542 Dynamic Theory. 



cles of the ribs and abdomen, back and loins. The sensory portion of 

 the dorsal nerves bring the sensation of touch from the skin of the ab- 

 domen, chest, loins and back. Of the lumbar nerves, the motor part 

 connect with muscles in the abdomen, groin, thigh, leg, loins, sacrum, 

 nates, and scrotum, while the sensory part receive the touch impres- 

 sions of the skin of the same parts. Of the sacral nerves, the motor 



FIG. 265. Brain and Spinal Cord of Human foetus, 3 months. (Kolliker.) 



^.Hemispheres of the Cerebrum. 



m. Midbrain Corpora Quadrigemiiia. 



c. Cerebellum. 



4. Fourth Ventricle. 



o Medulla Oblongata. 



5. Spinal cord. 



6. Its enlargement for the Arms. 



L Legs, or Lumbar enlargement. 



part serve the muscles on the external side of the leg, and 

 part of the toes and the foot, part of the thigh, the organs 

 of generation uterus &c. , and the bladder, nates, &c. The 

 sensory part convey the touch stimuli from the skin corres- 

 ponding to the muscular parts named. The relation be- 

 tween the spinal nerves, spinal cord and brain, are shown in 

 figs. 262 & 263. 



The nerves of all the senses, except touch, and some nerves of that 

 sense, too, pass directly into the skull, and there connect with the great 

 FIG. 265. central nervous system of the body. These nerves of sense, together with 

 others which pass out from the brain as motor nerves, constitute what are called the 

 Cranial or Cerebral Nerves. In the vertebrates generally, including man, there are 

 twelve pairs of them, as follows: 



First pair are the olfactory nerves or lobes. They always, among the vertebrates, retain 

 a connection with the hemispheres of the cerebrum, and often have a cavity in each 

 which connects with the cavity of the corresponding hemisphere called the lateral ven- 

 tricle. These olfactory lobes rest upon the ethmoid bone of the skull, through holes in 

 the bottom of which the nerve fibres pass, connecting the olfactory lobes with the pit- 

 uitary membrane of the nose. The lobes are to be regarded as parts of the brain, being 

 real ganglia containing gray vesicular matter. This relationship is more obvious in the 

 lower vertebrates in which the lobes are relatively more prominent, and the sense of 

 smell relatively more highly developed. The nerves pass backward from the lobes to 

 the Optic Thalamus, in which there are additional olfactory ganglia. The olfactory 

 nerves are exclusively sensory, none of their fibres possessing any motor influence with 

 any of the muscles. 



The second pair are the optic nerves. These likewise start from lobes or ganglions, 

 viz., the Corpora quad ri gem in a, in which each one has two roots, one in the natis and one 

 in the testis. They pass forward from these ganglions by way of the optic thalamus, 

 from which they receive large accessions of nerve fibres, which connect with the thala- 

 mus by two more roots on each side, thence along the base of the brain, each one of the 

 pair con verging toward its fellow. They cross each other, forming the optic chiasm, or 

 decussation, in front of a grayish tubercle at the base of the brain, called the Tuber 

 cinereum. After crossing, the nerves pass into the orbits of the eyes, and terminate 

 upon the retina. They are sensory nerves, but are also closely connected with the third 

 pair, through fibres of which reflex action is set up in the iris in the contraction of the 

 pupil. Sneezing is also excited by reflex action of light, through connection with the 

 fifth pair. 



The third pair, the motores occulorum, move the majority of the muscles of the eye. 

 They divide into two branches, the superior branch going to the rectus superior oculi, or 

 superior straight muscle of the eyeball, and to the upper eyelid, and the inferior branch 

 going to the lower and inner straight muscles, and the lesser oblique muscles of the eyeball. 

 They are motor nerves, but have fibres which are sensory, receiving touch stimuli from 

 the skin and sensitive surfaces about the eyebu II. These nerves connect with the pedun- 

 cles of the brain (crura cerebri ) below the corpora albicantia, each one by two roots like 



