64 NORMAL HISTOLOGY. 



Bursae. In situations in which a muscle or tendon moves over a bony 

 prominence, or in which two tendons glide upon each other, the intervening 

 areolar tissue contains a space filled with fluid, termed a bursa. Such bursae, 

 whose evident purpose is to diminish friction, are abundant in connection 

 with the limb-muscles and in the vicinity of the large articulations may com- 

 municate with the synovial cavities. Conspicuous examples of such relation 

 are the subscapular bursa, which opens into the shoulder-joint, and the supra- 

 patellar bursa, which communicates with the cavity of the knee-joint. Sub- 

 cutaneous bursce are also developed in the areolar tissue separating the super- 

 ficial and deep fasciae in situations in which the skin glides over a bone and 

 is subject to pressure, as over the tip of the elbow. The immediate lining 

 of the bursal sac consists of an incomplete endothelial investment, while the 

 wall of the sac itself is composed of dense fibre-elastic tissue. 



NERVOUS TISSUE. 



The nervous system the complex apparatus by which the organism 

 is brought into relation with its surroundings and by which its various parts 

 are united into one coordinated whole consists of structural units, the 

 neurones, held together by a special sustaining tissue, the neuroglia, assisted 

 by ingrowths of the surrounding connective tissue. The neurone, the 

 morphological unit of the nervous system, includes a nucleated protoplasmic 

 accumulation, the cell-body, and the processes. The former, usually spoken 

 of as the nerve-cell, presides over the nutrition of the neurone and, in many 

 cases, is the seat of the subtle changes giving rise to nervous impulse. The 

 processes originate as outgrowths from the cell-body during development and 

 provide the conduction paths along which impulses are conveyed. They 

 are very variable in length, some extending only a fraction of a millimeter 



beyond the cell-body, while others continue 

 many centimeters to distant parts of the body. 

 The longer processes usually acquire protect- 

 ing sheaths and are known as nerve-Jibres. 

 These are associated into bundles and consti- 

 tute the nerves that pass to the muscles and 

 various other organs. 



Reduced to its essential parts, the nervous 

 system consists of two units. The one, the 

 sensory neurone, takes up the stimulus received 

 upon the skin or other sensory surface and, 

 by means of its process (nerve-fibre), conveys 

 such impulse from the periphery towards the 



FIG. 83. Diagram showing funda- r ,i .1 . < ' . 



mental units of nervous system. A, aggregations of nerve-cells that lie in the 



sensory neurone, conducting afferent vicinitv of the bodv-axis Funrtiormllv surri 

 impulsesbyitsprocess(a) from periph- Juy axis, naiiy, bl 



ery (S) , , motor neurone sending a path consists of an afferent fibre. The 



efferent impulses by its process (*) to i . i j ^ r j ^ 



muscle. impulses thus carried are transferred to the 



second unit, the motor neurone, which, in 



response, sends out the impulse originating within the cell-body (nerve-cell) 

 along the process known as the efferent fibre to the muscle-fibre and causes 

 contraction. The assumed simple relations of the foregoing nervous appa- 

 ratus are, in fact, superseded by much greater complexity in consequence 

 of the introduction of additional units, by which the sensory impulses are 

 distributed to nerve-cells situated not only in the immediate vicinity of the 

 reception-cell, but at different and often distant levels. 



