572 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



through the spinal nerves below the point of section are cut off. If the cord 

 be cut just below the medulla oblongata or above the origin of the phrenics, 

 both costal and diaphragmatic movements immediately or very soon cease, but 

 respiratory movements may continue in the larynx, and when dyspnoea occurs 

 they may be observed in the muscles of the face, neck, and mouth. In rare 

 cases, after section at the junction of the medulla oblongata and the spinal cord, 

 respiratory movements may continue in the thorax and the abdomen, but these 

 instances are exceptional and the movements are of the nature of reflex spasms. 



During each respiratory act there flow to the larynx impulses which open 

 the glottis during inspiration. The pathway of these impulses is through the 

 laryngeal branches of the vagi, almost solely through the recurrent or inferior 

 laryngeal nerves. (See section on the Physiology of the Voice.) If the pneu- 

 mogastrics are cut above the origin of these branches, respiratory movements 

 in the larynx cease, and, owing to the paralysis of the laryngeal muscles, the 

 vocal cords are flaccid, the glottis is no longer widened, and thus great resist- 

 ance is offered to the inflow of air, causing difficulty during inspiration. 



During forced breathing, besides the above nerves a number of others may 

 be involved, especially the spinal nerves, which supply the extraordinary respi- 

 ratory muscles of the chest, abdomen, pelvis, and vertebral column, and the 

 facial, hypoglossal, and spinal accessory nerves. 



L. THE CONDITION OP THE RESPIRATORY CENTRE IN THE FETUS. 



During intra-uterine life the child receives O from and gives CO 2 to the 

 blood of the mother. No attempt is made by the child to breathe, because the 

 centre is in an apnoeic condition, due to a low condition of irritability and to 

 the relatively large amount of O in the blood. The fetal blood contains a 

 larger percentage of haemoglobin than the blood of the mother; Quinquaud 

 has shown that the fetal blood has a larger respiratory capacity than adult's 

 blood ; and Kegnard and Dubois have proven the same to be true of the calf 

 and the cow. Were it not for these two conditions, the child would continu- 

 ally attempt to breathe. While such efforts do not occur under normal cir- 

 cumstances, they may be present if we interfere in any way with the supply of 

 oxygen, as by pressure upon the umbilical vessels. The child has been seen 

 to make respiratory efforts while within the intact fetal membranes. It seems 

 evident, therefore, that all that is necessary to excite the respiratory centre to 

 activity is a venous condition of the blood. In utero, and as long as the child 

 is bathed in the amniotic fluid, respiratory movements cannot be carried on 

 even though the respiratory centre be excited to activity, the reason being that 

 with the first movement of inspiration amniotic fluid is drawn into the nasal 

 chamber; the fluid acts as a powerful excitant to the sensory fibres of the 

 mucous membrane, thus causing inhibitory respiratory impulses. From this 

 fact we learn the practical application that it is desirable immediately after birth 

 of a child, if spontaneous respirations do not immediately and effectively occur, 

 to carefully remove mucus or other matter from the nose, so that the inhibitory 

 influences generated by nasal irritation shall be discontinued. 



