lo Dec, 1908.] 



Elements of Animal PIivsiolos. 



y- 



717 



the spinal cord and is probably concerned with skilled movements. The 

 shape of the grey matter on cross section of the cord is characteristic 

 (Fig. 59). On each side will be found a blunt ventral horn containing 

 amongst many others, large nerve cells which are those that give rise to 

 motor nerves ; a sharp dorsal horn into which stream afferent fibres ; and 

 in some regions of the cord a small lateral horn probably connected 

 with the preganglionic fibres of the autonomic system. The spinal cord, 

 both white and grey matter, is almost divided into two by a deep dorsal 

 cleft or fissure and a shallower and more open ventral fissure. If the 

 spinal cord be dissected out it will be seen that on each side and, at a 



f b 



Fig- 59- Transverse section of spinal cord (semi-diagrammatic). a, ventraT 

 fissure; b, dorsal fissure; c, d, e, g, white matter; h, ventral rootlets; i, dorsal 

 root; m, ventral horn; n, dorsal horn; o, lateral horn. (After Erb.) 



series of different levels, the peripheral nervous system arises by means 

 of '' roots." (Fig. 60). On the dorsal side are the dorsal roots, each with 

 its ganglion attached, in which the nerve cells of the afferent fibres are 

 situated. Each dorsal root is made up of a large number of fibres whollv 

 afferent in character and is connected with the dorsal horns of grey matter 

 and w^ith the dense mass of white fibres on each side of the dorsal fissure. 

 The ventral roots arise, as the name indicates, from the ventral aspect 

 of the cord and are connected with the cells of the ventral horns. In 

 all regions of the cord they contain motor fibres for voluntary muscle but 

 in the thoracic and sacral regions they include preganglionic fibres as well. 

 Each ventral root joins a dorsal root a short distance beyond the ganglion 

 on the latter, and thus a mixed nerve trunk is produced. This condition 

 mav be taken therefore as typical of the w'hole cord — a dorsal series of 

 groups of neurons bringing in afferent impulses and a ventral series 

 allowing an outflow of impulses to voluntary muscle and the organs 

 innervated bv the autonomic system. 



Afferent impulses reach the spinal cord from receptors in the skin of 

 the trunk, limbs and tail, from the pelvic viscera, to some degree from the 

 abdominal alimentary canal, and from the proprioceptors in the tendons, 

 ligaments, muscles and joints of the trunk, limbs and tail. The motor 



