GENERATION AND DEVELOPMENT 695 



the influence of higher centres in the brain. Erection and 

 ejaculation in the dog may be produced by stimulation of a 

 definite area of the cortex of the cerebrum, and they may also be 

 produced after section of the spinal cord in the lumbar region. 

 The sensory nerves in the penis, by which erotic sensations are 

 carried, are the pudic ; if the pudic nerve be cut, erection, is 

 impossible ; if the central cut end be stimulated, it leads to 

 ejaculation. 



The first portion of the penis which receives the excess of 

 blood in erection is the corpus cavernosum ; the spongiosum 

 and glans are not fully erect in the stallion until the penis is 

 introduced into the vagina. The erector penis muscle com- 

 presses the penis against the pelvis, and by blocking the return 

 of blood assists in maintaining erection (Sisson) . At the moment 

 of ejaculation in this animal the glans swells enormously, appar- 

 ently to cover or grasp the os uteri. After intercourse the organ 

 is withdrawn into its sheath by the contraction of a pair of un- 

 striated muscles, known as the retractor penis* Though the organ 

 in the horse assumes such considerable proportions, in the bull 

 this is not marked. The peculiar penis in this animal comes to 

 a narrow point without any of the swelling observable in the 

 stallion. In the act of erection, the S-shaped curve of the penis 

 is removed, and the organ elongates ; at the same time the 

 retractor muscles of the sheath draw back the prepuce and the 

 organ is exposed. In the ram, also, the penis is narrow and 

 pointed, and the vermiform appendix at its extremity appears 

 essential for successful impregnation, for if it be removed it is 

 said the animal proves sterile. In the boar the penis terminates 

 in a peculiar corkscrew-like ending. In the dog the increase in 

 the size of the penis is mainly at its posterior part, and the bulbous 

 swelling there observable f is the portion grasped by the spasm 

 of the sphincter cunni of the female, rendering withdrawal im- 

 possible until complete relaxation occurs. The bone in the 

 penis of the dog facilitates its introduction into the vagina. 



Sexual Intercourse. — Copulation is not necessary in all animals, 

 nor indeed in any. What is required is merely an interchange 

 of elements from the nucleus of two different cells. To this last 

 statement a slight exception might be taken, because there is 

 a condition, parthenogenesis, where the access of a second element 

 is not required, but this method of development is unknown in 

 the higher animals. The act of intercourse is of short duration 



* The retractor penis muscle is found in the horse, ox, dog, and cat. 

 It has a double source of innervation : motor from the lumbar sympathetic, 

 and inhibitory from the nervi erigentes. 



f The bulbous swelling is actually the termination of the glans, which 

 in the dog is of considerable length, and extends over the entire length 

 of the os penis (Sisson). 



