MAMMALIA— PHYSIOLOGY 467 



The nerve centers and peripheral nerves which direct somatic 

 movements constitute a distinct portion of the Nervous System 

 which we may call the somatic {efferent) motor division. 



The visceral activities consist of contractions of the visceral 

 muscles, secretory processes and vaso-motor regulation. All these 

 contribute directly or indirectly to the processes of nutrition in 

 the widest sense or of reproduction. Just how far they may be 

 called forth by somatic stimuli is not known. We know that the 

 sight of food may cause salivary secretion. Respiration is called 

 forth by rise in temperature within the body. The nerve centers 

 and fibers which control the activities of the viscera make up the 

 visceral efferent or viscero-motor division of the Nervous System. 



To summarize, in any vertebrate animal there are four kinds of 

 nervous activity: (i) Reception of somatic stimuli. (2) Direction 

 of somatic movements. (3) Reception of visceral stimuli. (4) 

 Direction of visceral movements and activities. 



The mammalian brain exhibits marked superiority over that of 

 the lower vertebrates in the development of large richly convoluted 

 cerebral hemispheres and a cerebellum not surpassed in any form 

 except the bird. The olfactory lobes situated ventrally at the ante- 

 rior portion of the cerebral hemispheres are highly developed in 

 many of the lower mammals. In common with the reptiles and 

 birds, the mammals have 12 pairs of cerebral nerves. There are j/ 

 pairs of spinal nerves in man, and 38 pairs in the cat. 



Both the brain and the spinal cord consist of gray and white 

 nervous matter but in the brain the gray is on the outside and the 

 white is within while in the spinal cord the gray is on the inside and 

 the white is external. White matter is chiefly composed of nerve 

 fibers while gray matter is much more vascular and composed of 

 nerve cells which give rise to nerve fibers. Nerve fibers are non- 

 medullated — {a) Naked axis cylinder, {b) Axis cylinder with primitive 

 sheath, and medullated — {c) Primitive sheath absent, {d) Primitive 

 sheath present. Medullated fibers are present in greater quantity 

 than non-medullated in the cerebro-spinal system. The mass of 

 white substance of the brain, spinal cord, and optic nerve has 

 medullated fibers without the primitive sheath. Both brain and the 

 spinal cord are covered by three membranes, the outer dura mater 

 with a thin delicate membrane, the arachnoid^ which serves as a 

 sheath for the nerves and is separated from the dura mater by a 

 narrow sub-dural space. A vascular membrane, the pia mater^ is 



