Abstract
The wall of the common carotid artery of the domestic fowl possesses a baroreceptor zone around the origin of the artery to the carotid body. The zone comprises about 20 small foci which have undergone a fibrous modification. Within these foci there are bare endings, which closely resemble ultrastructurally the baroreceptors of the mammalian carotid sinus in their mitochondrial content, relationship to collagen and elastic fibres, and abnormal organelles. These should be slowly adapting baroreceptors, similar functionally to the baroreceptors of the mammalian carotid sinus. Also present in the baroreceptor zone are a few encapsulated endings, resembling in structure the simplest encapsulated mechanoreceptors of mammals. These, too, should be slowly adapting but may be capable of slightly more subtle responses to pressure changes in the arterial wall. Some presumptive efferent axonal endings are associated with smooth muscle cells, and may modify the tension of the arterial musculature. In general, the afferent innervation of the avian carotid baroreceptor zone appears to be less profuse than that of the mammalian carotid sinus.