

PHYSIOLOGY. 



blood undergoes in its passage through 

 the lungs; for when respiration 1 is ob- 

 structed, the heart's action ceases, and it 

 may be recalled by again introducing air 

 into the lungs. Hence arises the impor- 

 tance of inflating the lungs in instances of 

 apparent death from drowning, &c. in 

 order to excite the heart to action. The 

 mode in which the arteries and veins con- 

 tribute to the circulation, will be under- 

 stood from the account of those vessels 

 in the article As ATOMY. 



The circulation of the blo.od is different 

 in the foetus, in consequence of differ- 

 enpes in the structure of the organs de- 

 voted K) this function. Se-e FOETUS. 



The situation of the child in utero pre- 

 cludes the access of atmospheric air to 

 the lungs; these organs arc consequently 

 small and collapsed, and the lesser cir- 

 culation can hardly be said to take place 

 in the foetal state. Although its circulation 

 might, in this respect, be considered as 

 more simple than that of the adult, this 

 function becomes considerably compli- 

 cated by the connection with the pla- 

 centx. A portion only of the child's blood 

 circulates through this part, and it is no 

 doubt so altered or modified by this pas- 

 sage through the vessels of the placenta, 

 -as to become more fit for the growth and 

 nourishment of the child. No such altera- 

 tion or modification has, however, been 

 actually demonstrated in the foetal blood. 

 Physiologists have discovered no differ- 

 ence in this fluid in the various vessels of 

 the foetus. It is of the same dark colour 

 in the arteries and veins. The interrup- 

 tion of the communication with the pla- 

 centa, before respiration has commenced, 

 is, however, suddenly fatal. Our igno- 

 rance of the functions of the placenta, and 

 of the liver, which is of immense size in 

 the foetus, as well as of the changes which 

 the foetal blood undergoes ia the compli- 

 cated system of organs, which are con- 

 nected with its circulation in this state of 

 existence, leaves many parts of the sub- 

 ject in doubt and obscurity. 



Respiration. The exposure of the blood 

 to the atmospheric air, by which the 

 chyle, that has entered the circulating 

 system from the thoracic duct, is con- 

 verted into blood, and by which those 

 changes are effected in the whole mass of 

 circulating fluid, which are essentially 

 necessary to the continuation of life, takes 

 place in the lungs. 



The respiratory organ has been aptly 

 compared to an empty bladder, placed in 



a pair of bellows, with its neck adapted 

 to the instrument, and giving entrance to 

 a coiumn of air, when the sides are sepa- 

 rated. In breathing, the d'latation of the 

 chest occasions the lungs to enlarge by 

 the entrance of air into them from with- 

 out; these viscera not possessing any 

 means of enlargement in themselves :^ thi's 

 is termed inspiration. The expulsion of 

 the air, after it has served the purposes 

 of respiration, by means of a process 

 exactly contrary to the former, is called 

 expiration. The diaphragm and the ab- 

 dominal muscles are the chief agents in 

 enlarging and diminishing the chest. The 

 former muscle in its relaxed state is 

 strongly arched, and the convexity of 

 this arch is towards the chest. Its curved 

 fibres become straight by the contraction: 

 the whole muscle descends towards the 

 abdomen, and pushes the abdominal vis- 

 cera, which lie against its under or con- 

 cave surface, downwards and forwards. 

 Hence the surface of the belly rises when 

 we draw air into the chest. In the next, 

 moment, the abdominal muscles contract 

 and push back the viscera, and thereby 

 diminish the chest in a degree propor- 

 tionate to its former enlargement. The 

 increase of the thorax, effected in this 

 way, takes place in the perpendicular 

 direction; but it may also be enlarged in 

 its whole diameter by means of the in- 

 tercostal muscles, which, by elevating and 

 twisting outwards all the ribs, push the 

 sternum forwards, and enlarge the chest 

 in every direction. When the action of 

 these powers has ceased, the natural 

 elasticity of the parts restores the parietcs 

 of the thorax to their former position. In 

 natural respiration, both these methods 

 of altering the capacity of the chest are 

 employed ; but females seem to use the 

 intercostal muscles more than the male 

 subject, as the heaving of their bosom 

 demonstrates ; yet breathing can be car- 

 ried on by either method, to the exclu- 

 sion of the other; as we sometimes sec 

 under cir&imstances of accident or dis- 

 ease. In the case of a broken rib, where 

 the rubbing of the broken ends would be 

 highly painful, the chest is bound up so 

 as to render the ribs motionless, and the 

 diaphragm and abdominal muscles per- 

 form the whole business of respiration 

 When the diaphragm and abdominal 

 muscles act together, they compress the 

 viscera between them, and the pressure 

 thus produced, assists in the expulsion of 

 their contents. This effort is termed 

 straining, and is seen in vomiting, in the 

 act of discharging the feces and urine, 



