830 Comparative Animal Physiology 



portant the opportunity for lateral connections is increased by surface folding. 



Medulla Oblongata. In addition to serving as the point of entrance and 

 exit for most of the cranial nerves, and providing way-stations for their fib- 

 ers in numerous sensory nuclei, the medulla has some integrative functions. 

 It contains the respiratory center in all vertebrates, the motor outflow differ- 

 ing from group to group. Respiratory outflow in elasmobranchs is largely in 

 the tenth cranial nerve, partly in the seventh;^^" in mammals it is in the 

 phrenic arising from the third to fifth cervical roots and in nerves to the 

 intercostal muscles (thoracic roots). In the skate the right and left halves 

 control respiration homolaterally.-^^ That the respiratory center is spontane- 

 ously rhythmic was shown by Adrian and Buytendijk, who recorded rhyth- 

 mic electrical waves corresponding to breathing frequency from the medulla 

 of an isolated goldfish brain; this observation has not been confirmed. 



The respiratory center in mammals lies in the mid-dorsal part of the me- 

 dulla at about the level of entrance of the eighth cranial nerve, and con- 

 sists (in the cat, monkey, and dog) of a lower inspiratory portion and an up- 

 per expiratory center.^^^- ^-^ Anterior to the respiratory center, at the level 

 of the pons, is a pneumotaxic or tonic center. The respiratory center re- 

 ceives sensory impulses over several routes, primarily via the vagus from the 

 lung. The breathing cycle^-^ is as follows: The inspiratory center is spon- 

 taneously active continuously, it discharges impulses to the spinal motor cen- 

 ters, which cause the inspiratory movements of diaphragm, and of intercostal 

 and abdominal muscles; it also sends impulses to the pneumotaxic center. 

 As the lungs are distended afferent impulses via the vagus, together with 

 impulses from the now excited pneumotaxic center, inhibit the inspirator)' 

 center, stopping its discharge. The expiratory center acts principally by in- 

 hibiting the inspiratory center but partly by inducing active exhalation. 



The organization of the respiratory centers in non-mammalian groups is 

 not so well known. In turtles, breathing is apneustic (prolonged inspiration); 

 there is evidence for centers of inspiration and expiration but not for a pneu- 

 motaxic center.-'^^ In amphibians and reptiles, lungs have an outer layer of 

 smooth muscle, and this shows spontaneous tonic activity. In frogs and sala- 

 manders efferent impulses of the vagi inhibit the spontaneous lung contrac- 

 tion during breathing,'-*^ and breathing is a swallowing act. In the reptiles 

 (turtle and snake), on the other hand, the vagi are excitatory to the lung 

 musculature, causing lung contraction between respirations, and the center 

 for this contraction is inhibited during the respiratory act."-^^ The respiratory 

 center is a swallowing center in adiaphragmatic animals, and in its evolu- 

 tion it is associated with movements of esophagus and glottis. The chemical 

 and reflex control of the respiratory center is considered in Chapter 8. 



The medulla also contains other regions which regulate autonomic func- 

 tions. A vasomotor center is found in mammals in the anterior mid-medulla, 

 just caudal to the pons; this center contains neurones which have a vaso- 

 constrictor action, also probably some vasodilator cells. Stimulation of the 

 vasomotor center is by specific sensory paths, descending tracts, and by di- 

 rect chemical excitation, there being also some interaction with the respira- 

 tory center. The comparative physiology of vasomotor reflexes has been dis- 

 cussed previously (Ch. 15), also the temperature sensitivity of the vasomotor 

 center in the turtle (Ch. 10). 



