GENERAL PRINCIPLES APPLICABLE TO THE STUDY OF ALL THE BONES. 19 



Nerves. — These belong to the cerebro-spinal and ganglionic system of nerves ; 

 the latter are always vaso-motor nerves. 



Almost constantly, a somewhat voluminous nerve enters the medullary canal, 

 by passing through the nutrient foramen, and is distributed to the medulla. 

 The compact tissue receives few nerve filaments ; while, on the contrary, the 

 spongy tissue at the extremities of the long bones, as well as the short bones,, 

 contains many. Certain short bones, such as the vertebrte, are especially 

 remarkable for the numerous nerves they receive. 



DEVELOPMENT OP BONES. 



The bones, before arriving at the condition in which we see them in the 

 adult animal, pass through several successive phases, the study of which consti- 

 tutes Osteogeny. 



Nearly all the bones were originally cartilaginous, those of the roof of the 

 cranium and the face being only represented by fibrous tissue. We will examine 

 the development of these cartilaginous and fibrous bones. 



A. Development of the Cartilaginous Bones. — In the embryo at an early period^ 

 the bones are composed of a mucous material analogous to that which enters into 

 the composition of all the other organs ; this matter is constituted by a mass of 

 embryonic cells. Later, they become harder, white, and elastic — that is, cartila- 

 ginous. Certain portions of the skeleton persist in this condition during the life 

 of the animal. These permanent cartilages are found where the bony skeleton 

 must have a certain amount of flexibility, and on the articular surfaces. 



The temporary cartilages, like the permanent, have a fundamental amorphous 

 01 hyaline substance, in which are embedded round cells containing one or several 

 nuclei. But they soon undergo modifications, which result in giving to the 

 pieces they form the hardness and structure of perfect osseous tissue. 



These modifications gradually cause the cartilage to disappear, and to be 

 replaced by bony tissue, without the skeleton ceasing for an instant to preserve 

 its form and functions. The process commences by calcification of the peri- 

 chondrium and of the middle part of the diaphysis, which ensures the stability 

 of the organ. Then the blood-vessels, which are constantly present in temporary 

 cartilages, pass through the diaphysary nucleus, ramify, and are directed in a 

 parallel manner towards the extremities. Simultaneously, the chondroplasts are 

 arranged in parallel series in front of the vessels {rivulation of the cartilage), and 

 finish by merging into large, irregular, elongated cavities, incompletely separated 

 by bands of fundamental substance of the calcified cartilage, designated the 

 directing lines of ossification. 



The blood-vessels enter these cavities, carrying to their surface cells which 

 proliferate, and are differentiated into osteoblasts or productive cells of bone. 

 The osteoblasts are deposited, layer upon layer, against the walls of the spaces 

 limited by the lines of ossification, and form, by a kind of secretion, concentric 

 strata of the osseous fundamental system which surrounds them ; then the 

 osteoblasts become osteoplasts. The cavities made in the cartilage finish by 

 becoming filled up, except at the centre, where a central cylindrical, space 

 remains (Haversian canal), and a blood-vessel. 



In the extremities of the bones, where there are no Haversian canals, the 

 preparatory modifications are the same as in the diaphysis, only the cartilaginous 

 cells collect in small irregular masses, and not in parallel piles. Each of these 



