RESPIRATION. 



333 



tory membrane is closely packed in a par- 

 ticular part of the organism, and the function 

 of respiration is at the same time energetic as 

 in the Mammalia, the blood is circulated with 

 great activity, and in great quantity, through 

 vessels distributed in this membrane, and 

 appropriated solely to this purpose. When 

 the respiratory membrane is extensively dif- 

 fused, as in insects, throughout the organism, 

 and the atmospheric air is brought into contact 

 with it in the different tissues, a particular set 

 of canals for carrying the nutritious juices to 

 and from the respiratory membrane is not 

 required ; and were we, in such animals, to 

 examine the circulatory apparatus without 

 any reference to the nature of the respiratory 

 apparatus, we could not understand how a 

 circulatory apparatus, apparently so imperfect, 

 is yet equally efficient in carrying on the nutri- 

 tive processes as in other animals where its 

 mechanism is much more complicated. 



Apparatus for renewing the air in the lungs 

 in the human species. In man, as in the other 

 Mammalia, this consists of three distinct parts : 

 1st, of a movable framework composed of 

 articulated bones and cartilages, but chiefly of 

 the former, termed the thorax ; 2dly, of muscles 

 for enlarging and diminishing the capacity of 

 the thorax ; 3dly, of nerves through which the 

 movements of these muscles are excited and 

 regulated. The uses of this apparatus are 

 not, however, restricted to respiration. The 

 bones of the thorax furnish a certain degree 

 of protection to the lungs, heart, and other 

 important parts enclosed by them ; and 

 during certain violent efforts of the voluntary 

 muscles, as in lifting a weight, they are 

 no longer mobile as in the respiratory move- 

 ments, but are rendered fixed, and afford a 

 firm and steady point d'appui to the powerful 

 muscles passing between the external surface of 

 the thorax and the thoracic extremities, during 

 their contraction. The same muscles which 

 act involuntarily in dilating and contracting 

 the chest in respiration, are frequently engaged 

 in the performance of voluntary muscular 

 movements, as in articulate speech, straining, 

 &c. They also, in connexion with other 

 muscles, or even alone, perform various in- 

 voluntary muscular movements which are not 

 respiratory, as in the excito-motory movements 

 of coughing, sneezing, defecation, and urin- 

 ation, and in the sensational and emotional 

 involuntary muscular movements of laughter, 

 sighing, yawning, vomiting, &c. 



The thorax can be enlarged in all its dia- 

 meters by the action of its muscles, in the 

 vertical or atlanto-sacral, in the antero-pos- 

 terior or vertebro-sternal, and in the trans- 

 verse. Its enlargement in the antero-posterior 

 and transverse directions is effected by the 

 elevation of the ribs, and its enlargement in 

 the vertical direction by the descent of the 

 diaphragm, and by the elevation of the upper 

 part of the thorax, but chiefly by the former. 

 As the ribs in the human species differ in 

 length, in the degree of their inclination to the 

 spine," in the form and extent of their curva- 

 ture, in the manner in which the anterior 



extremities of their cartilages of prolongation 

 terminate, and in some other anatomical points 

 which must influence their mode of action, 

 the phenomena attending the elevation of the 

 ribs are not the same over all parts of the 

 chest, but it will be sufficient for our present 

 purpose to state the general effects of these 

 movements.* As the osseous arches formed 

 by the ribs are so inclined upon the vertebral 

 column that their lower edges form acute 

 angles with that column, and their anterior or 

 sternal are placed lower than their vertebral 

 ends, and as their vertebral or posterior 

 ends have a very limited extent of motion -f-, 

 their elevation brings them to or near the 

 horizontal plane, and carries forward their 

 sternal extremities ; and as the greater num- 

 ber of the ribs are attached to the sternum 

 through their cartilages of prolongation, this 

 bone must by this movement be pushed for- 

 wards, and the antero-posterior diameter of 

 the thorax be enlarged. 



The transverse diameter of the thorax is 

 increased by the circumstance that the ribs 

 during their elevation do not simply ascend, 

 but perform a slight rotation round an axis 

 passing between their anterior and posterior 

 extremities, by which two effects are pro- 

 duced ; 1st, their lower, which form a segment 

 of a somewhat larger circle than their upper 

 edges, are turned somewhat outwards, and 

 the upper slightly inwards, so that the con- 

 cavities of the arches formed by the ribs are 

 now perpendicular, or nearly so, to the median 

 plane of the body, instead of being oblique as 

 before their elevation ; 2dly, the middle portion 

 of the greater number of ribs, which before 

 was placed below a straight line passing 

 through their two extremities, in consequence 

 of the shaft of the rib bending upwards near 

 the sternal end at what has been termed the 

 anterior angle, is now placed on the same 

 plane with the two extremities, and the whole 

 rib rendered horizontal. This rotatory motion 

 is greater at the middle of certain of the ribs 

 as they rotate upon their two extremities, so 

 that each rib in the performance of this move- 

 ment may be considered as forming two levers, 

 the two extremities being the pivots, and the 

 middle of the ribs the ends of the levers most 

 remote from the pivots.^ The forward move- 

 ment of the sternum is greater at its lower 

 than at its upper part, in consequence of the 

 greater length and inclination of the lower 

 vertebro-sternal or true ribs, and the greater 

 length of their cartilages, and the more acute 



* Mr. Sibson has lately (Philos. Transact, of 

 London, Part IV. for 1846, p. 528) given an elabo- 

 rate analysis of the movements of the thorax in 

 respiration. Dr. Hutchinson has also lately (Me- 

 dico-Chirurgical Transactions of London, vol. xxix. 

 1846, p. 183) published some of the results of his 

 observations on this subject. 



f According to the observations of Haller (Ele- 

 ment. Physiologiae, torn. iii. p. 23. 1766) the great- 

 est movement at the vertebral extremity of a rib is 

 scarcely the one-sixth part of a line. 



J These observations do not apply to the inferior 

 ribs, especially the two last or floating ribs, as they 

 are depressed in inspiration, and not elevated. 



