=Vessels and Nerves.=—It is customary to recognize two sets of =arteries=—the =periosteal= and the =medullary=. The former ramify in the periosteum and give off innumerable small branches which enter minute openings (Volkmann’s canals) on the surface and reach the Haversian canals of the compact substance. Other branches enter the extremities of the long bones and supply the spongy bone and marrow in them. In the case of the larger bones—and especially the long bones—the large =medullary= or =nutrient artery= enters at the so-called =nutrient foramen= (Foramen nutricium), passes in a canal (Canalis nutricius) through the compact substance, and ramifies in the marrow; its branches anastomose with the central branches of the periosteal set. The larger =veins= of the spongy bone do not, as a rule, accompany the arteries, but emerge chiefly near the articular surfaces. Within the bone they are destitute of valves. The =lymph vessels= form perivascular channels in the periosteum and the Haversian canals of the compact substance. Lymph-spaces exist at the periphery of the marrow. The =nerves= appear to be distributed chiefly to the blood-vessels. Special nerve-endings (Vater-Pacini corpuscles) in the periosteum are to be regarded as sensory, and probably are concerned in mediating the muscle sense (Kopsch). DEVELOPMENT AND GROWTH OF BONE[2] The primitive embryonal skeleton consists of cartilage and fibrous tissue, in which the bones develop. The process is termed =ossification= or =osteogenesis=, and is effected essentially by bone-producing cells, called =osteoblasts=. It is customary, therefore, to designate as =membrane bones= those which are developed in fibrous tissue, and as =cartilage bones= those which are preformed in cartilage. The principal membrane bones are those of the roof and sides of the cranium and most of the bones of the face. The cartilage bones comprise, therefore, most of the skeleton. Correspondingly we distinguish =intramembranous= and =endochondral ossification=. [Illustration: FIG. 3.—LEFT FEMUR OF YOUNG PIG, EXTERNAL VIEW, TO SHOW DIVISION OF A LONG BONE INTO SHAFT (_s_) AND EXTREMITIES. Proximal extremity consists of two parts, head (_h_) and trochanter major (_t. m._), which have separate centers of ossification. Distal extremity consists of trochlea (_t_) and condyles (_c_); _e.l._, epiphyseal cartilages; _s.f._, supracondyloid fossa. ]