94 



OBSTETRICAL PHYSIOLOGY. 



the remainder of this production, particularly its lower part, preserves its 

 yellow color until the young animal is born. In solipeds the chesnuts 

 are seen at mid-term, in the form of thin brownish plates, which soon be- 

 come darker. The structure of the hoof is not tubular until after birth, 

 when the foetal hoof, gradually disappearing, the horn that replaces it is 

 fibrous and tubular, and much more consistent. 



The corneous substance is developed at the expense of the blastema 

 which the capillaries of the modified derm throws out on its surface. In 

 this material appear nucleated cells which, pressing against each other, 

 become at first polygonal in shape and flattened, then lose their nucleus 

 and are confounded with each other. At a later period, cells of a new 

 formation are moulded on the papillae of the coronary cushion and plantar 

 surface of the foot of solipeds and ruminants, giving it that fibrous ap- 

 pearance which is so striking during extra-uterine life. 



The Locomotor ^ Apparatus. 



The development of bone, and with it the locomotory apparatus in gen- 

 eral, next demands our attention. Bone is developed in the blastema or 

 primitive basis — a transparent glairy mucus matter containing numerous 

 minute corpuscles. This progressively acquires increased firmness ; some- 

 times assuming a membraneous or ligamentous condition^ usually a gristly 

 consistence, before its conversion into bone. The change into cartilage 

 is noted by the appearance of minute nucleated 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 vascu- 

 lar 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 diap/iysis, 

 towards another centre — the epiphysis, situated at each end. Particular 

 parts or processes are furnished with a separate centre of development, 

 named the apophysis. Length occurs at the extremity of the diaphysis, 

 and bulk by deposition on the surface, the medullary cand of certain 

 bones being due to internal absorption. 



The spinal stalk is the first portions 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- 

 vertebrae 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 membraneous axis, but not for long ; as it becomes segmented in 

 order to form the vertebrae, and these segments are gradually converted 

 into cartilage. Each persisteiit vertebra does not correspond to a proto- 

 vertebra, the latter dividing into two portions to constitute two vertebrae. 

 The body of the veirtebra is developed more quickly than the spinous 



