832 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



VII. THE CRANIAL NERVES. 



The cranial nerves, twelve in number on each side, arise from differ- 

 ent parts of the brain and pass through foramina in the base of the skull, 

 their number being given to them from the order in which they pass out 

 from the base of the brain ; other names are, also, given them from the 

 parts to which they are distributed or from their functions. 



The cranial nerves are either pure sensory nerves, pure motor nerves, 

 or mixed nerves. The pure sensory nerves are the olfactory (1), optic 

 (2), and acoustic (8); the pure motor nerves are the oculo-motor (3), the 

 pathetic (4), and the abdncens (6); the trifacial (5), or trigeminus, is a 

 mixed nerve, arising from a distinct motor and a distinct sensory root 

 comparable to the spinal nerves. Regarding the functions of the other 

 cranial nerves as determined by the functions of their roots, the pneu- 

 mogastric and glosso-pharyngeal are sensory nerves and the facial, 

 spinal-accessory, and hypoglossal nerves are motor. The trunks of these 

 last five nerves, however, contain both afferent and efferent fibres, the 

 pneumogastric receiving efferent fibres from the spinal-accessory and the 

 glosso-pharyngeal from the facial and motor branch of the trigeminus; 

 while the facial receives afferent fibres from the trigeminus, pneumogas- 

 tric, and glosso-pharyngeal, and the hypoglossal sensory fibres from the 

 trigeminus, vagus, and three upper cervical nerves. 



The functions of these nerves, although already considered under 

 different subjects, will be here recapitulated. 



1. The Olfactory Nerve. (See Sense of Smell.) 



2. The Optic Nerve. (See Sense of Vision.) 



3. The Oculo-Motor Nerve. — This nerve arises from the oculo- 

 motor nucleus under the aqueduct of Sylvius, the fibres of origin being 

 traced into the corpora quaclrigemina and through the cerebral peduncle 

 into the lenticular nucleus. It contains three sets of fibres which are 

 distributed (1) to all the muscles of eyeball, with the exception of the 

 superior oblique and external rectus muscles and the levator palpebral 

 muscles, (2) to the circular muscles of the pupil, and (3) to the ciliary 

 muscle. Hence, it is the path for voluntary motor impulses to all the 

 muscles of the eyeball, with the exception of the muscles above mentioned, 

 it is the efferent nerve for the reflex contraction of the pupil from 

 the action of light on the retina, and it contains the voluntary motor fibres 

 to the muscle of accommodation (ciliary muscle). 



4. The Pathetic Nerve ( Trochlear^).— The fibres of the fourth 

 cranial nerve may be traced back from their apparent origin on the outer 

 side of the cms cerebri in front of the pons varolii, beneath the corpora 

 quadrigemina, to the valve of Vieussens (behind the fourth ventricle), on 

 the upper surface of which it is connected by commissural fibres with its 



