406 



KNOWLEDGE • 



[Dec. 29, 



amoug the maize, consisting of a tlimsy web somewhat 

 tightly enveloping the chrysalis, which was of a light 

 yellow ish-browTj, with the wing-cases largely developed and 

 one-third of an inch in longtli. 



" The perfect insect took wing in October, and is three- 

 fourths of an inch in expanse, and active in its movements. 

 The superior wings wore elongated, the costal margin 

 arched, the apices roiuided. General colour greyish-brown, 

 of a silvery hue, with stigmata and strig;e of a darker 

 colour. Inferior wings of a light semi-transparent silvery 

 hue, with a deep marginal fringe. Thorax similar in 

 colour to the anterior wings, and not crested. Abdomen 

 yellowish ; the whole of the under side light silvery grey. 

 Tlie wings are slightly convoluted in repose." — Si-i:')it\r!c 



"OUR BODIES:" 

 SHORT PAPERS ON PHYSIOLOGY. 



Br Dr. Andrew Wilson, F.R.S.E. 



THE name- "joint" is given to the movable surfaces 

 Ijctween two or more bones ; and, as a matter of fact, 

 muscular movements (to be discussed in another paper) 

 are directed towards the movements of the bones concerned 



is an 

 of this 



in forming the joint. The scientific name of a joint 

 arturii'/ition ; and the reason for the application ol 

 name, at once to the connected series of syllables and words 

 which fonu "speech," and to bodily mechanics, can be 

 readily appreciated. There are some five stnictures which 

 enter into the formation of an ordinary "joint." Firstly 

 come the bonrs. Tlien, secondly, bones are tied together 

 by stout fibrous cords, kno\vn as H'jniiiinls. The beautiful 

 ligaments of the knee, uniting tiie thigh to the top of 

 the shin-bone, or those of the elbow joint, illustrate the 

 cords in question. Ligaments are related to the sinews 

 (or tendon-s) of muscles in their composition ; both being 

 fibrous cords. Tliirdly comes the layer of gristle or 

 carlila'i". which coats the ends of the bones which rub on 

 one another in the movements of the joint. This cartilage 

 is called arllr„!ar cirtHagp. It is of a beautifully smooth, 

 glistening structure (as may be seen in a Ijutcher's shop, 

 when we inspect the end of a bone which has just lieen turned 

 out of its socket), and is of a bluish-white colour. This 

 cartilage acts the part of a " bufier," and serves to limit 

 jarring and concussion in joints, besides rendering the 

 movenienta of the bones smooth. The fourth structure 

 found in a jouit is a peculiar membrane or layer, which 

 receives the name of xt/norinl tupinhran'; and which, prac- 

 tically, Iint« the cavity of the joint The office of this 

 membrane is to manufacture a gjairy fluid named n;/not:in, 

 which nerves as " jointroil," and which, being poured out 

 between the movable surfaces, lubricates the bones in 

 their movements. When, through inflammation of the 

 joint, the synovial fluid increases in extent, we suffer 

 from what is popularly called " dropsy " of the joint. 

 Tlic fifth set of structures wliich may be found asso- 

 ciat<.-<l with joints arc known as l/nmri:. These are little 

 cushions or pads, covered usually with synovial membrane, 

 and placed in situations where there is liability to friction. 

 The <rur<(,« are to the lK)dy, in fact, what the groov(;d wheel 

 uisule the sailor's " block " is to the tackle of a ship : they 

 dimmish friction. Wc discover them in situations where, 

 for example, there is a great play of muscle or sinew over 

 any surface. iJetween the skin and the knee-cap, for 

 example, where, it is evident, much friction must exist, a 

 Ours'e ia interposed. Another existe lictween the great 



projection near the head of the thigh-bone and the hip- 

 muscles which rub over the projection at every step we 

 take. 



Such being the general conformation of "joints," we 

 may now survey the various forms of articulations met 

 with in our frames. There are three classes of joints to be 

 found in the bodies of man and his neighbour animals. 

 Firstly come what seem to imply a contradiction in terms, 

 namely, iimiiovahk joiiils. These are well represented in 

 the dove-tailing which is seen between the bones of the skull, 

 a process securing inunense firmness of union. The next 

 variety of joints includes those which are named mixed 

 articulations. Here we find only a limited range of move- 

 ment, which may, as in the spine, confer general flexibility, 

 rather than exact movement, upon the parts concerned. 

 In the spine, for example, we have a series of bones, firmly 

 united together (for the protection of the npiwil cord) by 

 intervening plates of gristle or cartilage. The alternating 

 series of bone and gristle-pads thus seen confers a high 

 degree of flexibility on the spine, without permitting any 

 definite range of motion between the separate bones. 



But the most typical joints are those in which free move- 

 ment in one direction or another is permitted. Under this 

 head — that of movable joints — come the ordinary joints of 

 the body : elbow, shoulder, ankle, knee, fingers, toes, itc. 

 It is very obvious to any one who swings his arm round 

 and round at the shoulder, that the movement there is of 

 a diUerent nature from that seen in the knee or elbow ; 

 whilst the familiar "turn of the wrist" again represents a 

 movement of a third kind. Thus it becomes clear that we 

 find in the mechanics of our bodies — firstly, baU-andsockct , 

 or " universal " joints, capable of free movement in all 

 directions. The deeper the cup, or socket, the more limited 

 is the movement. Witness, in proof of this fact, the more 

 limited movement of the hip joint (where the cup is deep) 

 compared with that at the shoulder, where the cup is a 

 mere shallow "saucer" of the .shoulder-blade. At the 

 elbow, knee, ankle, and in the fingers and toes, him/ejoints 

 are represented. The motion is backwards and forwards 

 in these joints ; but such a joint as the knee performs more 

 complex movements than are included under this descrip- 

 tion. Lastly come the rolalory jointa in which the 

 movement takes place round a fixed point or invol. 

 When, after keeping the fore-arm fixed with the palm 

 of the hand turned forwards, we suddenly reverse the 

 palm and turn it backwards, one of the bones {radius) 

 of the forearm runs (or rotates) round the other bono 

 (nhui) of the forearm, and so reverses the position of the 

 hand. The radius thus conies to cross the ulna, as when a 

 speaker, addressing an audience, places both palms down- 

 wards on a table in front of him, in the familiar style. 

 Here there is seen the round head of the radius rotating 

 round the ulna, its neighbour-bone ; and it may bo added 

 that there arc certain animals (e.;/., elephants and dogs, 

 cats, and carnivora generally) in which the radius is permar 

 nently fixed in the crossed position, known scientifically as 

 pronation. The position in which the two bones lie side 

 by side (as when the palm is directed forwards) is called 

 gerpination. When we turn our head on our neck wo 

 receive a second illustration of a " pivot-joint" Then the 

 head, together with the first vertebra of the spine, or " atlas," 

 move a little around a little bony peg borne on the second 

 vertebra or nxin. 



It may be added tliat in the use of the various "joints," 

 highly instructive examples of " animal mechanics " may 

 be occasionally found. Thus the bones really constitute a 

 scries of levers of various kinds. When we pull the head 

 backwards on the neck, we are using a lever of the first 

 order. Here the wciyhl (the face) is on one side of the 



