STRUCTURE OF THE HUMAN BODY 53 



both efferent and afferent fibres, as is the case with the remark- 

 able tenth pair of cranial nerves, named vagus on account of their 

 wandering course. Running along the neck, these nerves pass 

 into the thorax, where they send branches to both heart and lungs, 

 and, piercing the midriff, end in branches supplying the stomach. 



The Sympathetic System. This brief survey of the nervous 

 system may be completed by referring to the sympathetic system, 

 which consists in the main of a couple of slender cords running 

 ventral to the backbone, and dilating at intervals into small 

 swellings known as the sympathetic ganglia, containing nerve- 

 cells, and therefore considered as collectively constituting a 

 part of the central nervous system. They are connected with 

 the spinal and some of the cranial nerves, and largely control 

 the internal organs and blood-vessels, to which they send 

 numerous branches. The working of the sympathetic system 

 is entirely involuntary, a fortunate circumstance to which we 

 owe the fact that we are very largely unconscious of the 

 internal movements of the body. 



SENSE ORGANS. The sense organs have been described as 

 the implements of the nervous system, gaining, as it does, 

 information about the external world by their means. A sense 

 organ may broadly be regarded as a more or less modified 

 piece of skin adapted to receive impressions from 

 some external agent (contact, heat, light, sound, 

 &c.) or stimukts (L. stimulus, an ox-goad), and 

 connected up with the central nervous system by 

 a sensory nerve. 



The Skin as a Sense Organ. The skin of the 

 neck, trunk, and limbs is richly supplied with 

 sensory nerves derived from the spinal cord, while 

 the skin of the head and the linings of the mouth- 

 and nose-cavities receive branches from the fifth 

 cranial nerve. Connected with many of the Fig 29 . _ Magnified 

 smallest twigs of these nerves are microscopic 

 bodies which are known as touch-corpuscles (fig. 29), 

 because they are believed to have to do with the sense of touch. 

 A finger-tip furnishes the best example. Upon it will readily be 

 seen a series of fine ridges arranged in a characteristic manner, 

 differing largely, however, in different individuals, and thus afford- 

 ing an important means of identification. Under these ridges, 



