506 



THE NERVOUS SYSTEM. 



the somatic efferent nucleus. Then there is a group of small multipolar cells, 

 the axons of which pass out into sympathetic ganglia, and indirectly control the 

 involuntary unstriped muscles and other active parts of viscera. These cells 

 form the splanchnic efferent nucleus. 



In the upper cervical and lower cranial region a portion of the somatic efferent 

 nucleus is set apart to innervate the striped muscles developed in the branchial 

 arches. This is the lateral somatic or intermediate efferent nucleus. Many recent 

 writers are of the opinion that this nucleus is splanchnic ; but its fibres directly 

 innervate striped voluntary muscles, which are developed from the same material 



ROOF-PLATE 



Splanchnic Terminal Nucleus. 

 ' Gustatory Nucleus. 



,Acousti'co-Lateral 



Terminal Nucleus. 



Somatic Terminal 

 Nucleus. 



--- Ear Vesicle. 



LAMINA) 

 BASALISJ 



Somatic -- 

 Efferent Nucleus 



Floor 

 Plate" 



Sensory Ganglion. 



Skin. 



Striped 

 Muscle 

 Sympathetic Ganglion - 



Unstriped ; 

 Muscle 



Visceral 

 Mucous Membrane. 



--Branchial 

 Striped 

 Muscle. 



FIG. 446. DIAGRAM OF A TRANSVERSE SECTION THROUGH THE EIGHT HALF OF THE FCETAL EHOMBEN 

 CEPHALON AND EPITHELIAL AREAS ASSOCIATED WITH IT TO ILLUSTRATE THE DIFFERENT CATEGORIES 

 OF NERVE COMPONENTS AND THEIR CENTRAL NUCLEI. 



(myotomes) from which the other striped muscles are formed (Agar and Grahan 

 Kerr). 



The alar lamina also can be subdivided into a series of functional area; 

 (Fig. 446). 



At the dorsal edge is the somatic afferent terminal nucleus, which receives im 

 pulses coming from the skin. In one region a part of this nucleus is specialise' 

 for the reception of impulses coming from the internal ear (acoustico-latera 

 terminal nucleus). Then there is a group of cells collected around the incomin 

 visceral sensory nerves the splanchnic afferent terminal nucleus. A part of this : 

 specialised to receive taste impressions the gustatory nucleus but this has IK 

 yet been clearly demarcated from the rest of the nucleus. 



This analysis of the various functional elements that may enter into tt 

 constitution of the various cerebral and spinal nerves is made use of in elaboratir 

 the theory of nerve components, which will help us to understand many featur* 

 of the structure of the nervous system that otherwise would be unintelligible. 



Nerve -cells. We have already noticed that there is a broad distinctk 

 between the nerve-cells which are found in the ganglia of sensory nerves and tho 



