368 THE PERIPHERAL NERVOUS SYSTEM 



its three characteristic layers is not completed until between the second and third 

 years. The inner reticular zone is formed first, next the fasciculate zone and last 

 the glomerular zone. 



When the cells of the medulla begin to produce an internal secretion they 

 give the chrome reaction. By using extract of the aortic bodies, which are entirely 

 composed of chromaffin cells, Biedl and Wiesel have proved that its effect, like 

 that of adrenalin, is to increase the blood pressure. The logical conclusion is 

 that the effect of adrenalin, an extract of the suprarenal glands, is due to an 

 internal secretion produced by the chromaffin cells of the suprarenal medulla. 



Portions of the suprarenal anlage may be separated from the parent gland and form 

 accessory suprarenals. As a rule, such accessory glands are composed only of cortical substance 

 and may migrate some distance from their original position, accompanying the genital glands. 



E. DEVELOPMENT OF THE SENSE ORGANS 



The nervous structures of the sense organs are derived from the ectoderm 

 and consist of the general sense organs of the integument, muscles, tendons and 

 viscera, and of the special sense organs which include the taste buds of the tongue, 

 the olfactory epithelium, the retina, optic nerve and lens of the eye, and the 

 epithelial lining of the ear labyrinth. 



I. GENERAL SENSORY ORGANS 



Free nerve terminations form the great majority of all the general sensory 

 organs. When no sensory corpuscle is developed, the neurofibrils of the sensory 

 nerve fibers separate and end among the cells of the epidermis. 



Lamellated corpuscles first arise during the fourth and fifth months as masses 

 of mesodermal cells clustered around a nerve termination. These cells increase 

 in number, flatten out and give rise to the concentric lamellae of these peculiar 

 structures. In the cat these corpuscles increase in number by budding. 



The tactile corpuscles, according to Ranvier, are developed from mesenchymal 

 cells and branching nerve fibrils during the first six months after birth. 



II. TASTE BUDS 



The sense of taste resides chiefly in the taste buds of the tongue. The de- 

 velopment of the tongue has been described (p. 158) and we may speak here only 

 of the development and distribution of the taste buds. 



In the fetus of five to seven months taste buds are more widely distributed 

 than in the adult. They are found in the walls of the vallate, fungiform and foliate 



