32 SUMMARY OF CURRENT RESEARCHES RELATING TO 



the sympathetic system. Some become distributed along the nerve- 

 tibres and give rise to the neurilemma ; therefore the cells of the 

 neurilemma are homologous with the neuroglia-cells in the central 

 nervous system. The cells taking part in the development of the 

 sympathetic nervous system and the neurilemma do not all actually 

 migrate as such from their sources in the cerebro-spinal system. Doubt- 

 less many arise by the mitotic division of "indifferent" cells alone- the 

 course of migration. The existence of sympathetic sensory neurones in 

 the sympathetic trunks and the pre-vertebral plexuses has not been 

 demonstrated. Experimental evidence, however, indicates the presence 

 of sympathetic sensory neurones in the sympathetic plexuses in the walls 

 of the digestive tube. 



While it is impossible by direct observation to trace either sympathetic 

 excitatory or sympathetic sensory elements back to their specific source in 

 the cerebro-spinal nervous system, indirect embryological and anatomical 

 evidence warrants the conclusion that the sympathetic excitatory neurones 

 arise from cells which migrate from the neural tube along the fibres of 

 the motor nerve-roots, while the sympathetic sensory neurones, wherever 

 such neurones exist, arise from cells which migrate from the cerebro- 

 spinal ganglia. This interpretation makes the sympathetic neurones 

 homologous with the afferent and the efferent components of the other 

 functional divisions of the peripheral nervous system. As the cells 

 migrating peripherally from the cerebro-spinal nervous system are the 

 " indifferent " cells and " neuroblasts " of Schaper, Schaper's conception 

 of the developmental relations of the neurones and the supporting 

 elements in the central nervous system may be extended to the sym- 

 pathetic neurones and the cells of the neurilemma. The nervous system 

 is a unit, of which the sympathetic system is a part homologous with the 

 other functional divisions. The sympathetic system may be looked upon 

 as an accession to the nervous system, which has arisen comparatively 

 late in the evolution of Vertebrates in response to the conditions of the 

 vegetative life. The fact that the sympathetic system is homologous 

 with the other functional divisions of the nervous system lends a new 

 aspect to the problems involved in its functional relations. The fact 

 that the vagal sympathetic plexuses have their origin in the hind-brain 

 and the vagus ganglia will doubtless have an important bearing on 

 certain physiological and psychological problems involving the heart 

 action and the digestive functions. 



Development of Sympathetic Nervous System in Birds.* — Fol- 

 lowing up his recent work on the development of the sympathetic 

 nervous system in Mammals, A. Knntz publishes the results of an inves- 

 tigation into the development of the nervous system in Birds. The 

 primary sympathetic trunks in the chick arise about the beginning of 

 the fourth day of incubation, as a pair of cell-columns lying along the 

 sides of the aorta, and along the dorsal surfaces of the carotid arteries. 

 The " anlagen " of the secondary sympathetic trunks 'arise about the 

 beginning of the sixth day, as ganglionic enlargements on the median 

 side of the spinal nerves. These ganglionic enlargements are at first 



* Jouru. Comp. Neurol., xx. (1910) pp. 283-308 (10 figs.). 



