Peripheral Nervous System in Human Embryo 289 



ground bundle has been modelled out and forms the support to the 

 reconstruction shown in Plate II. 



Cekebral Nerves. 



For purposes of description the nerves of the head will be grouped 

 in accordance with their relation to the four functional systems of 

 Gaskell, the significance of which has been emphasized in the recent 

 writings of Johnston, C. J. Herrick, and others. According to Gaskell 

 the functional systems are determined by the two chief activities of the 

 organism; first, actions in relation to the external world (somatic), and 

 second, internal activities having to do with the processes of nutrition, 

 etc. (visceral). In each case there is the double activity on the part 

 of the nervous system, sensory and motor, making in all four primary 

 functional divisions. Now some of the cranial nerves consist of 

 elements belonging exclusively to one functional division, for example, 

 the n. abducens which consists entirely of somatic motor fibres, while 

 others are complex nerves containing elements of more than one system, 

 such as the n. glossopharyngeus which contains elements from three 

 functional divisions, somatic sensory, visceral sensory and visceral motor. 

 The nerves will therefore be grouped according to the predominance of 

 their functional elements as follows: 



Somatic Sensory. Somatic Motor Visceral 



Olfactory Oculomotor Trigeminal 



Optic Trochlear Facial 



Acoustic Abducens Glossopharyngeal 



Hypoglossal Vagus and Accessory 



In general the basal plate of the neural tube is motor and the lateral 

 plate is sensory; thus the somatic motor group arises entirely from the 

 basal plate, while the visceral group arises in large part from the lateral 

 plate and in lesser part from the basal plate. The somatic sensory group 

 is specialized and these nerves have individual processes or lobes of the 

 nervous system devoted to them, i. e., the olfactory bulb, the eye bulb 

 and stalk, and the tuberculum acusticum. 



Nerves of Special Sense Organs. 



These nerves belong to that large group of afferent nerves that con- 

 nect the integument with the central nervous system, the somatic 



