Book II. MUSCULAR STRUCTURE OF ANIMALS. 285 



1849. Bone is readily reproduced, in small quantities, especially in youth. In the case of fracture, the 

 periosteum inflames and swells, the crevice is filled up by a cartilaginous basis, abounding in vessels, and 

 the earthy matter is at length deposited, giving to the fractured part, in many cases, a greater degree of 

 strength than it originally possessed. In animals of the deer kind, the horns, which are true bone, 

 are annually cast off; a natural joint forming at their base, between them and the bones of the cranium, 

 with which they are connected. They are afterwards reproduced under a skin or periosteum, which the 

 animal rubs off when the new horns have attained their proper size. In some cases of disease, the 

 earthy matter is again absorbed into the system, the cartilaginous basis predominates, and the bones be- 

 come soft and tender. This takes place in the disease of youth termed rickets, and in a similar com- 

 plaint of advanced life, known under the name of mollifies ossium. In other instances, bone is formed 

 as a monstrous production, in organs which do not produce it in a state of health, as the brain, the heart, 

 and the placenta. {Monro's Outlines of Anatomy, p. 65.) 



1 850. Cartilage can scarcely be said to differ in its nature, from the cartilaginous 

 basis of the bone. It is of a fine fibrous structure, smooth on the surface, and re- 

 markably elastic. It covers those parts of bones which are exposed to friction, as the 

 joints, and is thickest at the point of greatest pressure. By its smoothness, it facilitates 

 the motion of the joints, and its elasticity prevents the bad effects of any violent con- 

 cussion. It is intimately united with the bone, and can scarcely be regarded as different 

 from an elongation of the cartilaginous basis. Where it occurs at a joint with consider- 

 able motion, it is termed articular or abducent cartilage. In other cases, it occurs as a 

 connecting medium between bones which have no articular surfaces, but where a variable 

 degree of motion is requisite. The ribs are united to the breast bone in this manner. 

 Between the different vertebrae, there are interposed layers of cartilage, by which the 

 motions of the spine are greatly faciUtated. As these connecting cartilages are com- 

 pressible and elastic, the spine is shortened when the body remains long in a vertical 

 position, owing to the superincumbent pressure. Hence it is that the height of man is 

 always less in the evening than in the morning. All these cartilages are more or less 

 prone to ossification, in consequence of the deposition of earthy matter in the interstices. 

 To this circumstance may be referred, in a great measuie, the stiffness of age, the 

 elasticity of the cartilages decreasing with the progress of ossification. 



1851. The articulations of bones, exhibit such remarkable diflTerences, in respect to 

 surface, connection, and motion, that anatomists have found it difficult to give to each 

 manner of union an appropriate name and character. We shall only notice the most 

 obvious kinds and motions, and these admit of two divisions, the true joints and the 

 motionless junctio7is. 



1852. In the motionless junctions, the connecting surfaces come into close and per- 

 manent contact, as in the serrated edges of the bones of the human skull, or the even 

 edges of the bones of the heads of quadrupeds and birds. Sometimes a pit in one bone 

 receives the extremity of another like a wedge ; as in the case of the human teeth : in other 

 cases the one bone has a cavity with a protuberance at its centre, which receives another 

 bone, as in the claws of cats, seals, &c. The human ribs are united with the breast- 

 bone by the intervention of cartilage, as are the two sides of the lower jaw with such 

 otlier in vertebral animals. 



1853. In true joints the articular surfaces are enveloped with cartilage, remarkable 

 for the smoothness of its free surface, and its intimate union with the bone, of which 

 it forms a protecting covering. The periosteum is not continued over the surface of 

 the cartilage, but is prolonged like a sheath over the joint, until it joins that of the 

 opposite bone. It thus forms a close bag at the joint, in which nothing from without 

 can enter, and from which nothing can escape. Into this bag the lubricating liquor 

 termed sj/nocra is conveyed. It is secreted by a mucous membrane on tlie interior, on 

 which account, as it in some cases appears like little bags, the term cursa mucosa has 

 been bestowed upon it. 



18.54. Ligaments. Besides the sheath formed by the continuation of the periosteum, and which is too 

 slender to retain the bones in their proper place, the joints are furnished with ligaments. These are 

 membranes of a dense fibrous texture, flexible, elastic, and possessed of great tenacity. They have their 

 insertion in the periosteum and bone with which they are intimately united. The motions which joints 

 of this kind are capable of performing, may be reduced to three kinds flexion, twisting, and sliding. 

 Inflexion, the free extremity of the bone which is moved, approaches the bone which is fixed, describing 

 the segment of a circle, whose centre is in the joint In twisting, the bone which is moved turns round 

 its own axis, passing through the articulation. In sliding, the free extremity of the bone moved, ap- 

 proaches the bone which is fixed, in a straight line. 



SuBSECT. 2. Muscular Structure of Animcils. 



1855. The muscles are the organs by which motion is executed: they unfold the most 

 singular mechanism of parts, and an infinite variety of movements. The muscles 

 appear in the form of large bundles, consisting of cords. These, again, are formed of 

 smaller threads, which are capable of division into the primary filaments. Each muscle, 

 and all its component cords and filaments, is enveloped by a covering of cellular mem- 

 brane, liberally supplied by blood-vessels and nerves. At the extremities of the 

 muscular fibres, where they are attached to the more solid parts, there are usually threads 

 of a substance, diflPering in its appearance from the muscle, and denominated tendon 

 or sinew. The tendons are, in general, of a silvery-white colour, a close, firm fibrous 



