408 COMPARATIVE ZOOLOGY OF CHORDATES 



a stable " internal environment " ; and stability of conditions 

 is essential for highly specialised and co-ordinated processes 

 of life. 



The relation between the quantities of oxygen and C0 2 

 in the blood is regulated by the respiratory system, controlled 

 by a centre in the brain. If the blood is rich in C0 2 the 

 respiratory movements are accelerated, and conversely they 

 are retarded if the quantity of C0 2 is low. In this connexion 

 it must be remembered that the respiratory movements of the 

 fish and amphibia are effected by the muscles of the visceral 

 arches. These are visceral muscles, innervated by visceral 

 efferent fibres in the dorsal cranial nerve-roots, and the centre 

 which controls them is in the visceral sensory lobe of the 

 medulla oblongata. In the Selachian (Raia) it is perhaps 

 better to speak of several centres, one corresponding to each 

 of the 7th, 9th, and 10th cranial nerves. Each of these seg- 

 mental centres in Raia has a degree of autonomy of its own, for 

 if separated from the others by cutting across the medulla, 

 it continues to regulate the muscular movements in the visceral 

 arch or arches to which it is connected. 



In the amniotes, however, the respiratory movements are 

 effected by the intercostal muscles (moving the ribs) and the 

 muscles of the diaphragm. These are somatic (myotomic) 

 muscles innervated by somatic efferent fibres through ventral 

 nerve-roots in the region of the neck and trunk. Neverthe- 

 less, the " respiratory centre " is still in the medulla oblongata, 

 in the primitive position which it occupied in the fish and 

 amphibia, but it no longer shows the simple segmental 

 arrangement. 



Lastly, attention may be paid to two features which the 

 higher vertebrates possess, and which though not strictly 

 regulatory (compensating) mechanisms, nevertheless serve 

 to ensure maximum constancy of conditions. The first of 

 these is concerned with the fact that the ovary and testis in 

 birds and mammals serve not only for the production of 

 reproductive cells, but they also furnish a chemical secretion 

 which evokes and maintains the development of the secondary 

 sexual characters. 



