If intrinsic is the body's natural aging process, then extrinsic is the exact opposite.
Extrinsic aging is caused by external factors such as sun exposure and smoking.
This trophic influence which one neuron exerts upon others, or upon the cells of an extrinsic tissue, such as muscle, is exerted in that direction which is the one normally taken by the natural nerve impulses.
In the region of the oesophagus these muscles are more strongly developed to perform the movements of deglutition, and, where a gastric mill is present, both intrinsic and extrinsic muscles co-operate in 3a producing the movements of its 36 various parts.
- The alimentary canal in Scorpio, as in Limulus, is provided with a powerful suctorial pharynx, in the working of which extrinsic muscles take a part.