l68 FROM FISH TO PHILOSOPHER 



tenor part of the orbit, this ganghon communicating 

 primarily with the VII cranial (facial) nerve. The post- 

 ganglionic fibers to the gland are probably cholinergic, 

 and secretion is stimulated by Mecholyl and acetylcho- 

 line, and inhibited by atropine. Secretion is also blocked 

 during anesthesia. 



The gland excretes only sodium chloride in hypertonic 

 solution (with traces of potassium chloride), and it can- 

 not excrete osmotically free water (i.e., a hypotonic solu- 

 tion such as is secreted by the tear glands, etc.). Its 

 structure is such as to exclude any filtration-reabsorp- 

 tion process, and to demand a pinely secretory opera- 

 tion, as in the aglomerular kidney. Unlike the latter, the 

 salt gland, as Schmidt-Nielsen has called it, secretes a 

 solution (specifically of sodium chloride) hypertonic to 

 the blood, and no other organ in the vertebrates is known 

 to accomplish this except the bird and mammalian kid- 

 ney (and here hypertonicity is achieved indirectly by the 

 active reabsorption [secretion] of sodium chloride into 

 the interstitium of the renal medulla). 



The kidneys of the cormorant respond to a load of 

 pure water by the increased excretion of dilute urine 

 (water diuresis), and to a load of sodium chloride or sea 

 water by the increased excretion of osmotically concen- 

 trated urine with a high sodium chloride content (os- 

 motic diuresis). However, the capacity to concentrate 

 the urine osmotically is at best slight (as indicated by 

 the maximal salt content); consequently the salt gland 

 serves as a safety device insuring that any excess sodium 

 chloride, ingested as sea water or in the diet, can be 

 disposed of without excessive loss of water. The stimulus 

 which excites secretory activity is an increase in the os- 

 motic pressure of the blood, not in the sodium concen- 

 tration, because the intravenous administration of hyper- 

 tonic sucrose solution is as effective in inducing secretion 

 as is the injection of hypertonic sodium chloride or the 

 oral administration of sea water. The high osmotic con- 

 centration of the secretion permits the salt to be excreted 



