70 ANIMAL BIOLOGY. [Part I. 



decalcifying the bone by dilute acid (chromic or hydrochloric), 

 and then cutting sections in the usual manner. 



We may begin by studying a section of a frog's femur. 

 Externally the living bone is invested by a layer of fibrous 

 connective tissue, the periosteum, on the inner surface of which 

 is a layer of special bone-forming cells or osteoblasts. Within 

 the living bone is a very vascular fatty substance the marrow 

 also containing osteoblasts. A section of the dry bone (25, ii.) 

 shows a number of dark elongated lacunce, from which radiate a 

 number of fine canaliculi. They are clearly arranged in two sets, 

 an inner and an outer. The inner are smaller, and their fine 

 canaliculi pass down towards the marrow cavity. The outer are 

 larger, and their canaliculi passing towards the circumference of 

 the bone, form branching tufts intersecting and uniting to form 

 a network. The lacunae run in rings round the bone, and thus 

 mark out the lamellce. Between the inner and the outer lamellae 

 is a middle lamella, in which there are no canaliculi. In the 

 young growing bone each lacuna has a bone cell sending out 

 processes into the canaliculi. There are a few Haversian canals 

 near the nutritive foramina for the entrance of the blood-vessels- 



A section of a metatarsal bone of the rabbit (25, iii.) shows us 

 an inner and an outer set of circumferential lamellce with small 

 lacunae. But between these two regions there is a wider space, 

 in which there are a number of groups of lamellae surrounding a 

 small cavity. Between the lamellae are lacunae. The central 

 cavity (from which fine radiating lines proceed outwards) is the 

 cross-section of a Haversian canal, containing blood-vessels, the 

 lamellae round which are called concentric lamellce. The Haversian 

 canals are in connection either with the medulla or with the 

 periosteum. One of the lacunae is figured under a high power 

 in 25, iv. 



A section of the femur of a rabbit shows us in the main an 

 aggregate of Haversian systems, one of which is shown in Fig. 

 25, v. There may be some circumferential lamellce externally, 

 and between the Haversian systems some interstitial lamellce. 



Close to the marrow cavity, especially towards the ends of 

 the growing bone, the structure is much more open than that of 



