made out with the aid of reagents and a powerful microscope. This is called the axis cylinder. In some nerve fibres (mostly in the brain and spinal cord) the axis cylinder is naked, and even a single fibril may so pass from one cell to another in the brain matter. In other parts the axis cylinder is generally covered by a thin membrane, called the primitive sheath, or with a soft, oillike substance, called the medullary sheath, or, as is commonly the case in most peripheral nerves, by both. The primitive sheath encloses the medullary sheath, which surrounds the axis cylinder. These fibres are made of peculiarly modified cells, which are, however, so elongated as not to be very easily recognized as such in adult tissue. The nerve or ganglion cells vary extremely in general form and size. The commonest in the nerve centres are large bodies with a clear, well-defined, vesicular, single nucleus, and distinct nucleolus; they have two or more processes, which are connected by nerve fibres to other cells, and to the axis cylinder of nerves. The peripheral nerve cells are generally much modified, and often small compared with those in the centres. Besides the cells in the sporadic ganglia, which are large rounded corpuscles with but few processes, there are many other bodies connected with the peripheral nerves which cannot be called ganglion corpuscles. They are nevertheless nerve cells. Muscles or Contractile Tissues.-When changes take place in protoplasm adapting it specially for contraction, it is termed muscle tissue. The large masses of this tissue attached to the skeleton so as to move its various parts, form the flesh of the higher animals. Muscle tissue is, almost invariably, connected with nerve tissue, and acts in response to stimuli communicated from the nerves. In some of the lower animals, the two tissues are so intimately related that it is not easy to distinguish them, and the development of both progresses equally as ascend the scale of animal life. They are nearly related in their origin, or even spring from the same primitive tissue. In fact, as has already been mentioned (vide p. 46), they form but one structure in some of the more simple and less differentiated animals. The neuro-muscular tissue, which is formed from the outer layer of the embryo, is the forerunner of the muscles as well as of the nerves of the embryo of the higher animals. In the higher animals and man muscle tissue consists of two distinct kinds of textures, known as— (a) Smooth, or non-striated muscle. (b) Striated muscle. In the smooth muscle the individual elements present the characters of an elongated and flattened cell, and contain a single long nucleus. They contract very slowly, and require a comparatively long time for the nerve influence to affect them, so that an obvious interval exists between the moment of their stimulation and their contraction. They are found in the internal organs and in situations where gradual and lasting contractions are required. They receive their nervous supply generally from the sympathetic system, and perform their duty without our being conscious of their activity or being able to control it by our will. Striated muscle tissue is made up of cylindrical fibres of such length that both extremities cannot be brought into the field of the microscope at the same time. Their exact relation to cells is not so easily made out as in smooth muscle, and doubtless varies in different muscles. Sometimes the fibres are made up of single cells, and in other cases they are formed by the permanent fusion FIG. 23. FIG. 22. of several cell elements which never differentiate into separate elements, owing to the imperfect division of the cells, but make up one mass, the multiple nuclei of which alone make its mode of origin apparent. The contractile substance is made up of two kinds of material, one of which refracts light singly, while the other is doubly refracting. These are ranged alternately across the fibre, making the transverse markings or striæ from which it gets its name. This striated material is quite soft and is encased in a thin homogeneous elastic sheath called sarcolemma, which fits closely around the soft contractile substance. This form of muscle is the widest departure from the primitive protoplasmic type, being specially modified so as to perform strong and quick contractions. It moves with wonderful rapidity, contracting almost the instant its nerve is stimulated. It forms the great mass of the quick-acting skeletal muscles, being attached to the bones by bands composed of a form of fibrous tissue, which form the tendons and fascia. Muscles made of striated tissue are commonly under the control of the will, and hence are frequently spoken of as voluntary muscles, but this term is misleading, for many striated muscles are not governed by voluntary control. The Connective Tissue group, coming exclusively from the mesoblast, exhibits very great varieties of form. Its cells differ much from the epithelial cells both in their character and their relations, and particularly in the adult tissues. Under the heading Connective Tissues are generally classed all those which support the frame and hold together the various other tissues and organs. They are 1. Mucous and retiform connective tissues. 2. White and yellow fibrous tissue. 3. Cartilage. 4. Bone. 5. Endothelium. The cells of all these tissues have the property of manufacturing some material which does not generally enclose them as a cell wall, but remains between the cells and forms the intercellular |