Handbook of physiology / by W. D. Halliburton.
- William Dobinson Halliburton
- Date:
- 1900
Licence: In copyright
Credit: Handbook of physiology / by W. D. Halliburton. Source: Wellcome Collection.
Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)
838/903
![ch. lviii.] FORMATION OF LIMBS AND HEAD. 8l I The Limbs.—The muscles of the body developed from the lateral extensions of the protovertebra? are, at first, like the vertebra?, arranged in successive segments or myotomes. This is very well seen in the ringed condition of the muscles in such simple vertebrates as amphioxus. Even in fishes, where the limbs are not in a high state of development, the muscular segments are well seen. They are seen also in man in the intercostal muscles, and in the abdominal region are indicated by the transverse septa across the rectus abdominis, but here, as in other mammals, this simple metameric segmentation is masked by the great develop- ment of the large muscles which attach the limbs to the four corners of the trunk. The limbs are lateral extensions of segments or somites in certain situations. They consist of parietal mesoblast covered by epiblast. At first there is simply a bud, but this grows, and in time divides into three segments, arm, fore-arm, and hand in the upper limb ; thigh, leg, and foot in the lower limb. The hand and foot then give rise to buds corresponding to the digits. Each limb is connected to a limb girdle. The epiblast here, as else- where, forms the epidermis ; the true skin, subcutaneous tissues, muscles, blood-vessels, and cartilages (subsequently replaced by bone) are formed by differentiation from the mesoblast. In further development the positions of the limbs become shifted by rotation, so that the anterior (radial) border of the upper limb becomes outermost, and the anterior (tibial) border of the lower limb becomes internal. Formation of the Head. In the formation of the head, a number of elements are con- cerned. There is first the notochord, which extends as far forwards as the dorsum sellce ; this, however, as in the vertebral column, is transitory, and is soon replaced by a primitive cartila- ginous cranium developed from the mesoblast around it, as the vertebrae are developed from the protovertebrse. This forms the base of the skull. The roof or cranial vault is formed by mem- brane bones, that is, bones not preceded by cartilage; sense capsules which are formed around prolongations of the brain, and the visceral arches and slits contribute to the formation of that part of the head which is called the face. The mesoblast, which continues up the protovertebra? into the head region on each side of the notochord, is not separated into parts corresponding to vertebra?. Cartilage is formed in it; two cartilaginous bars, one on each side of the notochord, are called the parachordal cartilages,](https://iiif.wellcomecollection.org/image/b21286346_0839.jp2/full/800%2C/0/default.jpg)
