102 OBSTETRICAL PHYSIOLOGY. 



cells, which increase in number and size, and are aggregated in rows, 

 with intercellular tracts where the ossification is about to begin. These 

 rows, in the cartilaginous basis of long bones, are vertical to its ends ; 

 in that of flat bones they are vertical to the margin. The cells furthest 

 from the seat of ossification are flattened and in close contact ; nearest 

 that seat they become enlarged and separated. The first appearance of 

 bone is that of minute granules in the intercellular tissue. Canals are 

 next formed in the bone, by absorption ; these ultimately receive blood- 

 vessels, and become the " vascular canals." The immediate nutrition 

 of bone is provided for by the production of minute "plasmatic canals," 

 " lacunae," or " bone-cells " from the vascular ones. Ossification begins 

 at the centre of round bones, and proceeds towards the surface ; in flat 

 bones it extends between two membranes, and from a central point 

 towards the periphery ; in short bones, towards the circumference; and 

 in long bones, from a central point or diapliysis, towards another 

 centre — the epiphysis, situated at each end. Particular parts or pro- 

 cesses are furnished with a separate centre of development, named the 

 aporihysis. Length occurs at the extremity of the diaphysis, and bulk 

 by deposition on the surface, the medullary canal of certain bones being 

 due to internal absorption. 



The spina is the first portion of the skeleton observed in the embryo, 

 it being represented by the chorda dorsalis, which is composed of a mass 

 of cells in the interior of a transparent sheath. The proto-vertebraB 

 appear on each side of the cord, and ultimately enclose it and constitute 

 the spinal canal ; in this way results the external sheath of the cord, and 

 the superior uniting membrane. The vertebral stalk now exists as a 

 membranous axis, but not for long, as it becomes segmented in order 

 to form the vertebras, and these segments are gradually converted into 

 cartilage. Each persistent vertebra does not correspond to a proto- 

 vertebra, the latter dividing into two portions to constitute two 

 vertebrae. The body of the vertebra is developed more quickly than 

 the spinous portion ; at the end of the second month all tiie vertebral 

 bodies are cartilaginous, while the laminae are yet in a membranous 

 condition. In the third month ossification commences, and during this 

 process the dorsal cord disappears, except between the vertebrae, where 

 it is developed to form the intervertebral fibro-cartilage. 



The /ace and cranium are formed by a membrane that envelops the 

 encephalon, and which is due to the proto-vertebral laminaB. This 

 cranial membrane becomes partly cartilaginous and partly fibrous, the 

 cartilage existing at the base of the cranium, and appearing to be a 

 prolongation of the bodies of the vertebrce : indeed, there is a resem- 

 blance between a vertebra and the cranium, in so far as the latter can 

 be resolved into four portions, each corresponding to a vertebra. This 

 cartilage is slowly transformed into bone ; while the fibrous part, 

 answering to the roof and sides of the skull, passes directly into the 

 osseous state. The bones of the face are formed by the j^haryngeal, 

 branchial, or visceral arches — a name given to four laminae which, 

 springing from the anterior extremity of the dorsal cord, curve down- 

 ward to meet those of the opposite side ; the spaces between them are 

 named the " pharyngeal clefts." The upper jaw, mouth, and nasal 

 cavities — composed by the nasal, maxillary, and palate bones— come 

 from the first pharyngeal arch ; while Meckel's cartilage, which passes 

 from the handle of the malleus towards the lower jaw, is also an ap- 

 pendage of it. This cartilage disappears about the sixth or seventh 



