HUMAN PHYSIOLOGY. 



127 



in very similar to it in form and composition. It consist* of 



f..ur fused vertebra, and is a radimentary representative in 



of tho quadruped. Articulated one on eaoh 



i tho sacrum are tho oasa innominata or haunoli bonoH. 



These are a p.ur t tliiok, flat, peculiarly-curved bones of groat 



ii, which moot eaoh othur in front, and thus, in < 

 tton with the Hucrum and coooyx, enclose tho baain-ahaped 

 , open above and below and imperfectly cloned in front 

 . of tho OHKU innominata presents on it* outer 

 a (1 ><>]> riip-flhaped deproaaion which receives the head 

 D|" tli<- thigh -bone Those, then, are the three groat cavities of 

 tho body, and it will bo aa well to oonaider a little tho purposes 

 they are deatinod to fulfil, an<l tho particular fitnesa of eaoh 

 harge ita respective dutiea. The walla of tho head, aa 

 we have aeon, aro firm and unyielding ; tho articulation** of the 

 bone* iv.inj.ii-in^ it being alao calculated more for strength 

 thiin mobility; tho reason for thin, that the office of tho 

 skull is to runt-iiii and protect tho brain and ita dependencies 

 organs HO delicate that the least injury to thorn might bo dan- 

 gerous to life. The thorax has two main duties to perform 

 namely, to contain and protect the lungs and tho heart, and 

 to execute tho greater part of tho function of respiration. To 

 jHTf.inn tho first duty, it ia necessary that its walls should 

 be strong, and wo have soon that in front it is protected by tho 

 powerful buckler tho breast-bone, and in tho roar by tho spinal 

 vertebriu, whilst at its sides the ribs, by their elasticity and 

 strength, aro able to resist a largo amount of violence. Horo 

 it is also protected by tho shoulder-bone, and the position of 

 tho upper extremities. Tho second duty requires that tho 

 capacity of tho chost should be capable of being alternately 

 increased or diminished : this is accomplished by tho groat 

 freedom of movement allowed to the ribs in their articulation 

 with tho vertebra and sternum, tho latter of which, having 

 no other bony attachment, moves with them. 



The pelvis having to sustain and transmit to the extremities 

 the whole weight of tho trunk, it is necessary that its com- | 

 ponent parts should bo of great strength ; and this wo find I 

 is the case, tho sacrum and tho ossa innominata being pecu- j 

 liarly massive and stroug, and tho articulation between them 

 and the spinal columns remarkable for its firmness and little 

 liability to bo disturbed. 



To complete our examination of the skeleton, it only remains 

 that wo should describe the four extremities. These ore divided 

 into two pairs, the upper and tho lower. 



The upper extremity is composed of four parts, the shoulder, 

 the arm, the fore-arm, and hand. The shoulder consists of two 

 bones the scapula or shoulder-blade, and the clavicle or collar- 

 bone. The scapula is a flat bone of triangular shape, which 

 rests on the hinder and side walls of the thorax, extending j 

 from the first to the seventh ribs. It is connected to the trunk 

 by numerous powerful muscles, but by no bony attachment, 

 and consequently has great freedom and extent of movement. 

 At its upper and outer angle it presents a shallow cup, shaped 

 like a pear, called the glenoid cavity, in which the head of tho 

 long bone of the arm is received. Running across the upper 

 flat surface of the scapula is a strong ridge of bone, terminat- 

 ing in a process which overhangs the glenoid cavity, and by 

 a smooth surface articulates with one end of the clavicle. 

 Another strong process of bone springs from the upper border 

 of the scapula, and also overhangs the glenoid cavity. From 

 each of these processes strong muscles and ligaments arise 

 that help to strengthen the joint and retain the arm in ita 

 connection with the shoulder-blade. 



The clavicle is a cylindrical and comparatively slender bone, 

 peculiarly twisted, and of great strength. It is situate at the 

 front part of the chest, one end articulating with the shoulder- 

 blade, and tho other with tho upper surface of tho sternum. 

 Its great use is to support the shoulder and keep it from falling 

 forwards, but it also serves to strengthen the upper part of 

 the chest, and protect the vessels and nerves passing down from 

 the neck. 



The arm contains but one bone, the humerus. It has a 

 cylindrical termination superiorly in a smooth globular head, 

 which is received into the cup of the scapula, and is held in 

 its place by the ligaments and muscles surrounding the joint, 

 but, owing to the shallowness of the cup in which it plays, 

 great freedom of movement is allowed in almost every direc- 

 tion. The lower end of its shaft is flattened out, and presents 



in the middle two rounded aurfaoea for articulating with the 

 bone* of tho fore-arm. Immediately above thaae proesssss are 

 three depression* two, small and hallow, on the front, and 

 one, larger and deeper, on the posterior surface of the bone. 



Tho fore-arm ia made np of two bonea, the radius and the ulna, 

 tho radiuM being placed on tho outer or thumb aide of the arm; 

 I '..i!i <>f them have cylindrical abaft* and a large and mall 

 extremity ; tho radius ia small above and increase* in size from 

 above downwards, whilst the ulna is large at tho elbow and 

 gradually tapers to tho wrist. The upper end of the ohm is a 

 half -moon shaped cavity, the hinder border of which in prolonged 

 upwards into a strong curved process; the cavity receives the 

 inner of tho two rounded eminences of the humerns ; the proooes, 

 when tho arm in straightened, fita into the cavity at the back 

 of that bone, and prevent* a greater degree of movement in 

 that direction. The radios terminates superiorly in a flattened 

 circular hood, tho top of which in slightly hollowed oat to rc- 

 oeivo the smaller and outer of tho condyles of the bomerua, 

 whilst laterally it articulates with tho side of the upper extre- 

 mity of the ulna, being hold in position by a ligament which 

 encircles it. Tho elbow may thus bo said to be composed of 

 two separate joints : one, tho articulation between the bones of 

 tho fore-arm and tho humerus, which in its action is much like 

 ; the other, tho articulation of the head of the radius 

 with tho ulna : by tho first we are able to bend and straighten 

 tho fore-arm ; by tho second wo rotate tho fore-arm and hand. 



Tho hand, tho smallest but most important of the divisions 

 of the upper extremity, ia composed of three parts the carpus, 

 metacarpus, and phalanges. 



The carpus or wrist is made np of eight small, short, irregu- 

 larly-shaped bones arranged in two horizontal rows. Tho upper 

 row articulates above with the radius, and, by tho intervention 

 of a cartilage, with the ulna ; by ita lower surface it articulate* 

 with tho bones of the second row, and these in their turn with 

 the bones of tho metacarpus ; by their lateral surfaces they 

 articulate with each other. 



Tho metacarpus is composed of five cylindrical bones, which 

 articulate with tho carpus above and with tho phalanges below; 

 the one which goes to form the thumb differs from the rest in 

 being placed at a considerable angle to the others, so that it can 

 be brought into a widely divergent position, enabling tho thumb 

 to bo brought in opposition to the fingers ; the joint between it 

 and tho carpus also differs from the rest, which permit simple 

 flexion and extension, in having, in addition to those movements 

 in a greater degree, rotation or circumduction. 



The phalanges are cylindrical bones, fourteen in number, and 

 are distributed three to each finger, the thumb having only two. 

 The phalanges of each digit articulate with tho metacarpus 

 below and with each other. 



We must now turn to the lower extremity, which, like tho 

 upper, might strictly bo said to consist of four parts the 

 haunch, thigh, leg, and foot; but as the haunch has already 

 been described as part of the pelvis, three only remain for con- 

 sideration. The thigh, like the arm, contains but one bone, 

 tho femur, the largest, longest, and strongest bone in tho body. 

 It has a cylindrical shaft terminating superiorly in a rounded 

 head, which articulates with the side of the haunch bone, the 

 joint being strengthened by a ligament running from the upper 

 surface of the head, and attached to the bottom of the depres- 

 sion of the innominate bone. At its lower extremity it is 

 broad, and presents two large rounded processes called condyles, 

 which articulate with the head of the tibia, and form with it 

 tho greater part of the knee-joint. 



The bones composing the leg are three in number; two of 

 these, the tibia and fibula, ore analogous to the bones of the fore- 

 arm, the other is the patella or knee-cap. The main bone of the 

 leg is the tibia, next to the femur, the largest in the body. It is a 

 cylindrical bone with a triangular-shaped shaft, ending superiorly 

 in a large expanded square head, on the upper surface of which 

 are two shallow depressions to receive the oondyles of tho femur 

 on tho outer side is a surface which articulates with the head 

 of Iho fibula ; the lower extremity is quadrilateral in shape, and 

 hollowed out beneath to receive the head of the astragalus (one 

 of the bones of the foot), its inner surface being prolonged down- 

 wards into a process which articulates with the side of the 

 astragalus, and forms the prominence of the inner ankle. 



The fibula is a long slender bone placed along the outer side 

 of the tibia, articulating with it just below tho knee-joint, into 



