooner than in those advanced in life; in the upper extremities sooner than in the lower; and in smaller animals more speedily than in man. ”

“ These are tailless Amphibia, which except in a few instances, are devoid of an exoskeleton. The vertebrae are as a rule procoelous, and are very few in number. The post-sacral part of the spinal column ossifies continuously, forming an unsegmented cylindrical rod, the urostyle. Remains of the notochord persist, lying vertebrally, i.e. enclosed within the centra of the several vertebrae, and not as in Urodela lying between one vertebra and the next. The skull is very short and wide. The mandible is almost always, if not invariably, toothless. ”