MUSCULAR TISSUE. 53 



in most cases, contain loops of capillary blood-vessels, which, together with 

 the other capillaries within the synovial membrane, are important in pro- 

 ducing the fluid within the joint. Although this synovial fluid, or synovia, 

 consists almost wholly (94 per cent. ) of water, it is slightly viscid and, there- 

 fore, well adapted to lubricate the articulating cartilages. In addition to 

 salts, proteid and mucoid substances, the synovia contains oil drops and the 

 remains of cells displaced by abrasion. 



Blood-vessels and nerves are wanting within the articular cartilages, as 

 well as within the interarticular and the adaptation cartilages. The synovial 

 membrane, on the contrary, possesses numerous vessels and nerves. The 

 larger blood-vessels occupy the stratum of loose connective tissue, the capil- 

 laries penetrating into the innermost layer and the villi. The nerves include 

 vasomotor and sensory fibres, some of the latter being connected with special 

 endings (Vater-Pacinian bodies and Krause's articular end-bulbs). Definite 

 lymphatics are found within the synovial membrane immediately beneath 

 the joint-surface. 



MUSCULAR TISSUE. 



Although possessed to some degree by all living protoplasm, contrac- 

 tility is exhibited characteristically by muscular tissue. The latter is made up 

 of greatly elongated elements, which during contraction shorten in the direc- 

 tion corresponding with their long axes and, hence, exert a definite pull that 

 results in motion. In the higher animals muscular tissue occurs in two chief 

 kinds, the striated and the nonstriated, as distinguished by their histological 

 appearances. The former composes the muscles controlled by the will and 

 therefore is also termed voluntary muscle; the latter acts independently of 

 volition and is spoken of as, involuntary muscle. The association of striated 

 muscle with the will and, ^nversely, of the nonstriated variety with involun- 

 tary action must be made, however, with reservation, since in some animals 

 voluntary muscle is without striations. There is, indeed, reason to believe 

 that the histological differences are not fundamental, but are correlated with 

 function. Thus, the muscles of the oesophagus which in one group of ani- 

 mals are striated, in another group may be represented by nonstriated tissue; 

 further, some of the voluntary muscle of the head may be regarded as the 

 equivalent of the involuntary muscle of the trunk. The nonstriated or invol- 

 untary muscle represents a less highly specialized type than the striped, the 

 latter exhibiting to a conspicuous degree histological differentiation. As an 

 intermediate group stands the muscular tissue composing the heart, since the 

 cardiac muscle is beyond the control of the will although it possesses striated 

 fibres. The latter occupy a position, therefore, between the fibre-cells of the 

 involuntary muscle and the elongated striated fibres of the voluntary muscle. 



NONSTRIATED OR INVOLUNTARY MUSCLE. 



This variety of muscular tissue occurs in the form of bundles and thin 

 sheets chiefly within the walls of the hollow viscera and of the vessels and, 

 although enjoying a wide distribution in the body, seldom forms considerable 

 masses. Its distribution includes: i. The digestive tract the muscularis 

 mucosse from the oesophagus to the anus and delicate bundles within the 

 mucosa; the muscular tunic from the lower half of the resophagus to the 

 anus; in the lafrge excretory ducts of the liver, pancreas, and some salivary 

 glands, as well as in the wall of the gall-bladder. 2. The respiratory tract 

 in the posterior wall of the trachea and as encircling bundles in the walls of 

 the air-tubes. 3. The urinary tract in the capsule and pelvis of the kidney 



