MUSCULAR TISSUE. cxv 



skeleton are for the most part formed by the junction of two, but more generally of 

 three or more nuclei round the aperture or included space. The vertebral rings, the 

 acetabulum, the occipital foramen, and the cranium itself, are illustrations of this. 

 It is easy to conceive that in this way the ready and equable enlargement of such 

 cavities and apertures is provided for. 3. Bones of a complex figure, like the ver- 

 tebrae, have usually many nuclei; but the converse is not always true. 4. We can 

 frequently connect the number of nuclei with the principle of uniformity of type on 

 which the skeleton of vertebrated animals is constructed. Thus the typical form of 

 the sternum seems to be that of a series of distinct bones, one placed between each 

 pair of ribs in front, as the vertebrae are behind, and this is its permanent condition 

 in many quadrupeds. In man it conforms to the archetype in its mode of formation, 

 in so far as it is ossified from several centres, and for some time consists of several 

 pieces ; but, to suit the fabric of the human thorax, these at last coalesce one with 

 another, and are reduced in number to three. 



In the reunion of fractured bones, osseous matter is formed between and around 

 the broken ends, connecting them firmly together ; and when a portion of bone dies, 

 as happens in necrosis, a growth of new bone very generally takes place to a greater 

 or less extent, and the dead part is thrown off. The several steps of the process of 

 restoration in these instances are so fully described in works on Surgical Pathology, 

 that it is unnecessary to add to the length of this chapter by introducing an account 

 of them here. 



MUSCULAR TISSUE. 



The muscular tissue is that by means of which the active movements of 

 the body are produced. It consists of fine fibres, which are for the most 

 part collected into distinct organs called muscles, and in this form it is 

 familiarly known as the flesh of animals. These fibres are also disposed 

 round the sides of cavities and between the coats of hollow viscera, forming 

 strata of greater or less thickness. The muscular fibres are endowed with 

 contractility , a remarkable and characteristic property, by virtue of wliich 

 they shrink or contract more or less rapidly under the influence of certain 

 causes which are capable of exciting or calling into play the property in 

 question, and which are therefore named stimuli. A. large class of muscles, 

 comprehending those of locomotion, respiration, expression, and some 

 others, are excited by the stimulus of the will, or volition, acting on them 

 through the nerves ; these are therefore named " voluntary muscles," 

 although some of them habitually, and all occasionally, act also in obe- 

 dience to other stimuli. There are other muscles or muscular fibres which 

 are entirely withdrawn from the control of the will, such as those of the 

 heart and intestinal canal, and these are accordingly named " involuntary." 

 These two classes of muscles differ not only in the mode in which they 

 are excited to act, but also to a certain extent in their anatomical charac- 

 ters ; and on this account we shall consider the structure of each class 

 separately. 



Of the structure of voluntary muscles. The voluntary muscular fibres 

 are for the most part gathered into distinct masses or muscles of various 

 sizes and shapes, but most generally of an oblong form, and furnished with 

 tendons at each extremity, by which they are fixed to the bones. 



The two attached extremities of a muscle are named, in anatomical de- 

 scriptions, its origin and insertion ; the former term being usually applied 

 to the attachment which is considered to be most fixed, although the rule 

 cannot be always applied strictly. The fleshy part is named the belly, 

 which in some cases is interrupted in the middle or divided into two by 

 a tendon, and then the muscle is said to be biventral or digastric ; on 



