Text-book of the embryology of man and mammals / by Oscar Hertwig ; translated from the third German edition by Edward L. Mark.

Date:
1892
Catalogue details

Licence: Public Domain Mark

Credit: Text-book of the embryology of man and mammals / by Oscar Hertwig ; translated from the third German edition by Edward L. Mark. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

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    of the arch are fused. Each vertebra is then separable after destruction of the soft parts into two pieces, into the body and the arch. These are united between the third and eighth years. In addition to the pieces of bone just described, accessor]/ centres of ossijication appear on the vcrtcbrai in subsequent years ; it is in this way that there arise the epiphysial j)lates at the end-surfaces of tlie body and the small bony pieces at the ends of the vertebral processes (the spinous processes and the transverse processes). Schwegel gives detailed information concerning the time of their appearance and their fusion. Oartilaginoiis skeletal parts, which serve for the support of the lateral and ventral walls of the body, the ribs and the breast bone, contribute to the completion of the axial skeleton. The ribs are developed independently of the vertebral column, in Man during the second month, by the chondrification of strips of tissue in the intermuscular ligaments between the successive muscle- segments. They are at first visible as small bent rods in the imme- diate vicinity of the body of the vertebra, and from here they rapidly extend ventrally. In eai'ly stages of development ribs are established from the first to the last segment of the vertebral column (the coccyx in Man excepted), but only in the case of the lower Vertebrates (Fishes, many Amjahibia, and Rejatiles) are they developed into large bows supporting the wall of the trunk in a uniform manner in all regions, whereas in Mammals and in Man they exhibit in the separate regions of the vertebral column different conditions. In the neck, lumbar and sacral regions, they appear from the beginning in a rudimentary condition only, and undergo metamorphoses to be described later. It is exclusively in the thoracic region that they attain important dimensions, and here at the same time they give rise to a new skeletal part—the breast bone, or sternum. The sternum, which is wanting in Fishes and Dipnoi, but is present in Amphibia, Reptiles, Birds, and Mammals, is a formation derivetl from the thoracic ribs, and is originally established, as Rathke was the first to discover, as a paired structure, which early fuses into an U7ij)aired skeletal jnece. E.UOE has followed the development of the sternum in Man in a very thorough manner, and has found that in embryos 3 cm. long the fij'st five to seven thoracic ribs have become prolonged into the ventral surface of the breast and by a broadening of theu' ends have united at some distance from the median plane to form a cartilaginous band, whereas the following libs end free and at a greater distance from