. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates -- Anatomy. SKELETON. 39 Cartilages and bones are covered on their outer surfaces by an envelope of connective tissue, called respectively perichondrium or periosteum. These membranes form the means by which muscles are attached to the bones and by which blood-vessels obtain entrance to them. The periosteum is also a seat of bone formation. DERMAL SKELETON. When present, the dermal skeleton arises by a marked prolifera- tion of cells at definite points in the corium. These cells become specialized (scleroblasts, odontoblas
. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates -- Anatomy. SKELETON. 39 Cartilages and bones are covered on their outer surfaces by an envelope of connective tissue, called respectively perichondrium or periosteum. These membranes form the means by which muscles are attached to the bones and by which blood-vessels obtain entrance to them. The periosteum is also a seat of bone formation. DERMAL SKELETON. When present, the dermal skeleton arises by a marked prolifera- tion of cells at definite points in the corium. These cells become specialized (scleroblasts, odontoblasts or osteoblasts) for the deposition of salts of lime plus a varying amount of organic matter (ossein). Upon limy plates formed in this way other parts, also calcareous, may be laid down by the basal surface of the epidermis, so that the whole dermal element may be in part mesenchymatous, in part ectodermal in FIG. 31.—Cross-sections of developing scale of Acanthias. c, stratum corneum; d, dentine of scale; ee, enamel organ; m, stratum Malpighii; p, pulp. It is generally thought that the primitive dermal skeleton resembled that of existing sharks, and that from the hypertrophy or fusion of such scales the so-called membrane bones have arisen. Then the scales of other vertebrates are to be traced back to an elasmobranch ancestry, while teeth are thought to be modified scales. Hence the structure and development of the elasmobranch scale should be understood. At regular intervals in the skin of a shark there is a multiplication of cells of the corium, each aggregation forming a small papilla which projects above the surrounding corium, carrying with it the basal layer of the epidermis. The surface cells of the papilla and the region around it becomes converted into osteoblasts which secrete calcic. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustration
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectanatomy, bookyear1912
