504 A MANUAL OF VETERINARY PHYSIOLOGY 



may arise in relation to some novel situation ; hence, with what is 

 usually described as indicating intelligent as apart from instinctive 

 acts, the former being not merely accompanied, but controlled, by 

 consciousness ' (Watson). The region of the brain where these pro- 

 cesses are carried on is mainly in the prefrontal, or region of higher 

 associations. Of less importance in this respect is the visuo -psychic, 

 or region of lower associations ; and of least importance is the visuo- 

 sensory or projection -sphere region. Layer 3 deals with the recep- 

 tion or transformation of afferent impressions, whether from within 

 or without the brain. Layers 4 and 5 govern the lower voluntary 

 and instinctive activities of the body, which require neither experience 

 nor education. They deal with the preservation and perpetuation 

 of the species, such as food, shelter, and sexual consort, accompanied, 

 though not necessarily controlled, by consciousness. Preservation is 

 deeply, firmly, and early implanted in the brain of the lowest 

 mammals, and in them is as fully developed as in man. It is the 

 common platform on which they meet man. In Fig. 154 is shown 

 the microscopical structure of the Betz-cell area of man and the 

 dog, and the motor area of the pig, the depth of the laminae in each 

 case being indicated.* 



Coverings of the Brain. — The dura mater is a dense fibrous 

 membrane, which acts the part of a protective covering for the 

 brain ; between it and the arachnoid there exists a lymphatic space 

 known as the subdural. The arachnoid contains but few vessels 

 and no nerves, and covers the extremely vascular pia mater ; 

 between the arachnoid and the pia mater is formed the sub- 

 arachnoid space, which contains the subarachnoid or cerebral 

 fluid. 



Cerebral Fluid. — The subarachnoid space communicates with 

 the ventricles of the brain ; the lymph in it is also known to be 

 in communication with the perivascular spaces of the cerebral 

 vessels, and the lymphatic spaces in the perineural covering of 

 nerves. Through the fourth ventricle it communicates with 

 the central canal of the spinal cord, and there is also a connection 

 between the cerebral spaces and those formed on the exterior 

 of the cord. The subdural and to an extent the subarachnoid fluid 

 communicates with the sinuses of the dura mater. The cerebral 

 fluid is secreted by the pia mater and choroid plexus ; in horses it 

 is normally small in quantity, amounting to about 5 or 6 grammes 

 (80 or 90 grains) . The use of this fluid is to equalise the pressure 

 on the brain, and to afford protection to the latter ; by the manner 

 in which this organ is suspended inside the skull by the dura 

 mater it is saved from jar and concussion. Both cerebrum and 

 cerebellum half float on water-cushions. Withdrawal of the 

 cerebral fluid leads to convulsions, while an increase in the 

 amount may cause coma, owing to the pressure it exercises. 

 When the arteries in the brain dilate, the skull being unyielding, 



* For these I am indebted to Dr. G. A. Watson. The structure of the 

 cortex of the brain of the pig has not previously been published. 



